Numerous variants of Cirsium arvense have been named based upon such features as pubescence, extent of leaf division, and spininess. Although extreme variants can be strikingly different, they are connected by such a web of intermediates that there seems to be little value in according any of them formal taxonomic recognition.
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].
Cirsium arvense is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa and widely introduced elsewhere.[2][3][4][5] The standard English name in its native area is creeping thistle.[6] It is also commonly known as Canada thistle and field thistle.[7][8]
The plant is beneficial for pollinators that rely on nectar. It also was a top producer of nectar sugar in a 2016 study in Britain, with a second-place ranking due to a production per floral unit of (2609±239 μg).[9]
A number of other names are used in other areas or have been used in the past, including: Canadian thistle, lettuce from hell thistle, California thistle,[10] corn thistle, cursed thistle, field thistle, green thistle, hard thistle, perennial thistle, prickly thistle, setose thistle, small-flowered thistle, way thistle, and stinger-needles. Canada and Canadian thistle are in wide use in the United States, despite being a misleading designation (it is not of Canadian origin).[11]
Cirsium arvense is a C3 carbon fixation plant.[12] The C3 plants originated during Mesozoic and Paleozoic eras, and tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, and ground water is plentiful. C3 plants lose 97% of the water taken up through their roots to transpiration.[13]
Creeping thistle is a herbaceous perennial plant growing up to 150 cm, forming extensive clonal colonies from thickened roots that send up numerous erect shoots during the growing season.[14] It is a ruderal species.[15]
Given its adaptive nature, Cirsium arvense is one of the worst invasive weeds worldwide. Through comparison of its genetic expressions, the plant evolves differently with respect to where it has established itself. Differences can be seen in their R-protein mediated defenses, sensitivities to abiotic stresses, and developmental timing.[16]
Cirsium arvense is placed in the subtribe Carduinae, tribe Cardueae of the family Asteraceae. Unlike other species in the same genus, it is dioecious, although male plants sometimes produce bisexual flowers.[17] It also differs from other native North American species in having large roots and multiple small flower heads on a branched stem.[18]
Its underground structure consists of four types, 1) long, thick, horizontal roots, 2) long, thick, vertical roots, 3) short, fine shoots, and 4) vertical, underground stems.[19] Though asserted in some literature, creeping thistle does not form rhizomes.[20] Root buds form adventitiously on the thickened roots of creeping thistle, and give rise to new shoots. Shoots can also arise from the lateral buds on the underground portion of regular shoots, particularly if the shoots are cut off through mowing or when stem segments are buried.[20]
Stems are 30–150 cm, slender green, and freely branched,[20] smooth and glabrous (having no trichomes or glaucousness), mostly without spiny wings. Leaves are alternate on the stem with their base sessile and clasping or shortly decurrent. The leaves are very spiny, lobed, and up to 15–20 cm long and 2–3 cm broad (smaller on the upper part of the flower stem).
Every plant species has a unique floral fragrance.[21] The fragrance that C. arvense emits attracts both pollinators and florivores containing compounds that attract each respectively. Non-native honeybees are shown to have the highest visitation rate, following other bee species in the genera Halictus and Lasioglossum. Hover flies are also commonly seen pollinating the flower heads of this plant.[22] Florivores such as beetles and grasshoppers are commonly seen as well. The compounds found in the fragrance may not be in the highest abundance but they are highly attractive. P-anisaldehyde is found in less than 1%, yet it attracts pollinators such as honey bees.[23] This is thought to be the result of additive and synergistic effects from the blend increasing the attraction to the plant. After pollination, it can be seen that fragrance emission decreases in C.arvense. This is regulated through a regulatory feedback mechanism depending on the pollination status of the plant. This mechanism has only been observed in pistillate plants for dioecious C. arvense. Fragrance emission increases with age.[24]
The fragrance contains several compounds that attract diverse insects. Looking at certain butterflies species, it can be seen that the fragrance blend is highly attractive to them, being sensitive to their antennae. High antennal response are seen in consequence to the phenylacetaldehyde as well as the terpenes (oxoisophoroneoxide, oxoisophorone, and dihydrooxoisophorone) found in the blend. This was seen in both natural plants emitting the fragrance and emitting the scent synthetically.[25] It is believed that general arousal can be stimulated through exposure of a single compound, whereas the accumulated exposure of all the compounds influence the foraging behaviour of the butterflies.[21]
The inflorescence compound cyme is 10–22 mm (0.39–0.87 in) in diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers.[20] The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal.[26][27][28] One to 5 flower heads occur per branch, with plants in very favourable conditions producing up to 100 heads per shoot.[14] Each head contains an average of 100 florets. Average seed production per plant has been estimated at 1530. More seeds are produced when male and female plants are closer together, as flowers are primarily insect-pollinated.[14] The plant can bloom from seed in a year then subsequently the seeds produced can emerge in the following year.[29]
