Nile and Mediterranean regions.
Cosmopolitan.
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [32,46,47,48,61,73,98,99,109,116,138,154,165,169]). For a key to the prostrate knotweed subspecies recognized in North America, see: [32].
Prostrate knotweed is generally considered an annual [32,65,69,99,160], though some sources report it as occasionally perennial [44,70,116].
Prostrate knotweed plants exhibit highly variable architecture depending on both genetic and environmental factors [32]. In general, prostrate knotweed is a mat-forming plant [65], with mats reaching 4 to 48 inches (10-122 cm) in diameter [113]. Prostrate knotweed stems are prostrate to erect, 2 to 80 inches (6-200 cm) long. Leaves are alternate and vary in size and shape, but are generally ovate. Inflorescences are axillary cymes with 2 to 6 flowers. Flowers are bisexual [32]. Prostrate knotweed fruits are one-seeded nuts [105]. Seeds are achenes, 1.7 to 4.0 mm long [32].Prostrate knotweed has a taproot [32,65,116]. Taproots of mature prostrate knotweed plants in alluvial soil reached depths of 30 inches (70 cm). Dense horizontal secondary roots were distributed in the upper 5 to 10 inches (15-25 cm) of soil (Kutschera 1960 cited in [32]). On sand dunes in the deserts of Death Valley National Monument, prostrate knotweed taproots penetrated approximately 5 inches (13 cm) in the soil and roots exhibited very little lateral spread (approximately 1 inch (3 cm)). Ten plants had an average root to shoot ratio of 0.09 [59].
Prostrate knotweed is one of the most widespread weeds in the world (review by [32]). Its widespread distribution is attributed to several plant characteristics, including high genetic polymorphism, high phenotypic plasticity [104], prolific seed production [123], multiple means of seed dispersal, formation of a persistent seed bank, and allelopathy (review by [32]).
Prostrate knotweed is native to Europe [98,138] or Eurasia [81]. It was likely introduced to North America with the first colonists and was first collected in Canada in 1821 (review by [32]). One source suggests that it was introduced as a contaminant in agricultural seeds [96]. As of 2010, prostrate knotweed occurs in all 50 of the United States, though as of 2010, Plants Database does not report prostrate knotweed occurring in California. However, several other sources report it occurring there [14,25,59,64,100,129]. Plants Database provides a distribution map of prostrate knotweed in Canada and the United States.
Fuels: As of 2010, little is known about the fuel characteristics of prostrate knotweed. The potential for prostrate knotweed to alter fuel characteristics likely varies by plant community. It is not clear whether the persistence of dead mats of vegetation or stems from year to year would represent an increased fuel load or fire hazard.
FIRE REGIMES: It is not known what type of fire regime prostrate knotweed is best adapted to. Results from a study in a northeastern Kansas tallgrass prairie suggest that annual prescribed fire is more favorable to prostrate knotweed than fire at 4-year intervals or no fire [143]. However, it is impossible to make valid generalizations from a single study from a single plant community. As the Fire Regime Table indicates, prostrate knotweed occurs in a wide range of North American plant communities that exhibit a full range of fire regime characteristics. It is also likely that prostrate knotweed occurs in plant communities and associated FIRE REGIMES not presented in this table. See the full Fire Regime Table for information on FIRE REGIMES of other plant communities of interest. 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". The impacts of prostrate knotweed on FIRE REGIMES are unknown.
Prostrate knotweed seeds require moist-cold stratification for germination [9]. One source suggests that achenes produced in different seasons (summer and autumn) are fundamentally different in their dormancy and germination characteristics, but most studies do not specify which type of seed was tested. Personal observations of the authors suggested that the small, summer achenes have a strong primary dormancy and may constitute the persistent seed bank. These seeds must undergo a moist-cold stratification at 35 °F to 54 °F (1.6 °C-12 °C) for 12 to 110 days to break dormancy. In contrast, autumn achenes are larger, have a weak innate dormancy, and are capable of germinating immediately if exposed to temperatures of 70 °F to 80 °F ( 20-25 °C). If temperatures are lower, they germinate in the spring in a single flush at temperatures as low as 40 °F (5 °C). The authors suggested that most germination studies likely refer to summer seeds [32].
