The following description of toadflax presents characteristics that may be relevant to fire ecology, and is not meant for identification. Correct identification of toadflax is important if control strategies are planned, because it may be confused with native snapdragons (Antirrhinum spp.). Keys for identification are available (e.g. [14,31,43,117,119]).
Dalmatian and yellow toadflax are deep-rooted, short-lived, herbaceous perennials. They reproduce by seed, and can form colonies by means of adventitious buds on roots. Toadflax shoots are tough and somewhat woody near the base, and are produced singly or in clumps of up to 25 stems per crown, spaced along the shallow, lateral roots [1,88,95,114].
Dalmatian toadflax is variable in many characteristics [1]. Stems of Dalmatian toadflax are 16 to 36 inches (40-90 cm) tall or taller, somewhat woody at the base, and the upper portion is frequently branched. Decumbent to weakly ascending stems are produced in the fall [1,88]. Both leaves and stems are waxy. Leaves are usually heart-shaped, 0.8 to 2 inches (2-5 cm) long, 0.4 to 0.6 inch (10-16 mm) wide with clasping bases. Flowers grow in the axils of upper leaves, in loose, elongate, terminal racemes and have a spur that is about as long as the rest of the flower (0.6 to 0.9 inch (14-24 mm)). The fruit is a capsule, 6-7 mm long and 4-8 mm wide, bearing an average of 140 to 250 seeds [87]. Seeds are 1.2-2 mm long with wings [14,28,31,62,119].
Yellow toadflax has a variable habit, depending on environmental factors such as shading, grazing, or soil type, which may be expressed in flower and pollen grain morphology [95]. Yellow toadflax stems are usually 12 to 36 inches (30-90 cm) tall, simple to branched. Yellow toadflax leaves are narrow, linear, 1 to 2 inches (2.5-5 cm) long, 2-6 mm wide, somewhat pointed at both ends, and not clasping. Yellow toadflax flowers occur at first in clusters near the ends of the stems, growing at the bases of upper leaves, and becoming more widely spaced along the stem as the season progresses [57,119]. Yellow toadflax flowers also have a spur that is about as long as the rest to the flower (10-14 mm) [14,28,31,119]. Yellow toadflax fruit is a capsule, 5-12 mm long, bearing a highly variable number of viable seeds [6,12] about 2 mm long with broad wings [14,95].
Mature toadflax plants have extensive, well-developed root systems. Taproots of a mature Dalmatian toadflax plant may reach depths of 4 to 10 feet (1.3-3 m), and lateral roots can extend 12 feet (3.6 m) from the parent plant. Vegetative buds were found as deep as 6 feet (1.8 m) in coarse soil. However, most Dalmatian toadflax plants produced from vegetative buds occur on lateral roots that are found in the upper 2 to 12 inches (5-30 cm) of soil [1,88]. Mature yellow toadflax taproots may grow 3.3 feet (1 m) deep, and lateral roots can be several meters long. Shoots tend to grow from vegetative buds that are around 0.8 to 2 inches (2-5 cm) below the soil surface [95].
These extensive root systems allow toadflax plants to exploit water resources efficiently and provide an effective anchor that prevents grazing animals and cultivation methods from dislodging or destroying plants. Superficial yellow toadflax roots do not overwinter; however, the roots are able to survive freezing to 5 degrees Fahrenheit (-15 °C) after artificial acclimatization to low temperatures [95].
Roots of both Dalmatian toadflax and yellow toadflax are colonized by VAM fungi, probably less so in cultivated soil than in populations established in less disturbed areas [39].
A life span of at least 4 years for individual roots has been reported for yellow toadflax [95], while individual Dalmatian toadflax plants are thought to live an average of 3 to 5 years. Dalmatian toadflax patches can persist for 13 years or more under favorable conditions. Dalmatian toadflax stands sometimes disappear for several years, then re-establish, either from buried seeds, or perhaps from vegetative root buds [88]. In this way a stand may persist indefinitely [57]. The age of toadflax plants may be determined by observing growth rings in the secondary xylem of the main root [18,88].
The following discussion of the origin, introduction and distribution of Dalmatian toadflax and yellow toadflax is based on several review articles [11,57,95,114], unless other sources are cited.
Dalmatian toadflax is a native of the Mediterranean region from Yugoslavia to Iran [88]. Yellow toadflax is native to the steppes of southeastern Europe and southwestern Asia [95]. Both species have been cultivated as ornamentals for centuries and are widely distributed throughout the world [1,95,114].
It has been suggested that Dalmatian toadflax was introduced to North America as an ornamental in the late 1800s; however, the earliest authentic specimen was collected in California in 1920 [1,114]. Yellow toadflax was introduced to New England in the late 1600s as an ornamental and medicinal plant and continues to be sold in nurseries and seed catalogs. For example, Gutknecht [35] lists "Linaria vulgaris (common toadflax or butter-and-eggs)" as a plant that is well suited for xeriscaping. The spread of toadflax was facilitated by its use as an ornamental, medicinal, magical, and dye plant, although accidental introduction and distribution along roads and railway corridors, or in crop seed, baled hay, ship ballast, and clothing likely increased its spread [57,95].
Toadflax occurs throughout the continental U.S. and in almost every Canadian province [47,110]. Yellow toadflax is most common in northeastern North America, and localized in other parts of the continent, particularly the western Canadian provinces [57]. The northern limits of yellow toadflax's North American range are approximately 55° N to 65° N [95]. The latitudinal range of Dalmatian toadflax in North America is from about 33° N to about 56° N, as compared to a range of about 35° N to 47° N in Eurasia [1]. Dalmatian toadflax is most common in western North America, especially in California, Oregon, Washington, Idaho, Montana, Wyoming, Alberta, and British Columbia [11,57,77], and it is spreading in the Southwest. For example, it is estimated to have invaded 200,000 acres (80,000 ha) on the Coconino National Forest in northern Arizona as of 2001 [76].
Toadflax is most commonly found in cultivated fields, roadsides, railways, "waste areas", clearcuts, overgrazed pastures and rangeland, and in plant communities that are typically open or disturbed [6,57]. Neither species occurs as frequently in intact wildlands and natural areas.
The following lists include some ecosystems, habitats, and forest and range cover types in which toadflax is known or thought to be invasive, as well as several that may be invaded by toadflax following disturbances in which vegetation is killed and/or removed (e.g. cultivation, logging, fire, grazing, herbicide application, flooding). These lists are speculative and not exhaustive. More information is needed regarding incidents and examples of particular ecosystems and plant communities where toadflax is invasive.
Fire adaptations: Toadflax has a deep and extensive perennial, sprouting root system that is likely to allow it to survive even severe fire. Toadflax is also capable of establishing either from on-site seed, or seed dispersed into a burned area. Seed may be dispersed by animals into recently burned areas where it is adapted to establish under conditions of reduced competition. It is unclear what the effects of fire are on toadflax seed.
FIRE REGIMES: Dalmatian toadflax occurs in ecosystems with historic FIRE REGIMES of varied frequency and severity; from frequent, low-severity fires in ponderosa pine ecosystems, to less frequent and more severe fires in bunchgrass and sagebrush ecosystems, to frequent and severe fires in plains and prairie grassland ecosystems. Yellow toadflax occurs primarily in agricultural communities in the western U.S. and throughout Canada, and in disturbed areas in the north-central and northeastern U.S. A variety of native plant communities once dominated these areas, and historic FIRE REGIMES have been dramatically altered.
Toadflax was not widespread in these communities when historic FIRE REGIMES were functioning, but has established since habitat alteration and fire exclusion began. It is unclear how historic FIRE REGIMES might affect toadflax populations, and it is unclear how the presence of toadflax in native ecosystems might affect fire regimes.
In general, in ecosystems where toadflax replaces plants similar to itself (in terms of fuel characteristics), it may alter fire intensity or slightly modify an existing fire regime. However, if toadflax is qualitatively unique to the invaded ecosystem, it has the potential to completely alter the fire regime [15]. No examples of FIRE REGIMES altered by toadflax invasion are described in the available literature.
