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I- <br />rrkV <br />con <br />ca <br />cd <br />cd <br />Updated on: <br />08/08 <br />Colorado Dept. of <br />Agriculture <br />Conservation <br />Services Division <br />700 Kipling Street, <br />Suite 4000 <br />Lakewood, CO <br />80215 <br />303 - 239 -4100 <br />'R <br />Rangeland, pasture, and riparian site recommendations <br />Canada thistle Identification <br />and Management <br />stage typically occurs in the early summer. Seed <br />production will follow and effective management <br />options will then become limited. <br />Identification and <br />Impacts <br />Canada thistle (Cirsium arvense) <br />is a non - native deep- rooted <br />perennial that spreads by seeds and <br />aggressive, creeping, horizontal roots <br />(rhizomes). Canada thistle can grow <br />2 to 4 feet in height. The leaves are <br />oblong, spiny, bright green in color, <br />and are only slightly hairy on the <br />undersurface. Flowers occur in small <br />clusters that form on the ends of <br />branches. They are about 1 cm in <br />diameter, tubular shaped, and vary <br />from white to purple in color with a <br />strong vanilla scent (female flowers). <br />Canada thistle emerges from its <br />root system from late April <br />through May. It begins to flower <br />in late spring to early summer with <br />increase in day length. Canada <br />thistle only produces about 1,000 <br />to 1,500 seeds per plant. Typically, <br />it reproduces vegetatively through a <br />creeping root system, and can quickly <br />form dense stands. Every piece of <br />root, from 1/2 to 1 inch in length, is <br />capable of forming new plants. The <br />key to controlling Canada thistle is <br />to eliminate seed production and also <br />to reduce the plant's nutrient reserves <br />in its root system through persistent, <br />long -term management. <br />Canada thistle is one of the most <br />feared noxious weeds in the U.S. <br />as it can infest many land types, from <br />roadsides, ditch banks, riparian zones, <br />pastures, irrigated cropland, to the <br />most productive dryland cropland. <br />Forage production is severely reduced <br />because cattle will not graze near <br />infestations. <br />The key to effective control of <br />Canada thistle is combining <br />control methods. These weeds <br />need to be continually stressed, <br />forcing it to exhaust root nutrient <br />stores and eventually die. Of all <br />control methods, prevention is most <br />important. Maintain healthy pastures <br />and rangeland and continually monitor <br />your property for new infestations. <br />Details on the back of this sheet can <br />help to create a management plan <br />compatible with your site ecology. <br />Canada thistle is designated <br />as a "List B" species on the <br />Colorado Noxious Weed Act. It <br />is required to be either eradicated, <br />contained, or suppressed depending <br />on the local infestations. For more <br />information visit www.colorado. <br />gov/ag/csd and click on the Noxious <br />Weed Program link or call the <br />State Weed Coordinator at the <br />Colorado Department of Agriculture, <br />Conservation Services Division, 303- <br />239 -4100. <br />All photos © Kelly Uhing, Infestation map <br />above, Crystal Andrews, Colorado Department of <br />Agriculture. <br />v <br />ro <br />