Canada thistle is a perennial introduced forb. It is distinguished from other thistles by creeping horizontal lateral roots, dense clonal growth, and dioecious habit [49,121,240]. Descriptions and terminology of Canada thistle biology can be confusing or contradictory. For example, descriptions of leaf morphology, stem height, and number of flowering heads may differ somewhat between floras. The following discussion provides ranges of what may be encountered for these characteristics, which will vary under different field conditions. Donald [55] and Moore [150] provide comprehensive reviews of the biology of Canada thistle.
Canada thistle has a deep and wide-spreading root system with a slender taproot and far-creeping lateral roots. It often forms large patches, and individual clones may reach 115 feet (35 m) in diameter [55,75,126,186,248]. Most Canada thistle roots are in the top 0.7 to 2 feet (0.2-0.6 m) of soil, but roots can extend as deep as 6.5 to 22 feet (2-6.75 m) [113,152,157]. Carbohydrate reserves are stored in roots and can range from 3% of root fresh weight during spring to as high as 26% in late fall [137]. Roots are injured when directly exposed to freezing temperatures for 2 hours at -5 °C and killed after 2 hours at -7 °C [192]. Arbuscular mycorrhizal infection of Canada thistle roots has been observed in several studies [17,50,116]. Canada thistle does not form rhizomes, despite this assertion in some literature. Adventitious root buds that may form new adventitious shoots can develop along the root at any location, and at any time of the year with favorable growing conditions [55,85]. New plants can also form from root fragments as short as 0.2 inch (6 mm) [157]. Soil type, structure and horizonation may impact the anatomy, morphology and distribution of Canada thistle roots as well. This suggests that root morphology and distribution are site specific and greenhouse studies of root morphology may not apply [55].
Canada thistle has slender aerial shoots with leafy stems reaching 1 to 6.5 feet (0.3-2 m) tall [42,81,84,176,238]. Leaves are 1.2 to 7 inches (3-18 cm) long and 0.2 to 2.4 inches (0.5-6 cm) wide [81,134,238]. Canada thistle leaf morphology (texture, hairiness, lobing and spininess) can vary considerably, even within a geographical region [84,150]. Canada thistle has numerous aboveground branches that bear several, small flowerheads (0.4 to 0.75 inch (1-2 cm) in diameter) in clusters [49,81,121,126,176,240]. Seeds are 0.09 to 0.2 inch (2.4-5 mm) long, and 0.04 inch (1 mm) in diameter with a pappus of feathery bristles [42,75,176,238,240].
While allelopathy has not been conclusively demonstrated for Canada thistle, this species may produce phytotoxins that inhibit the growth of other plants [55,203]. Fructan metabolism in Canada thistle adds to its competitive advantages by allowing it to grow at relatively cool temperatures [37].
Canada thistle is native to southeastern Europe and the eastern Mediterranean area, and was probably introduced to North America in the 1600s as a contaminant of crop seed and/or ship's ballast [152]. It is probably the most widespread of all thistle species [152]. In addition to North America, Canada thistle is invasive in northern and southern Africa, the Middle East, Japan, India, New Zealand, Australia, and South America. It infests at least 27 crops in 37 countries and thrives in temperate regions of the northern hemisphere [146]. In North America, Canada thistle occurs from Alaska east to the Northwest Territories, Quebec, and Newfoundland and south to California, New Mexico, Kansas, Arkansas, and North Carolina [107]. The PLANTS database provides a map of Canada thistle's distribution in the United States.
Canada thistle has been identified as a management problem in many national parks and on The Nature Conservancy preserves in the upper Midwest, the Great Plains states, and the Pacific Northwest [214]. It is an invader in Mesa Verde National Park, Colorado [67], Yellowstone National Park, Wyoming [4,48,218], Wood Buffalo National Park, Northwest Territories, Canada [83,237], Theodore Roosevelt National Park, North Dakota [32], and the Camas Swale Research Natural Area in the Willamette Valley, Oregon [43].
Although Canada thistle is not usually found in undisturbed forested areas, it has the potential to colonize a wide variety of forest habitats within its range following overstory removal and soil disturbance. The following listings take this potential into account.
