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Appendix 4: Profiles of Colorado State-Listed Noxious Weeds 161 <br />Canada thistle (Cirsium arvense) <br />=List B <br />Canada thistle <br />Cirsium arvense (L.) Scop.; Breea arvensis (L.) Lessing <br />Family: Asteraceae (Sunflower) <br />Other Names: field thistle, Californian thistle <br />USDA Code: CIAR4 <br />Legal Status: Colorado Noxious List B (top ten worst). <br />Identification <br />Growth form: Perennial forb. <br />Flower: Flower heads are white to purple and borne in clusters of 1-5 <br />per branch, with a strong vanilla scent. Heads aze only about 1 cm in <br />diameter. <br />SeedslFruit: One-seeded fruits (achenes) are straw or light brown in <br />color, straight or slightly curved (Moore 1975). <br />Leaves: Leaves are spiny, alternate, oblong or lance-shaped, with the <br />base leaves stalkless and clasping, or extended down along the stem. <br />Stems: Mature plants range from 2-4 feet in height. <br />Roots: Canada thistle has two types of roots, horizontal and vertical. <br />The horizontal roots produce numerous shoots, while vertical roots <br />store water and nutrients in their many small branches. <br />Seedling: Eazly spring growth appears as rosettes with spiny-tipped, <br />wavy leaves. <br />Other: The floral bracts of Canada thistle are spineless. <br />Similar Species <br />Exotics: Bull thistle (Cirsium vulgare); flower bracts are somewhat <br />tapered and covered with spines. Scotch thistle (Onopordum <br />acanthium); stems appear the have wings, floral bracts are covered <br />with spines. Plumeless thistle (Carduus acanthoides); floral bracts aze <br />covered with sharp spines. Musk thistle (Carduus nutans); floml <br />bracts aze broad with spiny tips. Russian knapweed and Canada thistle <br />are often confused. <br />Natives: Wavyleaf thistle (Cirsium undulatum): flower bracts often <br />have a prominent white glandulaz dorsal ridge (often sticky to touch) <br />and minutely hairy mazgins (Whitson et al. 1996). Leafy thistle <br />(Cirsium foliosum): the leaves surrounding the terminal flowers are pink to white. Yellowspine thistle (Cirsium <br />ochrocentrum): flower bracts aze covered with cobweb-like hairs and have a spreading yellow spine at the tip. The <br />tall biotype of Colorado thistle (Cirsium coloradense) and Cirsium traceyi aze also similaz. Most native thistles aze <br />more hairy and lightergreen/blue in color. Canada thistle is compazatively dazker green. <br />Impacts <br />Agricultural: Canada thistle is an aggressive, creeping, perennial weed. It infests crops, pastures, rangelands, <br />roadsides, and riparian areas (Beck 1996). <br />Ecological: Canada thistle spreads rapidly through horizontal roots, which give rise to shoots (Moore 1975). Its <br />root system can be extensive, growing horizontally as much as 18 feet in one season (Nuzzo 1998). Most Canada <br />thistle patches spread at a rate of 3-6 feet/yeaz, crowding out more desirable species and creating thistle <br />monocultures. <br />Human: Spiny thickets of Canada thistle can restrict recreational access to infested areas. <br />