The following table provides fire return intervals for plant communities and ecosystems in which toadflax may be found. Find further fire regime information for the plant communities in which these species may occur by entering the species' names in the FEIS home page under "Find FIRE REGIMES".Fire as a control agent: Burning is not usually a recommended or effective control method for toadflax, because root buds and buried seeds are unaffected by fire, and burning may increase competitiveness of toadflax by removing desirable plants [11,57,81,95]. Removal of toadflax top-growth may even stimulate production of vegetative shoots [51,57]. Nonetheless, Nernberg [71] describes a mixed-grass prairie restoration program in Saskatchewan in which "appropriately timed" prescribed burning is used to control growth and eliminate seed production in yellow toadflax and other nonnative invasive species; although no specific details or results are provided.
According to a review [57], scorching of floral stalks using propane burners can help prevent toadflax seed production.
Prescribed fire can be used as a management tool on some sites in an effort to restore historic FIRE REGIMES and promote desirable species. The disturbance created by fire may, however, favor many invasive species. At the time of this writing, there are ongoing studies in western Montana designed to test the effects of prescribed fire combined with herbicide application to control Dalmatian toadflax and other invasive species [85]. Herbicides may prevent domination by invasive species after fire in the short term; however, they also affect nontarget forbs that can compete with invasive species [45].
On a big sagebrush-bluebunch wheatgrass site in western Montana, Jacobs and Sheley [45] tested the effects of prescribed burning with and without herbicide application on density, cover, and biomass of Dalmatian toadflax and native forbs as well as species richness, diversity, and evenness. Herbicides (picloram at 0.56 kg a.i./ha, and chlorsulfuron at 0.075 kg a.i./ha) were applied in October (prior to burning) and April (after burning). Burning was conducted in April. They found that fire effectively killed nearly all trees on the test plots (primary objective of prescribed burning), but did not affect species richness, diversity, or evenness. One season after burning, the biomass and cover of toadflax, but not its density, were 2 times greater on burned plots than on unburned plots. All herbicide treatments reduced biomass, cover, and density of Dalmatian toadflax to 90% of the control. Chlorsulfuron decreased overall forb biomass to 50% of the control, while picloram decreased forb biomass to nearly 0. Both herbicides reduced richness and diversity, but not evenness. Timing of herbicide application made no difference in toadflax control or diversity indices, although spring application resulted in higher grass production. The authors conclude that using an herbicide such as chlorsulfuron may provide short-term control of Dalmatian toadflax. However, the combination of open niches left by forbs killed by herbicides, trees and shrubs killed by fire, and pressure on grasses by wildlife leaves sites susceptible to reestablishment of Dalmatian toadflax from the soil seed bank [45].
Postfire colonization potential: Because of its propensity to establish in dry, open areas with little plant competition, toadflax has high potential for establishing after fire (when competition from other vegetation is removed or reduced) either by seed imported to the site or by soil-stored seed. Several examples follow where toadflax established following fire. It is not clear in any of these examples whether toadflax plants or seeds were on-site prior to burning.
Two fire case studies near Flagstaff, Arizona, (Chimney Spring and Limestone Flat) were established to determine a burning interval that would adequately manipulate fuels and stocking of a ponderosa pine stand so that it could survive wildfire. The initial objective was to reduce fuel loads by reintroducing fire in areas where it had long been suppressed and fuels had accumulated. The fire at Chimney Spring reduced forest floor fuels by 63%, compared to a 42% reduction of forest floor fuel at Limestone Flat. In both places, several invasive species were abundant after fire, including Dalmatian toadflax, common mullein (Verbascum thapsus), and thistle (Cirsium pulchellum). Common mullein and Dalmatian toadflax were dominant on heavily burned sites around large, old-growth trees that have died since the initial burns [92,93]. Similarly, Kyle [54] observed several sites in northern Arizona where large-scale, high-severity fires have burned over the past 4 decades and noted that many of these areas are dominated by nonnative plants such as cheatgrass, smooth brome (Bromus inermis), sweetclover (Melilotus spp.), orchardgrass (Dactylis glomerata), perennial ryegrass (Lolium perenne), and Dalmatian toadflax.
In ponderosa pine forest sites in northern Arizona in 1989, prescribed burning was conducted to investigate its effects on the rare plant, Flagstaff pennyroyal (Hedeoma diffusum). Dalmatian toadflax was not noted prior to burning or during counts from 1989 to 1995; but in 2000 it had invaded all of the spring burn plots, the control plot (which was adjacent to the spring burn plots), and none of the fall burn plots. It is unclear whether it was present prior to the initiation of the study, or if it was introduced during fire line construction, or following burning; however, it had not yet invaded the area surrounding the burn study area where grasses were very dense. The authors urge caution and awareness of Dalmatian toadflax and other weed species when prescribing fire treatments [76].
Yellow toadflax was found in a jack pine-lichen woodland of the upper boreal forest in northern Quebec, 2 years after wildfire [100].
Toadflax invasion after fire may also be related to soil disturbances brought about through fire suppression activities. For example, after a wildfire and suppression activities in Glacier National Park, Montana, Dalmatian toadflax was found in trace amounts in bulldozed areas, but was not present in either burned or undisturbed areas [8].
Preventing postfire establishment and spread: The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [109] provides several fire management considerations for preventing establishment of invasive species that apply to toadflax.
Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This can be accomplished through early detection and eradication by careful monitoring, and by limiting invasive plant seed dispersal into the burned area by [29,109]:
Early detection is key, and monitoring in spring, summer, and fall is imperative. Eradicate newly established toadflax plants and small patches adjacent to burned areas to prevent or limit seed dispersal into the site [29,109]
Revegetation necessity can be based on the degree of desirable vegetation displaced by invasive plants prior to burning and on postfire survival of desirable vegetation. Revegetation necessity can also be related to invasive plant survival as viable seeds, root crowns, or rhizomes capable of reproduction. In general, postfire revegetation should be considered when desirable vegetation cover is less than about 30% [29].
When prefire cover of toadflax is absent to low (and has been so for 10 or more years- i.e. no seeds in the seed bank), and prefire cover of desirable vegetation is high, revegetation is probably not necessary after low- and medium-severity burns. After a high-severity burn on a site in this condition, revegetation may be necessary (depending on postfire survival of desirable species), and intensive monitoring for invasive plant establishment is necessary to detect and eradicate newly established invasives before they spread [29].
When prefire cover of toadflax is moderate (20 to 79%) to high (80-100%), revegetation may be necessary after fire of any severity if desired vegetation cover is less than about 30%. Toadflax plants are likely to survive even severe fires, so intense weed management is also recommended, especially after fires of moderate to high severity [29].
Fall dormant broadcast seeding into ash will cover and retain seeds. If there is insufficient ash, seedbed preparation may be necessary. A seed mix should contain quick-establishing grasses and forbs (exclude forbs if broadleaf herbicides are anticipated) that can effectively occupy available niches. Managers can enhance the success of revegetation (natural or artificial) by excluding livestock until vegetation is well established (at least 2 growing seasons) [29].
When planning a prescribed burn, managers should preinventory the project area and evaluate cover and phenology of any toadflax present on or adjacent to the site, and avoid ignition and burning in areas at high risk for toadflax establishment or spread due to fire effects. Avoid creating soil conditions that promote weed germination and establishment. Discuss weed status and risks in burn rehabilitation plans. Also, wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines; monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding. Additional guidelines and specific recommendations and requirements are available [109].Lajeunesse [57] suggests that a high degree of genetic variation in toadflax allows it to adapt to a wide variety of conditions. These species evolved in areas where much of the land is cultivated, and are adapted to the periodic disturbances of agriculture [57]. In North America, they are most commonly found on disturbed sites such as roadsides, fencelines, areas near dwellings, vacant lots, cemeteries, gravel pits, croplands, clearcuts, pastures, waste areas, and other disturbed sites where removal of vegetation allows toadflax seedlings to establish [11,57,95]. Similarly, typical yellow toadflax habitats in Europe include vineyards, woodland clearings, and clearcuts. In Europe, large populations of yellow toadflax were observed on fields where competing vegetation was depressed by grazing or fire, and on some sites (e.g. between trees in orchards or in train yards) that had been subject to regular application of broad-action herbicides [95]. Dalmatian toadflax invasion is also encouraged by repeated grazing and herbicide applications [81].
Toadflax can also establish and spread in sparsely vegetated areas and sites with naturally-occurring disturbances, small openings, and/or little competition between species. Examples of such sites include dry, open areas in grassland and bunchgrass communities, sagebrush, open coniferous forests, sand dunes, riparian areas, and borders of woods (e.g. [2,11,16,56,57,61,67,77,81,82,107,113]).