Canada thistle is adapted to both survive fire on site, and to colonize recently burned sites with exposed bare soil. The extensive root system gives it the ability to survive major disturbances as observed, for example, at Mt. St. Helens, where Canada thistle was part of the initial community after the 1980 eruption. It survived landslide and resprouted from root and stem fragments after the blast [2,45,216]. It is likely to survive fire and sprout vegetatively from its extensive perennial root system (see Asexual reproduction), as was observed, for example, after an August wildfire in Mesa Verde National Park [64,67]. Additionally, there are numerous examples from the literature where Canada thistle seedlings established anywhere from 2 to 9 years after fire [3,56,122,138,158,190,219,242], presumably from wind-dispersed seed, although this is not always clear in the literature.
FIRE REGIMES: Canada thistle may change the fire ecology of the site in which it occurs by its abundant, flammable aboveground biomass. For example, in boreal wet-meadows, investigators suggest that Canada thistle has the potential to increase fire frequency and perhaps severity as a result of its abundant and readily ignited litter [100].
The following table provides some historic fire return intervals for habitats in which Canada thistle may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) silver fir-Douglas-fir Abies amabilis-Pseudotsuga menziesii var. menziesii > 200 grand fir A. grandis 35-200 [9] maple-beech-birch Acer-Fagus-Betula > 1000 silver maple-American elm A. saccharinum-Ulmus americana sugar maple A. s. > 1000 sugar maple-basswood A. s.-Tilia americana > 1000 [233] bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 118,168] Nebraska sandhills prairie A. g. var. paucipilus-S. s. bluestem-Sacahuista prairie A. littoralis-Spartina spartinae sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [168] basin big sagebrush A. t. var. tridentata 12-43 [191] mountain big sagebrush A. t. var. vaseyana 20-60 [10,30] Wyoming big sagebrush A. t. var. wyomingensis 10-70 (40**) [231,251] coastal sagebrush A. californica plains grasslands Bouteloua spp. blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii blue grama-buffalo grass B. g.-Buchloe dactyloides cheatgrass Bromus tectorum California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 [168] sugarberry-America elm-green ash Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica 233] curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [11,195] mountain-mahogany-Gambel oak scrub C. l.-Quercus gambelii northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [168] beech-sugar maple Fagus spp.-Acer saccharum > 1000 [233] California steppe Festuca-Danthonia spp. 168] black ash Fraxinus nigra 233] juniper-oak savanna Juniperus ashei-Quercus virginiana Ashe juniper J. a. western juniper J. occidentalis 20-70 Rocky Mountain juniper J. scopulorum tamarack Larix laricina 35-200 [168] western larch L. occidentalis 25-100 [9] yellow-poplar Liriodendron tulipifera 233] wheatgrass plains grasslands Pascopyrum smithii 168] Great Lakes spruce-fir Picea-Abies spp. 35 to > 200 northeastern spruce-fir P.-A. spp. 35-200 [57] Engelmann spruce-subalpine fir P. engelmannii-A. lasiocarpa 35 to > 200 [9] black spruce P. mariana 35-200 conifer bog* P. m.-Larix laricina 35-200 [57] blue spruce* P. pungens 35-200 [9] red spruce* P. rubens 35-200 [57] pine-cypress forest Pinus-Cupressus spp. 9] pinyon-juniper P.-Juniperus spp. 168] whitebark pine* P. albicaulis 50-200 [9] jack pine P. banksiana 57] Rocky Mountain lodgepole pine* P. contorta var. latifolia 25-300+ [8,9,187] Sierra lodgepole pine* P. c. var. murrayana 35-200 [9] shortleaf pine P. echinata 2-15 shortleaf pine-oak P. e.-Quercus spp. 233] Colorado pinyon P. edulis 10-49 [168] South Florida slash pine P. elliottii var. densa 1-5 [156,233] Jeffrey pine P. jeffreyi 5-30 western white pine* P. monticola 50-200 Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47 interior ponderosa pine* P. p. var. scopulorum 2-10 Arizona pine P. p. var. arizonica 2-10 [9] Table Mountain pine P. pungens 233] red pine (Great Lakes region) P. resinosa 10-200 (10**) [57,72] red-white-jack pine* P. r.-P. strobus-P. banksiana 10-300 [57,90] pitch pine P. rigida 6-25 [29,91] eastern white pine P. strobus 35-200 eastern white pine-eastern hemlock P. s.