Temperature: Low winter temperatures release seed dormancy while high summer temperatures reinforce dormancy [9,10,11,34]. In laboratory experiments, prostrate knotweed seeds required a 40 °F (5 °C) treatment in the dark to germinate. Optimum germination (100%) was obtained after a 90-day cold-stratification at 40 °F (5 °C) [83].
Moisture: Prostrate knotweed seed germination is favored by moisture [11,28]. Laboratory germination tests showed that seeds exposed to low moisture had low germination (<5%) and showed no response to light treatments. Fluctuating soil moisture improved germination rates. Seeds exposed to constant moisture at 35 °F (1.6 °C) had low germination rates (<5%) while those exposed to fluctuating soil moisture had higher germination rates (approximately 40%). Fluctuations in soil moisture also improved germination rates of seeds kept in the dark, suggesting that such fluctuations may allow prostrate knotweed seeds to bypass the light requirement for germination in some situations. The authors suggested that deeply buried seeds would not be exposed to such moisture fluctuations [11].
Light: While some sources report that prostrate knotweed seeds require light to break dormancy [10,11], one study suggests that light is not required but improves germination rates. In laboratory germination tests in Kentucky, prostrate knotweed seeds exposed to several thermoperiods germinated from January to June, at rates of 70% to 90% for seeds exposed to light, and 1% to 26% for seeds kept in the dark [7].
Depth: Seed burial depth may influence germination rates, though results from experiments are not consistent. A review states that most seedlings emerge from the top 1 inch (3 cm) of soil and emergence declines with depth of burial [32]. In growth chamber experiments, shallow burial (<0.5 inches (1.25 cm) increased germination while deep burial (1 to 4 inches (2.5-10 cm)) decreased it [66]. In contrast, other laboratory experiments showed that germination rates were higher for prostrate knotweed seeds buried from 5.5 to 6 inches (14-15 cm) compared to those buried at depths ranging from 0 to 4 inches (0-10 cm). Dormancy was induced earlier for seeds closer to the soil surface than those buried at various depths beneath the soil. The authors suggested that dry conditions near the soil surface could induce dormancy [34].
Disturbance: Soil disturbance and scarification may improve germination rates. In field experiments using potted seeds in Ireland, germination began in late February, peaked in April, and ceased by the end of May. Soil disturbance in March increased seed germination (from 4% to 21%), though germination still ceased by the end of May. Soil disturbance at times other than late March or early April had no impact on seed germination, nor did it impact the timing of seedling emergence the following year. Mechanical or sulfuric acid removal of the pericarp increased germination rates [34].
Salinity: The impacts of salinity on germination are not clear. In laboratory experiments, exposure of prostrate knotweed seeds to highly saline conditions led to higher germination rates; germination rates were higher at electrical conductivities of 200 mS/m and 250 mS/m compared to electrical conductivities ranging from 0 to 150 mS/m (P=0.05) [130]. Other laboratory experiments also showed prostrate knotweed seeds to be moderately salt tolerant; the cumulative germination percentage of seeds decreased as salinity increased, though some seeds did germinate at the highest salinity (300 mM NaCl) [83]. In contrast, germination of seeds removed from saline soils in Ohio varied little in relation to soil salinity, and laboratory trials showed germination rates decreasing with increasing salinity [58].
Site types: A weed identification guide reports that prostrate knotweed is commonly found in areas with trampled, compacted soil, and persists in areas where other species do not grow well or are damaged [150]. Floras report that prostrate knotweed occurs on a variety of disturbed sites [44,46,47,48,121,154,155,165,169] including roadsides [69,81,98,121,154], railroad embankments [154], and sidewalk cracks [98,154]. Prostrate knotweed also establishes in disturbed areas associated with hiking trails [93], eroded or overgrazed mountain meadows [113], and mine sites [72,125]. Prostrate knotweed is also common in cultivated fields [61,65,69,70,113,138], pastures [155], lawns [98,154], gardens [69], and near dwellings [116].