Toadflax populations may establish but not spread widely into nearby natural areas. At a disturbed site in northern Manitoba, for example, yellow toadflax is among the 45% of introduced species that persist after initial establishment, as indicated by surveys done in 1959 and again in 1989. These yellow toadflax populations reproduce vegetatively, but do not mature fruits during the short growing season. Yellow toadflax is completely absent from natural communities in the area, and rarely occurs away from points of introduction. It does occur away from these points in the vicinity of human habitations or human disturbance (e.g. roadsides and railway tracks) [101]. This study site is near the northern limits of yellow toadflax's range (approximately 55° to 65° north latitude) [95].
Toadflax is most often found in well-drained, relatively coarse-textured soils varying from coarse gravels to sandy loams, but is sometimes found in heavier soils in North America. Similarly, native Eurasian habitats of this species are predominantly sandy, gravelly, or rocky [1]. Dalmatian toadflax is highly competitive in areas where summers tend to be dry in the northwestern and intermountain areas of the U.S., particularly on south- and southeast-facing slopes [2,57,88].
In central Europe, yellow toadflax prefers dry to moderately humid sandy loam soils that are moderate to rich in nutrients and minerals. Yellow toadflax may exhibit heavy metal tolerance. Yellow toadflax is more commonly associated with relatively summer-moist, coarse soils in the northwestern and north-central U.S. Yellow toadflax may grow well in moist areas of high fertility, but is more likely to be displaced by other species than on drier, less fertile sites. Yellow toadflax plants growing on dry sites are stunted but tend to be comparatively more persistent [95]. In many areas in North America, yellow toadflax is infrequent except in areas disturbed by cultivation [56,95].
The following table provides some elevation ranges for Dalmatian toadflax by geographic area:
Area Elevation References Colorado up to 9,400 feet (2,900 m) [67] Intermountain area 5,200 to 8,200 feet (1,600-2,500 m) [14] Nevada 3,000 to 7,000 feet (900-2,100 m) [48] New Mexico 5,000 to 6,000 feet (1,500-1,800 m) [62] Utah 4,400 to 10,000 feet (1,300-3,100 m) [119] Wyoming (Yellowstone NP) <6,200 feet (<1,900 m) [2]The following table provides some elevation ranges for yellow toadflax by geographic area:
Area Elevation References Intermountain area 6,600 to 9,200 feet (2,000-2,800 m) [14] Nevada 5,000 to 6,500 feet (1,500-2,000 m) [48] New Mexico 6,000 to 7,500 feet (1,800-2,300 m) [62] Utah 6,400 to 9,200 feet (1,950-2,800 m) [119]Toadflax is likely to be top killed by fire, however its deep, extensive root system is likely to survive even severe fire and allow reestablishment of the population from vegetative buds on roots. Many root-sprouting plants, including toadflax, have high fire survival rates, regardless of burn severity. This is because even the most severe fires typically damage roots only to 4 inches (10 cm) below the soil [29], and toadflax roots typically penetrate the soil to a depth of several feet.
There is little information available regarding the direct effects of fire on toadflax plants, and no information available regarding heat or fire effects on toadflax seed.
Impacts: Toadflax can be a serious agricultural weed that is favored by reduced-tillage farming methods, is resistant to many herbicides, and provides over-wintering sites for cucumber mosaic virus and broad bean wilt virus in New York [95].
Its impacts in wildlands and natural areas are not as clear. Like any invasive plant, it can displace plant communities and associated animal life. This can result in a loss of forage in pastures and rangelands that can impact livestock and some big game species, especially on winter ranges. Where sod-forming or bunchgrass communities are replaced by toadflax, soil erosion, surface runoff, and sediment yield can be increased. However, on harsh, sparsely vegetated sites toadflax can actually help stabilize soil [57,95].
In a survey of ranchers and farmers in north-central Idaho, 1% of respondents perceived Dalmatian toadflax as a moderate threat in 1982, and none perceived it as a problem in 1988 [10]. Toadflax can, however, spread rapidly, and it is important to eradicate infestations while they are still small in order to contain their spread [24,120]. For example, Dalmatian toadflax was introduced at a mine site in the 1970s on private land adjacent to the Raymond Mountain Wilderness Study Area in Wyoming. From this site, Dalmatian toadflax spread over 15 years to cover a 10-mile radius in the wilderness area and adjacent private land [120].
Control: Keys to successful control of toadflax are prevention of seed production, depleting root reserves, and killing seedlings before vegetative reproduction begins (within 2-3 weeks of germination) [11,95]. Reducing seed production may limit the spread of toadflax, although is not likely to result in a reduction of toadflax stand size or density because of vigorous vegetative reproduction. Reduction of toadflax stand size or density is more likely achieved by reducing the number of stems, limiting toadflax stem and lateral root-bud production, and encouraging desirable plant populations that provide strong competition for resources [32].
The diverse geographic range of toadflax throughout North America and its genetic variability result in localized populations that can respond differently to the same management methods and environmental conditions. Determining the most effective and economical control methods for a particular toadflax population will require annual monitoring, mapping, evaluation, and follow-up treatments. In this way, site-specific management efforts can be adjusted to determine the best combination of control strategies for a particular site [57].
Monitoring treatment areas is best done when toadflax plants have formed buds and are beginning to flower (around early June in most areas). Many control techniques are most effective at this time because root carbohydrate reserves are at their lowest, which makes it more difficult for root systems to recover. Follow-up work within the first 2 months of initial treatment (in late June or early July) is recommended to locate and remove any late-flowering plants [11].
Because toadflax occurs and is competitive with several other invasive species (e.g. [16,25,38,40]), management to control it and/or other species must consider the possibility of succession to an equally undesirable species when plants are removed [38,40,58,73,114].
Prevention: Because toadflax species are expensive, labor-intensive, and difficult to manage once established, preventing infestations is the most time- and cost-effective management approach. This is accomplished by maintaining desirable plant communities, by preventing toadflax seeds and root pieces from entering uninfested areas, and by careful monitoring for and early eradication of newly established plants. This is especially important where toadflax is common in areas around the management site, especially along roads, trails, and rivers (e.g. [81]).
Maintain desirable plant communities by limiting spring grazing (since toadflax seedlings can more effectively compete with grazed plants), minimizing soil disturbance, and seeding disturbed sites with desirable species (see Fire Management Considerations and Cultural control) [57,81,95].
Prevent seeds and root pieces from entering uninfested areas as follows [57]:
Toadflax is a common roadside weed. Prevention of its establishment (and the establishment of many other invasive species) requires that road construction projects be avoided whenever possible, especially in nature reserves. When unavoidable, road construction projects should be treated and funded as 10- to 20-year biological projects rather than 1- to 2-year engineering projects, with biologists and resource managers overseeing road construction. Projects should not be considered complete until native vegetation is fully established. Topsoil removed during construction can be redeposited in roadside ditches, and roadsides reseeded with native species. Roadsides should then be regularly monitored and actively managed for control and eradication of nonnative species [107].
Integrated management: Successful management of toadflax requires integrating as many management strategies as possible. Management programs for toadflax should emphasize both the prevention of seed formation and vegetative spread [57].
Physical/mechanical: Removal of the aboveground portion of toadflax plants can eliminate seed production for that year (if done in spring or early summer, before seed set), and reduce the current year's growth, but it will not kill them. Removal each year for 5 or 6 years may be necessary to deplete the remaining root system of reserves, and 10 to 15 years may be required to remove seedlings produced from dormant seeds. Hand-pulling, mowing, and tillage can be effective in preventing seed production and starving toadflax roots, thereby controlling infestations under certain conditions only if done repeatedly and/or in combination with other control methods [11,57,95].
Pulling toadflax by hand can effectively remove seedlings and small infestations and limit spread of large infestations by pulling around patch perimeters. Pulling is most effective in sandy or moist soils, because more root length can be removed. It is important to pull as much of the root as possible and to follow lateral roots to their ends [11,57]. Managers at several nature preserves in Colorado, Idaho, and Oregon have used hand-pulling to remove toadflax and keep infestations from spreading. Most say that early treatment, removal of as much of the roots as possible, and persistence for several years are important for hand-pulling to be effective [51,81,82]
A decade-long hand-pulling experiment at Magnusson Butte Preserve, Washington, demonstrates the effectiveness of repeated hand-pulling. The experiment was begun in a 5 by 5 meter test plot, and was then expanded to the entire 28-acre preserve. During the 1st week of June, a team of about 30 volunteers walked the preserve pulling all toadflax plants found. At this time, flowers were beginning to appear making plants easier to locate, and soils were still moist, making plants easier to pull with minimal soil disturbance. In the beginning, stems were removed in bags to avoid a mulching effect on desirable native plants, but in later years when there were fewer plants they were dropped in place with no ill effect. A follow-up visit was conducted during the last week in June to remove any late-flowering plants that might have been missed. Teams were able to reduce the number of flowering stems each year by an estimated 90-95% preserve-wide. In the 3rd year, it was noticed that flowering stems were not only reduced in number, but were smaller in size and lower in vigor [11].