-Tsuga canadensis 35-200 eastern white pine-northern red oak-red maple P. s.-Quercus rubra-Acer rubrum 35-200 loblolly pine P. taeda 3-8 loblolly-shortleaf pine P. t.-P. echinata 10 to Virginia pine P. virginiana 10 to Virginia pine-oak P. v.-Quercus spp. 10 to 233] eastern cottonwood Populus deltoides 168] aspen-birch P. tremuloides-Betula papyrifera 35-200 [57,233] quaking aspen (west of the Great Plains) P. t. 7-120 [9,82,141] black cherry-sugar maple Prunus serotina-Acer saccharum > 1000 [233] mountain grasslands Pseudoroegneria spicata 3-40 (10**) [8,9] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [9] coastal Douglas-fir* P. m. var. menziesii 40-240 [9,153,183] California mixed evergreen P. m. var. m.-Lithocarpus densiflorus-Arbutus m. California oakwoods Quercus spp. 9] oak-hickory Q.-Carya spp. 233] oak-juniper woodland (Southwest) Q.-Juniperus spp. 168] northeastern oak-pine Q.-Pinus spp. 10 to 233] coast live oak Q. agrifolia 9] white oak-black oak-northern red oak Q. alba-Q. velutina-Q. rubra 233] canyon live oak Q. chrysolepis blue oak-foothills pine Q. douglasii-P. sabiana 9] northern pin oak Q. ellipsoidalis 233] Oregon white oak Q. garryana 9] bear oak Q. ilicifolia 233] California black oak Q. kelloggii 5-30 [168] bur oak Q. macrocarpa chestnut oak Q. prinus 3-8 northern red oak Q. rubra 10 to post oak-blackjack oak Q. stellata-Q. marilandica black oak Q. velutina live oak Q. virginiana 10 to233] interior live oak Q. wislizenii 9] blackland prairie Schizachyrium scoparium-Nassella leucotricha Fayette prairie S. s.-Buchloe dactyloides little bluestem-grama prairie S. s.-Bouteloua spp. tule marshes Scirpus and/or Typha spp. 168] redwood Sequoia sempervirens 5-200 [9,62,209] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla > 200 [9] eastern hemlock-yellow birch T. canadensis-Betula alleghaniensis > 200 [233] western hemlock-Sitka spruce T. h.-Picea sitchensis > 200 mountain hemlock* T. mertensiana 35 to > 200 [9] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 57,233] *fire return interval varies widely; trends in variation are noted in the species summaryAbundant evidence of postfire establishment of Canada thistle [16,138,163,193] suggests that managers need to be aware of this possibility, especially if a known seed source is in the area, and take measures to prevent the establishment of Canada thistle after prescribed burning and wildfires. Seeding with aggressive, introduced grasses such as crested wheatgrass, intermediate wheatgrass, orchardgrass, and smooth brome following a prescribed burn in Utah pinyon-juniper communities prevented establishment of Canada thistle, whereas unseeded areas supported Canada thistle seedlings [77]. Similarly, in disturbed forest sites where Canada thistle becomes established, it may be shaded out over time as trees reestablish [56].
Research in this report suggests that response of Canada thistle to fire is variable and it depends on vegetation and site characteristics, as well as frequency, severity and season of burning. Prescribed burns may be effective at stimulating growth of native species and thereby discouraging the growth of invasives such as Canada thistle [182], and may be best if timed to emulate the natural fire regime of a site [44]. Hutchison [105] states that prescribed burning is a "preferred treatment" for the control of Canada thistle, and that late spring burns effectively discourage this species, whereas early spring burns can increase sprouting and reproduction. During the first 3 years of control efforts, he recommends that burns be conducted annually [105], though it is unclear what evidence these recommendations are based on. Season of burn is an important consideration for prescribed burning, as the timing of the burn will determine species composition and cover in the post-fire community [101,102]. Dormant season burning may be a preferred treatment method in some areas, because in many habitats it stimulates growth of native vegetation that subsequently competes with Canada thistle [252]. However, dormant season burning may not be as effective as late spring burning [105]. Controlled studies comparing the effects of these variables in different natural areas are currently lacking in the literature.
