Cirsium arvense var. vestitum Wimmer & Grab.

Canada thistle was introduced to the United States, probably by accident, in the early 1600s and, by 1954, had been declared a noxious weed in forty three states. In Canada and the U.S., it is considered one of the most tenacious and economically important agricultural weeds, but only in recent years has it been recognized as a problem in natural areas.

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More info for the terms: cover, eruption, forbs, marsh, natural, shrub, tree

Canada thistle is an early successional species that emerges from seed or root fragments shortly after disturbance. It grows best in open sunny sites, though may be somewhat tolerant of shade (see "Site Characteristics"). Canada thistle may establish in natural areas as part of the initial plant community after logging [106,109,160,250], fire [16,80,138,163,188,193,242], volcanic eruption (debris deposit, landslide) [2,45,216,217], grazing [143], and road building [140]. Canada thistle and other introduced species are taking over large tracts of logged, burned, or otherwise disturbed land in British Columbia [220]. In northern Idaho, Canada thistle establishes following clearcutting with soil displacement. With low soil displacement, the plant community follows a successional sequence that favors the eventual establishment of tree and shrub species, but with heavy soil displacement, a persistent forb-rich community, including Canada thistle, develops with few tree species present, and very little species replacement over time [106]. Canada thistle may not establish immediately after logging and fire disturbances, but may be delayed for 2 or more seasons [3,56,164,242]. Canada thistle was among the 3 most common species to survive a debris deposit created by the 1980 eruption of Mount St. Helens, where it sprouted from transported root fragments, and from seed [2,45,216]. Canada thistle is also found among the emergent vegetation after drawdown in the Delta Marsh, Manitoba [142,229]. In a study comparing possible control methods for perennial pepperweed, Canada thistle established, along with cheatgrass, after disking and herbicide treatments that reduced cover of native forbs and grasses [112].

Canada thistle was accidentally introduced to North America in the 1600s and is designated as a noxious weed in 43 states.

More info for the terms: adventitious, competition, ecotype, marsh, natural, pappus, presence

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].