Mowing and cutting of toadflax may be used to help decrease seed production, but will not eliminate toadflax stands [81,95,114]. Cutting flowering stems every year for several successive years can stress plants and help control infestations [51]. Mowing and cutting are usually not recommended, because they do not prevent root growth or affect buried seed [57]. Cutting may even contribute to stand longevity by stimulating dormant buds in the axils of vestigial leaves at the bases of cut floral shoots, as was observed in Dalmatian toadflax in Washington [88,89]. Considerable secondary and tertiary branching can occur following injury to the main stem [1]. Mowing may be less effective than cutting since plants that are cut several inches above the soil surface may sprout more rapidly. In addition, mowing may seriously damage desirable plants and is usually not feasible or desirable in natural areas [11].
Consistent, intensive, clean cultivation for at least 2 years with 8 to 10 cultivations the 1st year and 4 to 5 cultivations the 2nd year, will control toadflax (reviews by [95,114]). Care must be taken not to transport toadflax root pieces to clean fields, as root pieces as short as 1 cm long can sprout new plants and expand rapidly [68]. Seedlings less than 2-3 weeks old are particularly susceptible to tillage since, at this stage, vegetative reproduction has not started [95]. Tillage is generally inappropriate for natural areas.
Fire: See Fire Management Considerations.
Biological: Tu and others [106] provide information and considerations for biological control of invasive species in general in their Weed Control Methods Handbook. Carpenter and Murray [11] provide detailed information on biological control of toadflax, including contacts for authorities on each of the insects used.
Several insect species that feed on toadflax have been purposely or accidentally released in the U.S. and Canada. Flower feeding beetles (Brachypterolus pulicarius and Gymnetron antirrhini) appear to be the most important insects for reducing seed production in toadflax. B. pulicarius larvae develop inside floral ovaries, and adults feed on buds and young stems. B. pulicarius can reduce seed production in yellow toadflax by 80 to 90%, and by 43 to 93% in Dalmatian toadflax [33]. G. antirrhini can reduce seed production in yellow toadflax by 85-90% [72]. A stem-boring weevil (Mecinus janthinus), whose larvae and adults feed on shoots of both toadflax species, seems to be the most promising biocontrol agent for toadflax as of this writing [37,108]. Of the biocontrol agents listed for toadflax, the toadflax moth (Calophasia lunula) is not suggested for redistribution since it attacks native snapdragons in California [108].
The following table provides information on some insects tested and/or released for control of toadflax:
Agent Plant attacked Status States established References toadflax flower-feeding beetle---
[72,95] root-boring moth---
[72,95] toadflax capsule weevil (Gymnetron antirrhini) mainly yellow toadflax with a "strain" that feeds on Dalmatian toadflax accidentally introduced widespread and common at yellow toadflax sites; eastern Canada, BC; ID, MT, OR, WA, WY [17,72,95] weevil (G. netum) mainly yellow toadflax with a "strain" that feeds on Dalmatian toadflax accidentally introduced; larvae develop inside fruit and adults feed on buds, leaves, and stemsestablished on yellow toadflax several states in northern U.S. and in BC
[17,95] stem-boring weevilDomestic sheep and goats have been used to control toadflax [57,106]. Preliminary results of field trials in Montana show that domestic sheep can help suppress stands of Dalmatian toadflax and limit seed production. In these studies, 1,000 ewes and lambs were placed in a hilly rangeland area of moderate to heavy infestations of Dalmatian toadflax (densities of 25 to 100% of the vegetation). Approximately 35 to 45% of toadflax foliage was stripped, including the terminal 6 to 10 inches (15-25 cm) of plant stems. Although domestic sheep only nibbled the plants initially, within 2 to 3 weeks they were consuming Dalmatian toadflax regularly, even though other forage was present. In these studies, the sheep did well and showed acceptable weight gain ([57] and references therein).
Chemical: Tu and others [106] provide information and considerations for chemical control of invasive species in general, and detailed information on individual chemicals and adjuvants in their Weed Control Methods Handbook. Carpenter and Murray [11] provide detailed information on chemical control of toadflax, including contacts for authorities on each of the chemicals discussed.
Lajeunesse [57] suggests that chemical control of toadflax is complicated by the plant's high genetic variability, the waxy leaf surface that can hinder herbicide uptake, and coarse soils (in which toadflax is often found) that can allow herbicides to leach below the root zone. Even when chemical control appears effective, reinvasion by toadflax may occur from buried seed. Where herbicides appear effective, it is necessary to treat an infestation every 3 to 4 years for as long as 12 years [57]. Permanent, long-term control of toadflax cannot be achieved with herbicide treatment alone ([95] and references therein).
Triclopyr, fluroxypyr, 2,4-D, MCPA, 2,4-DB, MCPB, and mecoprop are ineffective for toadflax control ([57,95] and references therein). Glyphosate did not effectively control toadflax, but did kill neighboring plants, thereby giving toadflax the competitive advantage [81]. Effective herbicides for Dalmatian toadflax include chlorsulfuron (results for 1-2 years) [45], dicamba (results for 1 year), picloram (although it is ineffective on some sites), and imazapic ([46,57,81] and references therein).
Picloram at the rate of 1.5 to 2 lb a.i. per acre, is purported to be the most effective herbicide treatment for Dalmatian toadflax in Montana, although it will not usually provide complete control [55], and it may also harm desirable plants.
Sebastian and Beck [97] measured percent control of Dalmatian toadflax and percent injury to crested wheatgrass (Agropyron cristatum) in plots treated with various rates of picloram, fluorxypyr, and a mix of the 2 chemicals applied at different time/growth stages for 3 years after application. Better control was achieved with lower rates of picloram (0.5 lb a.i./acre), possibly because there was less injury to competing wheatgrass at the lower rates. Fluroxypyr resulted in no control when applied alone, and did not improve control rates when mixed with picloram over picloram alone [97].
The reduction of desirable nontarget species by picloram may be unacceptably high at application rates recommended for toadflax control. At much lower rates (0.28 kg a.i./ha), fall applications of picloram depressed species richness and diversity on rough fescue grassland (Festuca spp.) and early seral Douglas-fir/snowberry forest (Pseudotsuga menziesii/Symphoricarpos albus) treated to control spotted knapweed in western Montana. In these cases species diversity recovered to untreated levels within 3 years of the initial applications, and the authors described these effects as "small and transitory." Spring applications, however, caused depressions in diversity that persisted through the last year of sampling (6 years after initial application) [84]. Similarly, applications of picloram at 0.56 kg a.i./ha (in combination with prescribed burning) to control Dalmatian toadflax on big sagebrush-bluebunch wheatgrass sites in southwestern Montana reduced biomass, cover, and density of Dalmatian toadflax to 90% of the control, and decreased overall forb biomass to nearly 100% of the control. Applications of chlorsulfuron had similar effects on Dalmatian toadflax in this study, but slightly less negative impact on native forbs. The authors conclude that while herbicides may help provide short-term control on burned rangeland, the combination of forb reduction, open niches from burned trees and shrubs, and pressure on grass from wildlife may leave sites susceptible to reestablishment of toadflax from the soil seed bank [45].
An experiment to determine the effects of 6 different herbicides at 3 rates on established spotted knapweed populations indicated that all rates reduced spotted knapweed biomass by 95-100%. However, suppression of spotted knapweed with clopyralid allowed yellow toadflax to become the dominant plant in those treatment areas. Picloram appeared to suppress yellow toadflax to some degree, as it did not become dominant in picloram treated plots [58]. Similarly, it appears that some agricultural herbicides encourage Dalmatian toadflax in Washington [81].