Equations for estimating fuel loading of forb communities including Canada thistle are available [27].
The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [224] provides several fire management considerations for weed prevention in general that apply to Canada thistle. To prevent invasion after wildfires and prescribed burns, re-establish vegetation on bare ground as soon as possible using either natural recovery or artificial techniques as appropriate to site objectives. When reseeding burn areas, use only certified weed-free seed. Monitor burn sites and associated disturbed areas after the fire and the following spring for emergence of Canada thistle, and treat to eradicate any emergent Canada thistle plants. Regulate human, pack animal, and livestock entry into burned areas at risk for weed invasion until desirable site vegetation has recovered sufficiently to resist weed invasion.
When planning a prescribed burn, preinventory the project area and evaluate cover and phenology of any Canada thistle present on or adjacent to the site, and avoid ignition and burning in areas at high risk for Canada thistle 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. 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 [224].Temperature: Canada thistle grows best between 32 and 90 degrees Fahrenheit (0-32 °C) [150,152]. Extended periods with temperatures over 90 degrees Fahrenheit (32 °C) reduce plant vigor and generally limit growth. High temperatures and shorter days keep Canada thistle from thriving in the southern U.S. Optimum day/night temperatures for growth are 77 and 59 degrees Fahrenheit (25 and 15 °C), respectively [85]. The northern limit of Canada thistle's growth corresponds to the 0 degrees Fahrenheit (-18 °C) mean January isotherm; flowering is also limited in the northern latitudes [150]. Canada thistle invasion of native rangelands appears to be a problem especially of highly productive, mesic habitats [179,203,252]. However, Canada thistle was able to infest subalpine fir/twinflower habitats in western Montana [68]. The temperature exposure of overwintering buds required to reduce survival of Canada thistle was 2 hours at 19 degrees Fahrenheit (-7 °C) and to reduce total dry weight was 2 hours at 23 degrees Fahrenheit (-5 °C) [192]. The ability of adventitious root buds to withstand freezing depends on their location in the soil profile [55,192]. In soil samples from a mid-boreal wetland subjected to increased temperatures, Canada thistle seedling emergence increased significantly (p<0.05) at higher temperatures [99,100].
Moisture: Canada thistle tolerates annual precipitation ranging from 12 to 40 inches (305-1015 mm) per year, and grows best with 16 to 30 inches (400-750 mm) of precipitation per year [83,150,152]. In range and pastureland, Canada thistle is often restricted to swales or other areas of deep, moist soils [128]. Canada thistle is concentrated in disturbed areas and along streams, rivers and other moist areas in Rocky Mountain National Park, although individual plants have been found on relatively dry, sagebrush-dominated sites [139]. A high water table limits root growth [185], but Canada thistle often occurs in wetlands where water levels fluctuate, and in degraded sedge meadows it may be found growing on tussocks elevated above the normal high water line. In a mid-boreal wetland subjected to drought, Canada thistle increased 5- to 13-fold over predrought levels [98,100]. Canada thistle survives well in dry places [185] and under extended periods of drought, but biomass and number of root buds decrease after several years [194]. Growth was increased by high relative humidity (90-100%) over low relative humidity (30-50%) [104].
Elevation and slope: Canada thistle occurs over a wide range of elevations from sea level [58] to elevations in excess of 8,000 feet (2,500 m) [49]. In the northern Rocky Mountains, it is found mainly by roadsides and other disturbed sites in the lower elevations and warmer, drier habitats, and escapes to undisturbed sites at upper elevations [140,235]. In Yellowstone National Park, Wyoming, Canada thistle occurs at elevations ranging from 5,970 to over 7,875 feet (1,820-2,400 m) [4]. In Rocky Mountain National Park, Colorado, Canada thistle coverage is greater at elevations around 8,375 feet (2,550 m) and decreases at elevations around 9,095 feet (2,770 m), but occurs up to at least 9,185 feet (2,800 m) [139]. Canada thistle grows best on shallow (9-30%) slopes [4,140].