Elevation: Prostrate knotweed occurs at a wide range of elevations (100 to 10,120 feet (30-3,080 m) in North America.
Elevation of sites with prostrate knotweed in North America Location Elevation (feet) Arizona 100 to 8,500 [113] Colorado 5,000 to 9,500 [70] Hawaii 3,280 to 6,820 [40,155] Montana 2,200 [17] Nevada 8,600 to 9,500 [142] New Hampshire 3,800 [54] New Mexico 2,700 [168] Utah 2,495 to 10,120 [160]Soil: Prostrate knotweed tolerates a wide range of soil conditions, the extremes of which may not be favorable to other plants. One review states that prostrate knotweed grows well in soils that are compacted, poorly aerated, poor to rich in nutrients, and of all types and textures. Prostrate knotweed also tolerates soils with a high salt content, high calcium content, heavy metal contamination, and a range of pH (5 to 8.4) [32]. In China, prostrate knotweed established and grew "prolifically" in soils with pH 3.5 [174]. Observations from Colorado suggest that prostrate knotweed was one of few species able to establish in heavily eroded areas following severe sheet erosion [77]. In Iran, prostrate knotweed established on dried lead and zinc mine waste pools that had elevated levels of cadmium, copper, iron, nitrogen, lead, and zinc [24]. Also in Iran, prostrate knotweed grew at higher densities than any other plant on soils contaminated with petroleum products, and contamination did not prevent germination [110].
Prostrate knotweed is reported on soils of various types and textures in North America. Several studies report it growing on sandy soil. In central Arizona, prostrate knotweed occurred on riparian silt and sand [166]. Along the Colorado River in Arizona, it established on loamy sand [134]. In northeastern Wyoming it occurred on sandy loam [3]. Prostrate knotweed also established on a sand bar in Lake Superior, Minnesota [90] and on exposed lake sediment in northwestern New Mexico [168]. In northwestern Colorado, is established on alluvial soil with a heavy clay content [6]. On the ridges of Monument Peak, Oregon, prostrate knotweed occurred in shallow, gravelly soils on rocky outcrops [4].
Several sources report prostrate knotweed growing on shallow soils [4,8,71]. Prostrate knotweed grows on both dry and moist soils [113]. In northwestern New Mexico, prostrate knotweed established adjacent to a lakebed and survived for a year despite partial submergence [168].
Prostrate knotweed occurs on saline sites [15,16,17,51,58,148]. In Nebraska, prostrate knotweed was widely scattered along the borders of salt pans, establishing in areas with low salinity (0.5% to 0.7% total salts) compared to areas where it did not establish [148]. On brine spill sites in Ohio, prostrate knotweed tolerated moderate salinity levels, though an increase in salinity was correlated with lower prostrate knotweed abundance, higher mortality, earlier senescence, lower aboveground biomass, and lower germination rates [58]. In northeastern Ohio, prostrate knotweed was not present in the extant vegetation but occurred in the soil seed bank of a highly saline (3.5% NaCl) saltpan [51].
Climate: Prostrate knotweed occurs in a wide range of climates, from subtropical to subarctic [32]. Precipitation varies across the range of prostrate knotweed.
Average annual precipitation for locations with prostrate knotweed in North America Location Average annual precipitation (mm) Arizona 215 [134] 444 [88] Colorado 310 [56] 540 [167] Idaho 280 [27] Montana 305 [17] Nebraska 686 [2] Ohio 1,010 [58] Oregon 1,780 [108] Texas 1,040 [128] Wyoming 226 [72]Prostrate knotweed can withstand drought [2,18,32,113], though slow growth and low survivorship was linked to low precipitation and soil moisture in the deserts of Death Valley National Monument [59].