Cultural: Vigorous, healthy plant communities can often outcompete toadflax seedlings and thus prevent their establishment. In areas where toadflax is already established, initial toadflax control should be followed by establishment of well-adapted, desirable plant species that provide competition throughout the season at all levels of the soil-root profile. This can provide longer-term suppression. Proper grazing management to maintain the competitive ability of these plant communities is important for long-term control of toadflax [57,68].
Communities that are in good condition may recover without replanting desirable species as long as follow-up control is conducted annually. Increases in native and nonnative annual grasses, forbs, and residual native perennial forbs were observed following toadflax removal at Magnusson Butte Preserve in Washington. However, replanting native grasses and forbs can help accelerate recovery of an area [11].
Rose and others [91] tested 5 grasses ('Hycrest' crested wheatgrass (Agropyron cristatum), 'Luna' pubescent wheatgrass (intermediate wheatgrass (Thinopyrum intermedium)), 'Critana' thickspike wheatgrass (Elymus lanceolatus), 'Bozoisky' Russian wildrye (Psathyrostachys juncea), and 'Sodar' streambank wheatgrass (Elymus lanceolatus)) for their competitive ability against Dalmatian toadflax. They began by spraying the area (a dry, disturbed grassland site invaded by Dalmatian toadflax) with picloram in the fall, then rototilled and seeded the following April and August. Areas seeded in the spring to crested wheatgrass, intermediate wheatgrass, and thickspike wheatgrass showed significant (p<0.05) reductions of Dalmatian toadflax. The August seeding did not establish as quickly as the April seeding, but by the 3rd year, late summer seedings were all equal to or greater in competition than those established in spring. Dalmatian toadflax production (above ground dry weight) was greater in control plots than in plots seeded with thickspike, pubescent (intermediate), and crested wheatgrasses. Production of Dalmatian toadflax in areas seeded with streambank wheatgrass and Russian wildrye did not differ from production in unseeded controls. Each grass has characteristics that make it desirable in different situations, and areas to be revegetated should be assessed to determine which grasses would be most suitable depending on soil types, moisture level and use. Russian wildrye, intermediate wheatgrass, and crested wheatgrass are all nonnative species [91] and some of these are invasive in natural areas.Very little information is available regarding the use of toadflax by livestock and wildlife. A review by Lajeunesse [57] notes that Dalmatian toadflax and yellow toadflax evolved in plant communities that are grazed moderately to intensely, primarily by domestic sheep and goats and to a lesser extent by cattle. Cattle will sometimes casually browse flowering shoots, while domestic sheep will use Dalmatian toadflax as a major food source [57]. Domestic sheep and goats have been used to control toadflax [57,106].
A review by Saner and others [95] points out that several secondary compounds present in yellow toadflax may explain why cattle avoid it. It contains glycosides that may be insecticidally active, flowers contain flavonoids, and seeds contain saponins. This review also indicates that cattle eat dried yellow toadflax, that yellow toadflax is not generally poisonous to livestock, and that it has been used as a medicinal plant for cattle that cannot ruminate [95]. According to Mitich [65], occasional cases of mild poisoning from yellow toadflax have been reported for cattle, but such cases are rare because cattle usually avoid toadflax.
Uresk and Lowrey [112] found no evidence through fecal analysis that cattle consumed yellow toadflax in an area where it was abundant. Estimates of toadflax use by cattle may be inaccurate if identified by microhistological analysis, since the toadflax epidermis apparently does not survive the slide making process [94].
Although deer have been observed to graze Dalmatian toadflax, and toadflax seed is used by some species of birds and rodents, it is not known to be heavily used by any native animal species [87].
Palatability/nutritional value: Toadflax is usually avoided by cattle as it is said to be distasteful, or mildly poisonous [57,65].
Cover value: Toadflax can provide cover for small animals [57].
Toadflax most commonly invades disturbed plant communities typical of cultivated
areas, roadsides and other "waste places." It also invades communities
with
naturally-occurring disturbances or small openings [6,57].
For example,
Dalmatian toadflax is said to be highly invasive and
competitive in sagebrush (Artemisia spp.) ecosystems [77], and is
often invasive in open ponderosa pine (Pinus ponderosa), bunchgrass, and
riparian communities.
Dalmatian toadflax may displace natives in shrub-steppe communities in Washington [81].
It was found in trace amounts in ponderosa pine-bluebunch wheatgrass (Pseudoroegneria spicata) and
ponderosa pine-Thurber needlegrass (Achnatherum thurberianum) habitat types in Washington
and Idaho in the late 1960s [16]. In Oregon, Dalmatian toadflax is found in bluebunch wheatgrass-Sandberg
bluegrass (Poa secunda) and Idaho fescue (Festuca idahoensis)-bluebunch
wheatgrass communities, and on gravel bars in riparian communities [81].
In Montana, Dalmatian toadflax forms large colonies in dry mountain grasslands of valleys and foothills [56],
on sites formerly dominated by native prairie species such as Idaho fescue
and bluebunch wheatgrass. These sites are
now dominated by Dalmatian toadflax, leafy spurge (Euphorbia esula), spotted knapweed (Centaurea
maculosa), cheatgrass (Bromus tectorum) [61], and
sulphur cinquefoil (Potentilla recta) (personal observation). In
Glacier National Park, Montana, Dalmatian toadflax occurs in rough fescue (Festuca altaica)-Idaho
fescue habitat types along roadsides [107].
In Yellowstone National Park, Wyoming, Dalmatian toadflax was found at Mammoth Campground in a big sagebrush
(Artemisia tridentata)/bluebunch wheatgrass habitat type. It was not found at
campgrounds at higher elevations [2].
Toadflax often occurs in riparian areas in Colorado. It is found, for example, on gravel bars
in the south fork of the San Miguel River that are flanked by riparian forests
of cottonwood (Populus spp.) and spruce (Picea spp.) [82]. Along the Yampa River area in Colorado,
Dalmatian toadflax spreads in from adjacent upland
areas and along the river. It is found in gravel bars, riparian pastures, and
open meadows, and may compete with cottonwood seedlings for establishment sites
on gravel bars. In Phantom, Colorado, the river and trails are
conduits to dispersal of Dalmatian toadflax. It may also invade mountain-mahogany (Cercocarpus
spp.) shrubland and
shortgrass prairie communities adjacent to riparian corridors [81].
In Utah, Dalmatian toadflax is found in oak (Quercus spp.), quaking aspen (Populus
tremuloides), sagebrush, mountain brush, and riparian communities [119].
Dalmatian toadflax is found, though rare, in sandy soils and washes in oak woodland in Fort Bowie
National Historic Site in southeastern Arizona [116]. It is also
found in northern Arizona in Bebb willow (Salix bebbiana) wet meadow,
fescue (Festuca spp.)-muhly (Muhlenbergia spp.) grassland, and mixed conifer understory communities [81].
Less information is available for plant communities in which yellow toadflax is found. It
is most common in agricultural areas
[95].
Yellow toadflax was found in a jack pine (Pinus banksiana)-lichen woodland of the upper boreal forest in northern
Quebec, 2 years after wildfire [100]; and was identified in a ponderosa pine/bluebunch wheatgrass community in Montana [83].
Yellow toadflax is used in folk medicine and homeopathy; as an insecticide; as a yellow dye; and as a plant of religious and magical attributes [65,95].
Yellow toadflax is said to be useful to prevent erosion, and in reclamation of mined areas, sites despoiled by dumping of heavy metal-laden sewage sludge, and abandoned gravel pit slopes [95].
Toadflax phenology can be variable, as it is dependent on environmental conditions.
In early autumn, 1st-year Dalmatian toadflax plants often produce prostrate stems, forming a mat-like rosette that survives into the following spring. This "rosette stage" is important for carbohydrate storage. The following spring, floral stems grow from the prostrate stems. This sequence also occurs to a lesser degree in mature plants [88]. With the exception of these prostrate stems, top-growth dies back in the fall and is regenerated from the root system each spring [57].
Toadflax seedlings emerge both in spring and fall [87,95,114]. Dalmatian toadflax seedlings emerging in spring appear to have a higher survival rate than those emerging in fall [87,114]. In eastern Washington, Dalmatian toadflax emerges on south-facing slopes during the 1st or 2nd week in March through the 3rd week in April. Emergence on level ground occurs 2 to 3 weeks later, coinciding with soil temperatures of 50 degrees Fahrenheit (10 °C) at a depth of 1 inch (2.5 cm) [57,87]. Yellow toadflax seedling emergence begins in early to mid-May in Alberta [69], and earlier in warmer regions.