Soils: The wide distribution of Canada thistle suggests that it is adaptable to many soil types [55,185]. It grows on all but waterlogged, poorly aerated, and peat soils, including clay, clay loam, silt loam, sandy loam, sandy clay, sand dunes, gravel, limestone, and chalk [161]. Rogers [185] suggests that Canada thistle grows best on limestone soils with abundant moisture. Some authors suggest that it is best adapted to clay soils [152]; others suggest that it prefers well-aerated soils [150]. Preliminary results in Rocky Mountain National Park indicate that soils supporting Canada thistle tended to have a surface (0-10 cm) texture higher in clay and silt than in sand [139]. Canada thistle was found growing on heavily saline soils in central Alberta, though it was absent from saline areas of Saskatchewan and Manitoba [24]. Hardpans, gravel, sand, or very alkaline soil horizons can limit root development of Canada thistle [185].
Competition and light: Canada thistle grows best in open sunny sites [150]. Canada thistle seedlings are much less competitive than established plants, and will survive only if competition is limited and the daytime light intensity remains above 20% of full sunlight [152]. In Rocky Mountain National Park, total canopy cover of vegetation within Canada thistle patches is less than outside the patches [139]. At Yellowstone National Park, Canada thistle was found in 6 out of 10 campgrounds, with occurrences most frequent under a canopy cover of less than 20%, although it was occasionally present under more closed canopy covers (up to 95%) suggesting that it is somewhat tolerant of shade. Twenty percent of the quadrats in which Canada thistle was present had no evidence of disturbance [4]. Because Canada thistle is relatively shade intolerant, it may be found growing along the edges of woods (both deciduous and coniferous), but is rarely found under forest canopy, in undisturbed prairies, good to excellent pastures, or woodland or sites that are shaded most of the day [83,105,161]. In the Delta Marsh in Manitoba, Canada thistle is present in communities dominated by common reed. It is capable of persisting on undisturbed plots, growing with stunted spindly stems and no flowers, but growth improves after disturbance [213].
Generally, Canada thistle establishes and develops best on open, moist, disturbed areas, including ditch banks, overgrazed pastures, meadows, tilled fields or open waste places, fence rows, roadsides, and campgrounds; and after logging, road building, fire and landslides in natural areas [4,45,106,115,122,138,158,163,188,193,216,220]. Roads, streams and ditches provide areas of disturbance and corridors for invasion. At Yellowstone National Park, Canada thistle was found in all levels of disturbance (along horse and foot trails, roadways, and campgrounds) and its abundance increased as disturbance cover increased [4,219]. Physically disturbed habitat in fragmented old growth in Indiana facilitated invasion by exotics including Canada thistle [26]. Canada thistle invasion was also enhanced by heavy grazing by bison [237], areas left barren during planting operations, and on earth mounds made by pocket gophers and badgers in North and South Dakota [93].
Canada thistle reproduces both sexually by seed and vegetatively by creeping roots. Generally, vegetative reproduction contributes to local spread and seeding to long distance dispersal. Introduction into new areas is mostly by wind- or water-borne seed, or by seed in contaminated crop seed, hay or machinery [55,105]. Canada thistle allocates most of its reproductive energy to vegetative propagation, and a patch can spread rapidly by vegetative means under favorable conditions. Total allocation of dry weight to sexual reproduction was only 7% for Canada thistle grown in pots [23]. However, the contribution of sexual reproduction to the survival and spread of Canada thistle may be underestimated and may be an important mechanism for initiating continued genetic diversity in a clonal population [89].