Impacts: Most reported impacts of prostrate knotweed are related to its establishment in crop fields [123]. Prostrate knotweed is problematic in >60 crop species worldwide. Its density in agricultural fields was as high as 28.3 plants/m², as was recorded in a barley field in Alberta. Prostrate knotweed establishment reduces yield for some crops. Its stems may inhibit the mechanical harvest of other crops (e.g., onions, carrots) (review by [32]) and may act as an alternate host for crop pathogens [123]. Prostrate knotweed is also considered a nuisance in lawns, sidewalks, and paved areas (review by [32]).
Prostrate knotweed's impact on native plant communities is not well documented. A weed information guide suggests that dense mats of prostrate knotweed may smother herbaceous species and small shrubs [157]. Prostrate knotweed also has allelopathic qualities (review by [32]). In laboratory tests, soil collected from under prostrate knotweed inhibited the growth of several plant species, including Bermuda grass, Madagascar dropseed (Sporobolus pyramidatus), lambsquarters, sorghum (Sorghum bicolor), and Creole cotton (Gossypium barbadense). The soil used in this study was collected 4 months after prostrate knotweed plants died in the fall, suggesting that toxins may persist in the soil. Prostrate knotweed aboveground parts, roots, and root exudates also inhibited germination and growth of several crop and nonnative plant species [1].
Control: In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [20]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [97].
Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.
Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [97,127] (e.g., avoid road building in wildlands [145]) and by monitoring several times each year [76]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [74].
Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [146]. See the Guide to noxious weed prevention practices [146] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.
Cultural control: Laboratory studies report that extracts from some cover crops, including rye (Secale cereale) and brown mustard (Brassica juncea), reduced germination of prostrate knotweed seeds and rootlet and shoot length of prostrate knotweed seedlings [52].
Physical or mechanical control: Mechanical control methods alone are usually not effective at controlling prostrate knotweed, but integration with other control methods (e.g., chemical) may improve treatment effectiveness. Soil solarization controlled prostrate knotweed in some areas (review by [32]). In interior Alaska, roadside prostrate knotweed seedlings establishing 2 years after fire were manually pulled in approximately 15 minutes. The following year, no prostrate knotweed seedlings were observed [31]. To prevent seed dispersal, a weed information guide suggests cutting plants prior to seed set [157] (e.g., late May in Pennsylvania [71]).
Prostrate knotweed's low stature makes mowing treatments largely ineffective [154]. Bark mulching favored prostrate knotweed in apple orchards (review by [32]). Flaming and hot-steaming did not control prostrate knotweed in Nova Scotia and Slovakia (Rifai and others 2001 as cited in [32])
Biological control: As of this writing (2010) no biological control agent has been identified to control prostrate knotweed. In North America, prostrate knotweed hosts several insects, nematodes, fungi, and viruses (review by [32]). In garden experiments in Pennsylvania, a fungal rust killed all prostrate knotweed seedlings. Seedlings emerging the following year also died, and the entire prostrate knotweed population was killed [71].
Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [151,162] and the Weed control methods handbook [144] for background information and important considerations for developing and implementing biological control programs.
Chemical control: Both pre- and postemergent herbicides are effective at controlling prostrate knotweed (review by [32]), though a flora reports that prostrate knotweed resists herbicides [44]. The effectiveness of chemical control decreased with time in one cropping system experiment [175]. In commercial agricultural fields in California, exposure to several soil fumigants reduced the percentage of viable prostrate knotweed seeds. In areas exposed to the fumigant, 2.7% of seeds were viable, compared to 36.4% viability in areas not exposed to the fumigant [67].
Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [23]. See the Weed control methods handbook [144] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Integrated management: No information is available on this topic.
Prostrate knotweed is consumed by a variety of wildlife species as well as some livestock. However, in Australia, the death of several horses from nitrite toxicity was attributed to eating prostrate knotweed [85].