In northern Germany (also relevant to phenology in Canada), yellow toadflax vegetative shoots usually emerge in early to mid-April, when soil temperatures reach 42 to 50 degrees Fahrenheit (5-10 °C), and possibly in mid- to late March in warmer regions of the U.S. [57,95]. Dalmatian toadflax plants observed in eastern Washington fields developed floral stems only after a winter's dormancy and exposure to temperatures below 68 degrees Fahrenheit (20 °C). Floral stems emerge a few days later than seedlings in the spring [88]. Weak floral stems and some seed can be produced by 1st-year seedlings [87].
Flowering of Dalmatian toadflax begins in May or June and continues until August or October, but can occur earlier in warm seasons or warm habitats [11,57,87]. When flowering stems of yellow toadflax are 16 to 24 inches (40-60 cm) tall, branching begins, which signals the beginning of flower bud formation [57,95]. In northern Germany, yellow toadflax flower buds occur at the end of June, flowering begins mid-July, and the peak of flowering is at the end of July. On warm sites in Canada, flowering may begin in mid-June [95].
The following table provides some flowering dates for Dalmatian toadflax by geographic area:
Area Flowering dates References Great Plains July to August [31] Illinois June to September [66] Intermountain area July to August [14] New Mexico May to September [62] northeastern U.S. July, August [28]The following table provides some flowering dates for yellow toadflax by geographic area:
Area Flowering dates References Blue Ridge (NC, SC, TN, VA) June to September [122] Carolinas June to August [79] Great Plains June to August [31] Illinois May to November [66] Intermountain area June to August [14] Nevada June to September [48] New Mexico July to September [62] northeastern U.S. May to September [28] north-central Texas May [19] West Virginia June to October [104]Dalmatian toadflax seed production begins in late June or early July and continues until September or early October in northern climates. Toadflax seed dispersal may begin on lower portions of the stem while flower buds are still forming on upper portions. Dalmatian toadflax seed dispersal begins as early as July in northern climates and continues into winter [88]. In a greenhouse study in Washington, most seeds were produced between late July and early September, and about 97% of seeds were produced in the first 5 weeks of production [87]. Yellow toadflax seed dispersal begins in August or September in northern climates and continues into winter [12,69,95]. In a study in Alberta, yellow toadflax capsules were formed and open by early August, when stem height was up to 3.3 feet (1 m), and seed shed started 2 weeks later. The majority of seed was shed between late September and October [69].
Yellow toadflax ramets die at freezing temperatures but the stronger stems, including some fruit capsules, may remain erect. Some seeds may remain in the fruit capsule until strong winds occur or until the stem falls over [95]. Dried floral stalks can remain standing for 2 years, retaining some seeds but dispersing most during the 1st year [88].Toadflax is able to recover after fire and may even be promoted by fire, especially if other species are reduced. The postfire environment is well suited to toadflax establishment by seed.
Three sites in big sagebrush-bluebunch wheatgrass communities in western Montana were burned under prescription in mid-March to reduce shrubs and trees. Dalmatian toadflax density, cover, biomass per square meter, per plant biomass and per plant seed production were measured on burned plots and immediately adjacent unburned plots 6 months after burning. Dalmatian toadflax density and cover were not different between burned and unburned plots. Biomass of Dalmatian toadflax per square meter varied depending on site, while biomass per plant was significantly higher (p>0.05) on all burned plots compared with unburned plots. Burning also significantly increased (p>0.05) Dalmatian toadflax seed production per plant at all 3 sites. Seed production ranged from 7 to 79 seeds per plant on unburned plots, and from 158 to 1,328 seeds per plant on burned plots [44].
Dalmatian toadflax and yellow toadflax reproduce by seeds and by vegetative buds on roots. New infestations of toadflax usually originate from seed. Established patches of Dalmatian toadflax expand by both seed and vegetative reproduction, while yellow toadflax patches expand primarily by vegetative reproduction.
Breeding system: Both Dalmatian toadflax and yellow toadflax are self-incompatible [6,20,95,114]. However, a review by Saner and others [95] suggests that a small proportion of yellow toadflax seeds can develop without cross-pollination.
Pollination: Dalmatian toadflax and yellow toadflax are both insect pollinated [6,95,114]. Conspicuous yellow flowers with bright orange palates (nectar guides) make toadflax well adapted to insect pollination. Yellow toadflax is pollinated mostly by bumblebees and it is only of minor importance for honeybees [95].
Seed production: Viable seed production in toadflax is variable due to availability of outcross pollen [12], pollinator limitation, predation by insects [6], resource limitations [12], and variable flowering phenology [95].
Robocker [87] studied seed production in Dalmatian toadflax and found that capsules on main stems averaged 250 seeds per capsule while those on branches contained an average of 140 seeds per capsule. He calculated that, with minimal competition and good moisture availability, a Dalmatian toadflax plant with 10 stems could potentially produce 500,000 seeds. A review by Vujnovic and Wein [114] cites a study finding that plants with 12-15 stems produced up to 400,000 seeds.
Studies of yellow toadflax report low seed set and low seed viability. The average number of seeds per capsule ranges between 3 and 47 in North America [6,12] and from 70 to 110 in central Europe. Capsules formed later in the growing season tend to produce more viable seed. Some populations of yellow toadflax may never produce more than 25% viable seed [12]. Average number of seeds produced per yellow toadflax stem may vary from 165 to 5,584. Seed counts on a per plant basis are difficult to compare as the definition of what constitutes an individual is unclear. Mean numbers from 1,500 to 30,000 have been reported ([95] and references therein).
Nadeau and King [69] found that seed production of 210,000 seeds per m2 within a 0.5 m radius around yellow toadflax parent plants was reduced to 1,410 seeds per m2 within a 0.5 m radius in the subsequent year when biological control insects were abundant.
Many seed studies fail to differentiate between viable and nonviable seeds. Clements and Cavers [12] observed seasonal differences in number of viable seeds produced by yellow toadflax and attributed these differences to differential seed development in response to variable resource availability.
Seed dispersal: Toadflax seeds are formed in capsules on the terminal portions of branched stems. The capsules dry over time, crack open and release seeds [69]. Although the seeds are winged, and wind has been considered a major means of seed dispersal for toadflax species [2,87,88], Nadeau and King [69] observed that over 80% of yellow toadflax seeds fell within an 18-inch (50 cm) radius of the parent plant, and "very few" seeds fell more than 5 feet (1.5 m) from the parent plant.
Average seed size is similar for yellow and Dalmatian toadflax. Dalmatian toadflax seeds are slightly heavier [87], and the papery wing surrounding the seeds is less developed on Dalmatian toadflax. This suggests that Dalmatian toadflax seeds probably also fall within short distances of the parent plant. When seeds of either species fall from upright dried floral stems onto crusted snow, they can be blown by wind across the surface of the snow ([57] and references therein).
A review by Saner and others [95] suggests yellow toadflax seeds may also be dispersed by water, ants, birds, and rodents. Seed dispersal via farm equipment is likely an important mode of dispersal in agricultural areas. Dalmatian toadflax may also be dispersed by cattle, deer and other browsing animals [87,114]. Dalmatian toadflax seeds can remain viable after passing through the gastrointestinal tracts of cattle, and possibly deer [87].
Seed banking: Nadeau and King [69] found that yellow toadflax seeds kept in the dark at 68 degrees Fahrenheit (20 °C) produced few seedlings, suggesting a strong dormancy in yellow toadflax seeds. Dormancy was broken with wet stratification, with maximum germination obtained after 8 weeks of wet stratification [69]. Seed stored at room temperature for 13 years had a germination rate of 67%, at optimum germination temperatures. Under field conditions at Spokane, Washington, researchers estimate Dalmatian toadflax seed longevity at 10 years, with duration of viability dependent on soil and climatic conditions [87].
Results presented by Grieshop and Nowierski [32] suggest Dalmatian toadflax produces enough seeds to exceed potential emergence sites. Therefore, many Dalmatian toadflax seeds may remain ungerminated each year [32]. Because only a portion of toadflax seeds may germinate in any given year, and because seeds can remain viable for several years [87,88], dormant, soil-stored seeds can reinfest a site following control applications or other disturbances that reduce plant competition [57].
Germination: Toadflax seed viability and germination rates are variable. Factors that may affect toadflax seed viability include resource limitation, seed predators, and pollinator limitation. Germination is affected by temperature, light, seed dormancy and after-ripening.