Sexual reproduction: Shoot elongation and flowering in Canada thistle are induced by 15-hour day length, therefore flowering and seed production will be limited or prevented in regions with shorter summer days [84]. A typical Canada thistle shoot may produce 32 to 69 flowerheads per shoot (1-5 per branch) on average, but can produce as many as 100 flowerheads in a season [150,152]. Canada thistle is "imperfectly dioecious" [55], with male and female flowers occurring on separate plants. Up to 26% of "male" plants are actually self-fertile hermaphrodites or subhermaphrodites that occasionally produce seed [108].
Seed production: Canada thistle is insect pollinated, primarily by honeybees [55,105,150]. Male and female plants must be located within a few hundred yards of each other for insect pollination and seed set to occur [84]. Seed set is highest when male and female plants are intermixed and decreases when female plants are more than 164 feet (50 m) from male plants [125]. Since Canada thistle can grow in large patches, it is not uncommon to find sterile heads of female flowers [125,152]. Canada thistle has a reputation for producing few viable seeds, but the literature gives a wide range of estimates for seed production with numbers ranging from 0 to 40,000 seeds per stem [38,89]. Reports of average seed-set per flowerhead range from 21-93 [89,152]. Kay [108] reports that females produce an average of 30 to 70 seeds/flowerhead and males average 2 to 10 seeds/head. The number of flowerheads per stem reported ranges from 0 to 100 [89]. In annual grasslands in northern California where biomass of Canada thistle was 13+ 8 g/m2, seed production was 1300 seeds/m2, seed rain was 80+ 50 seeds/m2, and germinable seeds in the top 2 cm of soil were 280+110/m2 [96]. Inefficient pollination and genetic variability may contribute to poor seed yields [89]. Seeds of Canada thistle are subject to predation by insects before dispersal, but information is more qualitative than quantitative [55,89]. Weather extremes (cool and moist or hot and dry) can interfere with pollination, so some years even female plants do not produce much seed [61].
Seed dispersal: Canada thistle seeds are released about 2-3 weeks after pollination [123]. They are equipped with a pappus, loosely attached to the seed tip, that enables wind dispersal, and have good aerodynamic efficiency [198]. Canada thistle seeds have been observed windborne on the prairie several hundred meters from the nearest source population [175]. Evidence from seed rain studies on Mount St. Helens, Washington suggests that Canada thistle seeds can travel several kilometers [249]. This dispersal mechanism accounts for the numerous examples of Canada thistle seedling establishment after disturbance in natural areas [45,106,109,216,220], especially after fire [138,163,188,193]. However, wind dispersal has not been considered a major factor in its spread, since the pappus readily breaks off, leaving the achenes within the seedheads [23]. In developed areas, seeds are more commonly spread by animals, in hay, contaminated crop seed, machinery, and irrigation water [161]. Observations in Rocky Mountain National Park indicate that trails, especially those used by horses, are major invasion pathways for Canada thistle [139]. Livestock consuming unprocessed hay before entering national forests will likely spread more Canada thistle seeds than those consuming feed pellets, since pellet manufacturing destroys 99% of viable Canadian thistle seed when it includes grinding and screening [35].
Viability and germination: Canada thistle seeds mature quickly and most are capable of germinating 8 to 11 days after the flowers open, even if the plants are cut when flowering. Moore [150] summarized research indicating that almost all Canada thistle seed can germinate upon dispersal, although germination is extremely variable (0-95%). Viability of seeds during the 1st season after dispersal may be as high as 90% [84]. Most seeds germinate in the spring after the year in which they are produced [97,188], with some seeds producing basal leaves before winter and emerging to flower the next spring [105]. However, Heimann and Cussans [89] indicate that seedlings are not always able to survive the winter. Germination may be affected by ecotype, temperature, day length, depth of seed burial, substrate stratification, and seed freshness [161]. Seeds from "male" plants are smaller and percent germination is lower [108]. Temperature requirements for germination were summarized by Moore [150]; the effects of light, pH, and salinity are summarized by Donald [55]. Canada thistle seeds germinate best in warm temperatures (68 to 104 degrees Fahrenheit (20-40 °C)), with alternating light and dark periods [22,188,245]. Germination in Canada thistle was best after 0.5 to 16 days at 88 to 108 degrees Fahrenheit (31-42 °C) [212]. At lower temperatures germination is aided by high light intensity [89,97]. Germination at higher temperatures can help ensure that maximum germination takes place during warmer periods of the year [89]. Canada thistle seeds are somewhat tolerant of heat, and some were still viable after 10 minutes at 216 degrees Fahrenheit (102 °C) and 2 minutes at 504 degrees Fahrenheit (262 °C), although viability was decreased at these temperatures compared to unheated controls [212]. Canada thistle seeds germinate over a wide range of soil moisture [245]. Heimann and Cussans [89] provide a summary indicating that Canada thistle seed can germinate on the soil surface, but that germination is best when seeds are buried 0.2 to 0.6 inch (0.5-1.5 cm) deep. Emergence as deep as 6 cm in some soil types has been reported [245]. Most germination studies have been done under artificial conditions, and factors influencing germination in the field are far more complex [89].