Palatability and/or nutritional value: Prostrate knotweed seeds are consumed by birds [32,138] including the American coot [14], mallard, killdeer [41], rock dove [114], sharp-tailed grouse [140], California quail [36], and American tree sparrow [12]. Leaves may be consumed by birds [138] such as the sharp-tailed grouse [140]. Small mammals may also consume parts of prostrate knotweed [101]. One black-tailed prairie dog stomach contained >20,000 prostrate knotweed seeds [86]. Eastern cottontails consumed prostrate knotweed in Missouri [87]. Prostrate knotweed is browsed by mule deer [38,75] and pronghorn [161,173]. Insects feed on the seeds [101] and nectar [22].
In Australia, prostrate knotweed is used as a fodder plant for pigs (review by [32]). Free-ranging domestic cattle consumed prostrate knotweed while foraging in ponderosa pine forests in central Colorado [38]. Domestic geese did not feed on prostrate knotweed in feeding trials, even when it was the only food available [170].
Cover value: No information is available on this topic.
Prostrate knotweed is reported to have many medicinal uses, including the treatment of gingivitis, cardiovascular conditions, infections, and immunity disorders (review by [32]). Prostrate knotweed tea has been used to treat asthma [44] and diarrhea [70]. One source reports that exposure to prostrate knotweed may cause dermatitis [78]. According to English medieval superstition, an infusion of prostrate knotweed stems and leaves could stunt the growth of young boys or animals. Such properties were recognized by Shakespeare, who referred to "knot-grass" in A Midsummer Night's Dream: "Get you gone, you dwarf;/You minimus, of hindering knot-grass made" (review by [32]).
Prostrate knotweed seeds are edible to humans, either whole or ground into flour [70,98]. In China, people eat young prostrate knotweed shoots and leaves and drink prostrate knotweed tea (review by [32]).
Prostrate knotweed has been used in phytoremediation of soils contaminated with heavy metals [24] or crude oil [110]. It may also be used in erosion control (review by [32]). In China, parts of prostrate knotweed are used as an insecticide to control the pear leaf weevil (Rhynchites coreanus) and to treat maggots and roundworms in pigs [171]. Prostrate knotweed is a valued honey plant in Greece [45] and Australia. In China, flowering stems are used as a textile dye (review by [32]).
The seasonal development of prostrate knotweed varies by both population and genotype [32,105]. In southern Canada, most prostrate knotweed seeds lose dormancy in March and April and germinate in a single flush between March and May [32]. In North America, prostrate knotweed flowers from March to November depending on location.
Flowering date of prostrate knotweed in locations throughout North America Location Flowering date Arizona March to October [113] Great Plains June to October [65] Illinois June to October [109] Kentucky June to November [69] New England June to September [98] North and South Carolina May to November [116] Texas May to November [44] West Virginia June to October [138]In southern Canada, prostrate knotweed plants produced seeds approximately 2 months after seedling emergence and produced both summer and autumn achenes [32]. In Pennsylvania, prostrate knotweed plants began producing seeds by late May and continued fruiting until killed by frost in the fall [71]. In north-central Arizona, prostrate knotweed produced seeds from early September to mid-November.
Prostrate knotweed plants are killed by frosts in the fall. A weed identification guide reports that clusters or mats of dead stems persist through the winter [150].
Prostrate knotweed seeds form a persistent seed bank [32]. Some prostrate knotweed seeds (<1%) were viable after 19.7 years of burial in subarctic conditions near Fairbanks, Alaska. Seeds buried at shallower depths lost viability faster than those buried at greater depths; over the course of the study, the annual rate of viability decline was 40% for seeds buried at 1 inch (2 cm) and 29% for seeds buried at 6 inches (15 cm) [30]. From mine sites in the United Kingdom, prostrate knotweed seeds germinated from soil samples stored for 4 years, and germinated from samples taken from as deep as 7 feet (2 m) in the soil [43].