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Pappus of Cirsium arvense
A creeping thistle with a "cuckoo spit"
Variation in leaf characters (texture, vestiture, segmentation, spininess) is the basis for determining creeping thistle varieties.[14] According to Flora of Northwest Europe[26] the two varieties are:
The Biology of Canadian Weeds: Cirsium arvense[14] list four varieties:
The seeds are an important food for the goldfinch and the linnet, and to a lesser extent for other finches.[30] Creeping thistle foliage is used as a food by over 20 species of Lepidoptera, including the painted lady butterfly and the engrailed moth, and several species of aphids.[31][32][33]
The flowers are visited by a wide variety of insects such as bees, moths, wasps and beetles[34] (the generalised pollination syndrome).[35]
The species is widely considered a weed even where it is native, for example being designated an "injurious weed" in the United Kingdom under the Weeds Act 1959.[36] It is also a serious invasive species in many additional regions where it has been introduced, usually accidentally as a contaminant in cereal crop seeds. It is cited as a noxious weed in several countries; for example Australia, Brazil, Canada, Ireland, New Zealand, and the United States. Many countries regulate this plant, or its parts (i.e., seed) as a contaminant of other imported products such as grains for consumption or seeds for propagation. In Canada, C. arvense is classified as a primary noxious weed seed in the Weed Seeds Order 2005 which applies to Canada's Seeds Regulations.[37]
Control methods include cutting at flower stem extension before the flower buds open to prevent seed spread. Repeated cutting at the same growth stage over several years may "wear down" the plant.
Growing forages such as alfalfa can help control the species as a weed by frequently cutting the alfalfa to add nutrients to the soil, the weeds also get cut, and have a harder time re-establishing themselves, which reduces the shoot density.[38]
Orellia ruficauda feeds on Canada thistle and has been reported to be the most effective biological control agent for that plant.[39] Its larvae parasitize the seed heads, feeding solely upon fertile seed heads.[40]
The weevil Larinus planus also feeds on the thistle and has been used as a control agent in Canada.[41] One larva of the species can consume up to 95% of seeds in a particular flower bud.[42] However, use of this weevil has had a damaging effect on other thistle species as well, include some that are threatened.[43] It may therefore not be a desirable control agent. It is unclear if the government continues to use this weevil to control Canada thistles or not.
The rust species Puccinia obtegens has shown some promise for controlling Canada thistle, but it must be used in conjunction with other control measures to be effective.[44] Also Puccinia punctiformis is used in North America and New Zealand in biological control.[45] In 2013, in four countries in three continents, epidemics of systemic disease caused by this rust fungus could be routinely and easily established.[46] The procedure for establishing this control agent involves three simple steps and is a long-term sustainable control solution that is free and does not involve herbicides. Plants systemically diseased with the rust gradually but surely die. Reductions in thistle density were estimated, in 10 sites in the U.S., Greece, and Russia, to average 43%, 64%, and 81% by 18, 30, and 42 months, respectively, after a single application of spores of the fungus.[47]
Aceria anthocoptes feeds on this species and is considered to be a good potential biological control agent.
Applying herbicide: Herbicides dominated by phenoxy compounds (especially MCPA) caused drastic declines in thistle infestation in Sweden in the 1950s.[12] MCPA and clopyralid are approved in some regions. Glyphosate is a non-selective herbicide that can be used when the plant has grown a few inches tall, where the herbicide can be absorbed by the leaf surfaces.[29]
Crop tolerance and weed control ratings were conducted in the spring of 2012, and the Prepass herbicide by DOW AgroSciences was found to be most effective at controlling the species as a weed problem in alfalfa fields.[48]
Like other Cirsium species, the roots are edible, though rarely used, not in the least because of their propensity to induce flatulence in some people. The taproot is considered the most nutritious part. The leaves are also edible, though the spines make their preparation for food too tedious to be worthwhile. The stalks, however, are also edible and more easily despined.[49] Bruichladdich distillery on Isle of Islay lists creeping thistle as one of the 22 botanical forages used in their gin, The Botanist.[50]
The feathery pappus is also used by the Cherokee to fletch blowgun darts.[51]
Cirsium arvense is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa and widely introduced elsewhere. The standard English name in its native area is creeping thistle. It is also commonly known as Canada thistle and field thistle.
The plant is beneficial for pollinators that rely on nectar. It also was a top producer of nectar sugar in a 2016 study in Britain, with a second-place ranking due to a production per floral unit of (2609±239 μg).