Seed banking: The soil seed bank does not usually contain large numbers of Canada thistle seeds [36,184], although there is evidence of seed banking in a coastal British Columbia coniferous forest soil [110], in mature forest sites in central Idaho [117], and in the Delta Marsh in Manitoba [229]. Length of survival is related to depth of burial, with seeds surviving up to 22 years when they are buried more than 8 inches (20 cm) deep [78]. Under more natural conditions of shallower burial and periodic soil disturbance, Canada thistle seeds are more short lived (<5 years), with most seed being lost from the soil seed bank by germination during the 1st year [55]. Seeds that have been in water for several months can still be viable [84]. Donald [55] summarizes the research on seed banking in Canada thistle and the effects of seed immersion in water.
Seedling establishment: Canada thistle seedlings usually start growing slowly and are sensitive to competition and shading [55,89,128]. Seedlings grow poorly in very moist, poorly aerated soils and do not tolerate drought stress [245]. Before seedlings become perennial, they are also highly susceptible to tillage [152].
Asexual reproduction: Vegetative spread of Canada thistle can occur from horizontal extension of the root system, from root fragments, or from subterranean stem tissue [131]. Spread can be rapid when there is little competition, with 13 to 20 feet (4-6 m) of horizontal root growth possible in one season [97,185]. Canada thistle can develop new aerial shoots at any location along the root length, from the original vertical root, or from buds on lateral roots. Within a few weeks of germination, a Canada thistle seedling with at least 4 true leaves can begin producing root buds that can eventually produce new shoots [84]. Buds on lateral roots may form new adventitious shoots as frequently as 0.3 to 1-inch (0.8 to 2.4 cm) intervals [103], although the number of root buds is likely to vary from place to place and year to year [157]. A single Canada thistle plant can potentially produce 26 adventitious shoots, 154 adventitious root buds, and 364 feet (111 m) of roots after 18 weeks of growth [152,157]. It is possible that a colony of male plants would maintain itself regardless of whether it produced fruits [240].
Root buds are inhibited by the presence of the main shoot, primarily due to a competition for water [104], and new root bud growth is highest during late fall and winter months following death of aerial shoots [137]. When the main shoot is removed (e.g. as by mowing) the root buds are released, and new shoots emerge rapidly, especially when humidity is high [104,157]. Wilson [245] found that some 19-day old plants were capable of regenerating top-growth after clipping, and that 40-day old plants could produce 2 or 3 shoots after clipping. Root fragments as short as 0.2 inch (6 mm) and more than 6 weeks but less than 2 years old can regenerate entire plants, regardless of whether they have identifiable root buds at the time [157]. Nadeau and Vanden Born [157] observed that an 18-week-old plant had the potential of producing 930 shoots if its root system was cut into 10-cm-long pieces.
Vegetative spread of Canada thistle may also occur from subterranean stem tissue that can produce shoot buds and adventitious roots at each node. Partially buried stem sections from the postbloom stage survived and produced adventitious roots that over wintered and produced new infestations the following spring [131]. Similarly, Canada thistle can survive disturbance to be part of the early successional community in natural areas by resprouting from buried root and stem fragments [2,45,188,216].