The density of prostrate knotweed seeds in the soil seed bank is variable, and may be high even in areas where prostrate knotweed does not occur in the extant vegetation. At saline sites in Ohio, the mean number of seeds found in 100 cm² of soil ranged from approximately 50 to 225 [58]. Seeds of prostrate knotweed were found at a low density (4.3 seeds/m²) in the seed bank of a forested woodlot in southern Ontario [21]. In northeastern Ohio, prostrate knotweed was not present in the extant vegetation but occurred in the soil seed bank (2,631.6 seeds/m²) of a highly saline saltpan [51]. In Argentina, prostrate knotweed was present in the soil seed bank of 2- to 4-year-old successional fields but was not present in the extant vegetation. It was a dominant species in nearby croplands [39].
Seedlings: A weed identification guide reports that prostrate knotweed seedlings grow slowly [150]. Prostrate knotweed seedlings may reach high densities, though as of this writing (2010) there were no quantitative descriptions of seedling densities in natural plant communities. In experimental winter wheat (Triticum sp.) fields in Spain, prostrate knotweed seedling density was 3 times higher in tilled than untilled fields (P<0.05), exceeding 100 seedlings/m². A few new seedlings were observed after precipitation events in all tillage systems and precipitation appeared to increase survival [152]. In laboratory experiments, high salinity appeared to improve the growth of prostrate knotweed seedlings [130]. In garden experiments in Pennsylvania, a fungal rust caused the mortality of an entire seedling population [71].
Mature plants: One flora describes prostrate knotweed as "vigorous" [121]. In dense lawns of Bermuda grass, prostrate knotweed patches increased 5 feet (1.5 m) in diameter in a growing season [1]. In garden experiments in Pennsylvania, prostrate knotweed had a higher survival rate in plots where it was planted with native species than where it was planted in monocultures (P<0.05) [71]. In the deserts of Death Valley National Monument, prostrate knotweed survival and reproduction was limited by precipitation and/or soil moisture [59].
Prostrate knotweed establishes in early successional plant communities, though it may persist into later successional stages. Prostrate knotweed established in early succession on heavily eroded buttes in the Badlands region of western North Dakota [79]. In abandoned fields in Colorado, prostrate knotweed occurred in a full range of field ages, including fields abandoned 3 months prior to sampling, and fields abandoned for 62 years [77]. In blue grama and buffalo grass grasslands in eastern Colorado, prostrate knotweed dominated abandoned roads in the early stages of succession. Prostrate knotweed density was highest on roadbeds 2 years after abandonment. It occurred infrequently >5 years after road abandonment [126]. In mixed-grass prairies in southeastern Wyoming, prostrate knotweed was one of several annuals dominating the vegetation in the first years following plowing or scraping and was seldom observed 10 years after disturbance [124]. At mine sites in Wyoming, prostrate knotweed was a dominant species 1 to 4 years following plantings of native shrubs and grasses at one location [72] and established within 2 years of soil placement in another location [125]. Prostrate knotweed has also been reported at numerous sites in the first few years following fire [27,35,42,60,64,88,106,112,115,143,167]. See Plant response to fire for more information.
Several sources report a preference for open sites [50,116,160] and light is generally though to improve germination.
Prostrate knotweed establishes on disturbed sites, including logged areas [156], revegetating mine sites [72,125], scraped and plowed mixed-grass prairie [124], roads, hiking trails [93], ski runs [142], backcountry shelters [54], heavily eroded areas [77,79], exposed sand bars [90], and lake shores [168]. Prostrate knotweed is often associated with locations disturbed by domestic and wild animals. It tolerates trampling [113,160,168] and is found in areas heavily grazed by cattle [149] and bison [143,158]. In old fields in Germany, prostrate knotweed established in areas grubbed by wild boars [107]. Prostrate knotweed also commonly establishes in the highly disturbed areas surrounding black-tailed prairie dog towns [93,149].