Nadeau and King [69] found 40 to 51% average seed viability (by tetrazolium chloride test) in yellow toadflax seed collected throughout the season in Alberta. Clements and Cavers [12] found most of the observed seasonal differences in yellow toadflax seed viability in their study were the result of differential seed development in response to variable resource availability. Insect predation can also affect toadflax seed viability. McClay [63] found that seed viability and individual seed weight were seriously reduced on plants attacked by Brachypterolus pulicarius, and Nadeau and King [69] observed reduced seed viability in a year when there was heavy predation by B. pulicarius and G. antirrhini (see Biological control for more information on these biological control insects).
Yellow toadflax seed germination rates vary greatly but are usually low. Clements and Cavers [12] report yellow toadflax germination rates ranging from 13.5% to 65.8%. Remaining seeds are either dormant or nonviable [12]. Some yellow toadflax seeds may germinate immediately, but most seeds remain dormant. Seed dormancies of 8 years have been reported. Yellow toadflax seeds germinate in the top 0.8 to 1.2 inches (2-3 cm) of soil and may also germinate on the soil surface. In most experiments, light appeared to induce germination. Germination rates of 88% were observed following chilling and subsequent exposure to alternating temperatures ([95] and references therein). Wet stratification can also induce germination of yellow toadflax seeds [69].
Germination rates may be as high as 75% for Dalmatian toadflax [57]. Robocker [87] found that Dalmatian toadflax seeds germinated within a narrow temperature range, and that many seeds were carried over from year to year. Seeds germinated if they were not buried deeper than 1 inch (2.5 cm) in clay and loamy sand soil, and 1.2 inches (3 cm) in sandy soil [1,87]. In the field, emergence of Dalmatian toadflax seeds usually occurs from depths less than 5 mm. Newly harvested Dalmatian toadflax seeds are usually dormant. Dormancy declines and germination rates increase with seed age up to 5 to 6 weeks after maturation. Of the Dalmatian toadflax seeds produced during the first 5 weeks of maturation, 67% germinated in October (about 3 months after collection). Low winter temperatures break dormancy [87]. Some Dalmatian toadflax seeds germinate in fall, but most germinate in spring, with peaks in April and May [57].
Seedling establishment/growth: Toadflax seedlings are vulnerable to dehydration, competition from other species, and certain control strategies.
Dalmatian toadflax seedlings died following rapid or extreme temperature changes or dehydration in field and greenhouse studies. When Dalmatian toadflax seeds germinate in autumn, seedling survival the next year depends on sufficient spring and early summer precipitation, or a lack of competition from other plants. Dalmatian toadflax seedlings are easily outcompeted by established plants, especially perennial species [87], and also by cheatgrass on fertile soils. Conversely, cheatgrass does not compete well with established Dalmatian toadflax plants [88]. Cultivation or soil disturbance that removes perennial plants may increase Dalmatian toadflax seedling survival [87].
Grieshop and Nowierski [32] found, under the field conditions of their 2-year study, that Dalmatian toadflax populations filled most "safe seedling emergence sites" and seedling recruitment of Dalmatian toadflax was limited more by interspecific resource competition than by seed numbers. Because Dalmatian toadflax can produce enough seeds to exceed potential emergence sites, and because Dalmatian toadflax plants reproduce vegetatively, it appears that individual seedling recruitment in Dalmatian toadflax is more important for establishing new populations than it is for maintaining established populations [32].
Similarly, vegetative reproduction in yellow toadflax is more important than seedling establishment for maintaining populations of yellow toadflax. Yellow toadflax can produce a large number of seeds. However, low viability and strong seed dormancy result in poor yellow toadflax seedling establishment, especially with competition. Once established, yellow toadflax seedlings have a rapid rate of growth and vegetative spread, enabling them to colonize a suitable niche quickly when competitive stresses are low [69].
Yellow toadflax seedlings (genets) can produce daughter shoots (ramets) from vegetative root buds as early as 3 weeks after cotyledon appearance. A single ramet or genet of yellow toadflax would be equally capable of initiating a new yellow toadflax infestation; however, young genets are highly susceptible to control techniques and crop competition in the first 3 weeks of growth, while the larger root system of ramets may resist the effects of control treatments such as mowing and tillage. Tillage operations could readily spread toadflax infestations because of the ability of small portions of underground plant parts to produce new shoots [70]. It has been reported that a yellow toadflax patch with a radius of almost 3.3 feet (1 m) developed from a first-year seedling in 1 growing season, and established patches can increase in diameter an average of 4 feet (1.2 m) per year in Saskatchewan [95].
Roots of Dalmatian toadflax seedlings grow an average of about 2 inches (6 cm) per week, with the uppermost primary branch tending to grow horizontally at depths of 0.8 to 4 inches (2-10 cm) [88].
Asexual regeneration: Toadflax can reproduce vegetatively by the formation of adventitious shoots from both the tap and lateral roots, and from root fragments [88,95]; and can sprout from buds in the axils of vestigial leaves at the bases of floral shoots when shoots are removed [89].
Vegetative reproduction is possible from toadflax root fragments as short as 0.4 inch (1 cm) and is common from fragments of 4 inches (10 cm). In Alberta, a 4-inch-long (10 cm) piece of yellow toadflax root with a 4-inch-high (10 cm) shoot produced a patch 3.3 to 6.6 feet (1-2 m) in diameter in one growing season, and produced an average of 75 or 694 shoots, in barley (Hordeum spp.) or on fallow land, respectively. Seedlings appear to be important in initiating new yellow toadflax infestations in nonarable lands, while in arable land, root pieces can cling to farm implements, easily initiating new infestations [68].
A review by Saner and others [95] describes the mature root system of yellow toadflax as having "long roots" that are perennial and bear buds, and "short roots" that are annual. The rate of bud formation in yellow toadflax roots is highly variable, stimulated by exposure to light, and somewhat inhibited by soil disturbance. Vegetative reproduction from yellow toadflax root buds can begin as early as 2 to 3 weeks after germination, and an extensive root system may develop within a single growing season. Yellow toadflax plants typically produce 90 to 100 secondary shoots from the root system in the 1st year and 200 to 250 shoots by the 2nd year ([95] and references therein).
Established infestations of yellow toadflax appear to expand mainly by vegetative reproduction rather than by seed. Since the majority of yellow toadflax seed is dispersed within 6.6 feet (2 m) of the parent plant, and vegetative spread is common, this species is likely to spread by increasing individual patch size [69]. Distribution of toadflax patches may be influenced by competition. Nadeau and others [68] observed small clumps of shoots consistently appeared in a ring around the original yellow toadflax stand on fallow land, whereas in a barley crop clumping was reduced and shoots did not form a ring. Yellow toadflax produced 20 to 75 shoots in a 1.3- to 3.3-foot-diameter (0.4-1 m) patch when growing with barley, as compared with 197-694 stems in 3.3- to 6.6-foot-diameter (1-2 m) patch growing on fallow land in Alberta [68].
In studies in Pullman, Washington, persistence of Dalmatian toadflax was dependent on both yearly extension of lateral roots and establishment of new seedlings [86,89].
Toadflax can readily establish on open and disturbed sites where competition from other plants is reduced [11,57,95]. While individual crowns of toadflax are relatively short-lived (~4 years), individual patches may persist for more than 13 years on some sites. Soil disturbance and freedom from competition are important for stand persistence. On some sites, stands may disappear for several years and then reestablish from either buried seeds or vegetative buds [88,114].
Dalmatian toadflax is often associated with several other nonnative, invasive species. In ponderosa pine/Idaho fescue associations in northeastern Washington, grazing pressure can shift understory dominance from native grasses to cheatgrass, Dalmatian toadflax, and St. Johnswort (Hypericum perforatum), apparently irreversibly [16,25]. Dalmatian toadflax is competitive with cheatgrass in large parts of eastern Washington on the eastern edge of the Columbia Basin [38]. Dalmatian toadflax may be more tolerant of control methods, especially herbicides, than other species, and may therefore establish or spread following control attempts. In British Columbia, Dalmatian toadflax appears to be spreading as diffuse knapweed (Centaurea diffusa) and spotted knapweed are controlled [40]. Similarly, Dalmatian toadflax invaded sites where biological control has suppressed St. Johnswort on rangeland in Washington and Idaho [114].