Some sources report prostrate knotweed occurring in disturbed areas but not in adjacent undisturbed plant communities. In southern Nevada, prostrate knotweed established on ski runs but did not spread into surrounding forests [142]. In deciduous riparian forests in southeastern Arizona, prostrate knotweed was present in the soil seed bank in areas that had some human disturbance but was absent from the seed bank in undisturbed areas [120].
Though examples of prostrate knotweed spreading from disturbed areas into undisturbed areas are lacking in the literature (2010), some sources report it occurring in adjacent disturbed and undisturbed areas, suggesting that such spread is possible. Prostrate knotweed occurred both along roadsides and in the interior of ponderosa pine forests in Arizona, though populations were more dense and occurred more frequently along roadsides [60]. In the northern Rocky Mountains, prostrate knotweed occurred in both disturbed areas (e.g., ditch banks and logged areas) as well as nearby undisturbed areas (e.g., subalpine meadows) [156].
The scientific name of prostrate knotweed is Polygonum aviculare L. (Polygonaceae)
[57,80]. The Flora of North America recognizes 6 subspecies:
Except for Polygonum aviculare subsp. boreale, the subspecies listed above
overlap in distribution and exhibit complex intergradation, resulting in populations with
intermediate characteristics [57]. Because identification at the
subspecies level is difficult, and sources either rarely report subspecies or identification may
be suspect, this review synthesizes information about prostrate knotweed at the species level.
For a review of the taxonomic issues of the prostrate knotweed complex, see [32].
Polygonum aviculare or common knotgrass is a plant related to buckwheat and dock. It is also called prostrate knotweed, birdweed, pigweed and lowgrass. It is an annual found in fields and wasteland, with white flowers from June to October. It is widespread across many countries in temperate regions, apparently native to Eurasia, naturalized in temperate parts of the Southern Hemisphere.[2][3][4][5][6][7]
Common knotgrass is an annual herb with a semi-erect stem that may grow from 10 to 40 cm (4 to 16 in) high. The leaves are hairless and short-stalked. They are longish-elliptical with short stalks and rounded bases; the upper ones are few and are linear and stalkless. The stipules are fused into a stem-enclosing, translucent sheath known as an ochrea that is membranous and silvery. The flowers are regular, green with white or pink margins. Each has five perianth segments, overlapping at the base, five to eight stamens and three fused carpels. The fruit is a dark brown, three-edged nut. The seeds need light to germinate which is why this plant appears in disturbed soil in locations where its seeds may have lain dormant for years. It is a common carrier of the parasitic pathogen powdery mildew,[8] which can give the leaves a whitish appearance.[3][9]
Polygonum aviculare has a wide distribution as an arable weed and plant of fields, shingle, sand, roadsides, yards and waste places. There is much morphological variation among different populations and several different sub-species are recognized:[1][2][3][9]
Widespread and common in Great Britain,[10] Ireland,[10][11] and Scandinavia.[12]
It is common on roadsides and arable ground in the British Isles.[11]
Polygonum aviculare contains the flavonols avicularin, myricitrin and juglanin.[13] The flavanoids astragalin and betmidin, and the lignan aviculin have also been found.[14] The diterpene alkaloid panicudine is another known component.[15]
One fossil fruit of Polygonum aviculare has been extracted from borehole samples of the Middle Miocene fresh water deposits in Nowy Sacz Basin, West Carpathians, Poland.[16]
It formed a traditional ingredient in porridge consumed by Germanic peoples of western Europe, and has been found in numerous autopsies of peat bodies, including the Tollund Man.
In Vietnam, where it is called rau đắng, it is widely used to prepare soup and hot pot, particularly in the southern region.
Polygonum aviculare or common knotgrass is a plant related to buckwheat and dock. It is also called prostrate knotweed, birdweed, pigweed and lowgrass. It is an annual found in fields and wasteland, with white flowers from June to October. It is widespread across many countries in temperate regions, apparently native to Eurasia, naturalized in temperate parts of the Southern Hemisphere.