Succession may also lead from toadflax dominance to dominance by other invasive species. Sites where Dalmatian toadflax has established in Washington, for example, may be susceptible to infestations by rush skeletonweed (Chondrilla juncea) [73].
While toadflax tends to be more common in open areas, Dalmatian toadflax also tolerates moderate amounts of shade. At Yellowstone National Park, Dalmatian toadflax was found under a wide range of canopy cover, from 0 to 85%, although it was predominantly found under low canopy cover [2].
The currently accepted genus name for toadflax is Linaria P.
Mill (Scrophulariaceae) [14,28,31,47,56,62,113,119]. This report summarizes
information on 2 species of toadflax:
Linaria dalmatica (L.) P. Mill. [14,28,31,43,47,49,56,62,113,119] Dalmatian toadflax
Linaria vulgaris P. Mill. [14,19,28,31,42,43,47,48,56,62,66,79,104,113,117,118,119,122] yellow toadflax
For the purposes of this review, the common name toadflax will be used when
discussing characteristics common to both species, or when it is unclear which
of the species was studied. When referring to individual species, the common
names listed above will be used to represent the respective scientific name.
A review by Saner and others [95] reports that hybrids between yellow toadflax and
Dalmatian toadflax can be produced in the
laboratory, and therefore occurrence of this hybrid in the field cannot be
excluded. Hybrids between yellow toadflax and striped toadflax (Linaria repens) commonly occur.
This hybrid was described as Linaria x sepium
J. G. Allman from the British Islands in 1843 and was discovered near St. John's,
Newfoundland, in the early 1900s ([95] and references therein).
Linaria dalmatica is a herbaceous, short-lived perennial plant native to western Asia and southeastern Europe that has become a weed in other areas. The family this plant now belongs to is the Plantaginaceae Family.[2] Previously, it belonged to the Scrophulariaceae (Figwort) family. Its common names include Balkan toadflax, broadleaf toadflax, and Dalmatian toadflax.[3][1][4][5] Linaria dalmatica has unique yellow flowers with an orange center that draw individuals to purchase them to display in their gardens. The distribution of L. dalmatica to North America can be attributed to use as a fabric dye, folk remedies and as an ornamental plant. However, it is now classified as a weed in both Canada and the U.S.A.[4][6][7]
Linaria dalmatica is listed as a noxious weed in fourteen U.S. states and three provinces in Canada.[6][8][9] However, it is present in thirty-one U.S. states and ten provinces and two territories in Canada. It is a grassland invader native to the Mediterranean region, and it was introduced to North America in the late 1800s. It has fast-growing strong, horizontal roots, and can withstand cold, and is a problem for farms and grasslands in the interior of British Columbia.[10]
The genus name Linaria, appears to stem from the Latin word linum, meaning flax, as some species have leaves that appear flax-like. The species name dalmatica means Dalmatia which is located on the Balkan Peninsula.[11][12][13]
L. dalmatica have adapted to a wide range of conditions and can commonly be found in growing on roadsides, valleys, gravel pits, fields, pastures and overgrazed or deteriorated rangelands to name a few. They tend to thrive in open areas that are not well-kept and disturbed soils.[9][14]
Seeds
Seed production of L. dalmatica is dependent on many factors such as environmental conditions and competition sites. In sites with favourable conditions (moisture availability) and low competition of other plants, up to 500,000 seeds can be produced per plant. Seeds are small and appears black to purplish-brown in colour.[4][6][7][15] Seed dispersal can be through a number of ways including wind, wildlife and seed ingestion by animals.[4][7] Seeds tend to germinate in the spring, however they can also germinate in the fall. The weight and development of seeds are influenced by the environment in which they grow, where growth is significantly impacted by moisture availability. With low moisture availability or drought conditions, growth may be inhibited as Linaria dalmatica are weak competitors in these conditions.[6][7] Development of seeds are typically slow to begin with but increase with time.[7]
Roots
L. dalmatica has an extensive root system. The roots of L. dalmatica make it a strong competitor as they can produce long and deep taproots that are able to obtain take control of resources required for growth such as moisture and nutrients. In addition, lateral roots also arise from the buds present on the roots that contribute to the plants ability to obtain resources. These lateral roots can grow up to 10 feet away from the parent plant. The roots are a great storage place for energy and they possess the greatest amount of energy storage in the fall, storing sugars (i.e. fructose and glucose). Contrastingly, the least amount of energy stored in the roots occur in June just as the flowers are set to bloom.[4][6][7]
L. dalmatica also produce creeping roots which appear white in colour. These roots allow for a rapid rate of reproduction of these plants.[4][7]
Leaves
Leaves are broad and heart-shaped and are arranged in an alternating pattern. The leaves are attached to the stem ascending outward and are smooth, waxy and glabrous. In addition, they do not have petioles.[4][7][16]
Flowers
Flowers are raceme and are composed of five sepals and five petals (tepals), four pairs of stamens of unequal length and pistils are bicarpellate.[4] Flowering of L. dalmatica takes place from late May to September.[4][6][7] The flowers are bilaterally symmetrical and have spurs pointing downwards.[5][17]
As L. dalmatica are regarded as a noxious weed, determining solutions to manage and control the growth is essential. Due the high adaptability of L. dalmatica, they are able to outcompete plants and thus remain present and utilize all the resources required for growth. It is critical to remove these plants at their most vulnerable stage which is when it is at its seed stage. Hence methods to remove L. dalmatica are essential to prevent seed formation preceding flowering.[4][6][7]
Chemical Control
The use of herbicides are effective to remove weeds, however, when used on L. dalmatica results of this chemical control vary. Herbicide control of L. dalmatica seems to be short-term as the leaves waxy surface help prevent herbicide uptake and its potency. Furthermore, its extensive root system aids in reproduction when herbicides manage to remove the flowering plant. It is recommended herbicide use is the most effective pre-bloom or in the death stages of L. dalmatica. Herbicide use as these stages can promote long-term control and have a greater impact on the removal of these plants as energy storage in the roots are at its least.[4][7]
Mechanical Control
Manual labour of removing L. dalmatica by digging up the roots by hand and with a shovel. It is important to note, this is effective but require many years to suppress and remove all L. dalmatica plants.
Biological Control
The release of insects feeding on the leaves and stems of L. dalmatica is an additional way to control the rapid reproduction of this plant and localize it to a certain area. The most prominent insect that aids in the suppression of L. dalmatica growth is the toadflax stem-mining weevil. The toadflax stem weevil has been vital in the disruption of nutrient and water transport as larvae laid by the toadflax stem weevil feed on the stem while the developed adults feed on the leaves. The disruption of the resources used for growth inhibits the bloom of some flowers and therefore reduces the production of seeds. Additionally, shoot growth is inhibited as they wither before they reach growth due to insufficient nutrients and water.[7][16]
Additional insects that feed on the L. dalmatica and used as a biological control are the Brachypterolus pulicarius, Gymnaetron antirrhini and Rhinusa neta and Mecinus janthinus.[14]
Cultural Control
The use of cultural control alone is not effective but in combination with other controls such as chemical promote greater success in controlling L. dalmatica. The methods of burning and tilling may be effective alongside the spraying of herbicides. The addition of other competitive plants to compete with L. dalmatica can also promote the removal of this noxious weed. It is important to note that control through cultivation must be persistent and engaged in for at least two years to improve effectiveness of removal.[7][16]
Regarding the toxicity level of L. dalmatica, there is a lack of information to determine if it is toxic or not to both animals and humans. For animals, exposure and consumption of this plant is observed to be relatively low as it is not there preferred forage although grazing does occur.[6][7][14] However, it is proposed that due to quinolizidine alkaloids present within the plant, it will cause acute poisoning and other health effects in animals as it has been displayed in sheep and cattle.[6][18] Further experiment and testing is required to determine the toxicity significance in both animals and humans.
Linaria dalmatica is a herbaceous, short-lived perennial plant native to western Asia and southeastern Europe that has become a weed in other areas. The family this plant now belongs to is the Plantaginaceae Family. Previously, it belonged to the Scrophulariaceae (Figwort) family. Its common names include Balkan toadflax, broadleaf toadflax, and Dalmatian toadflax. Linaria dalmatica has unique yellow flowers with an orange center that draw individuals to purchase them to display in their gardens. The distribution of L. dalmatica to North America can be attributed to use as a fabric dye, folk remedies and as an ornamental plant. However, it is now classified as a weed in both Canada and the U.S.A.
