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(IV-19) CANADA THISTLE (Cirsium arvense) Canada thistle emerges between May and June 1 in most areas of Colorado and grows rapidly after getting started. Because of its seeding habit, vigorous growth and extei...~ve horizontal and vertical underground toot system, control or • ~ eradication is difficult. New plants may sprout from deeply buried root pieces. .. When the roots are cut into pieces, each-piece may develop into a new plant. DESCRIPTION: The flowers of Canada thistle are small; heads are about 3j4" or less in diameter and are light pink to rose purple in color. The leaves are dark green and very crinkly. Sharp spines are numerous on the outer edges of the leaves and on the branches and main stem of the plant. Canada thistle has an extensive, underground root system. It is sometimes confused with some of the common pasture thistles. The common thistles have a white cottony material on the leaves and stems; Canada thistle is usually dart: green, but some varieties may be grayish-green. CONTROL: Canada thistle is difficult to control once it becomes well established. Competitive crops, such as alfalfa, winter wheat, and seeded grass pastures, will compete with Canada thistle, but will probably not eliminate it. Increasing seeding rates by one-half and fertilizing the crops at heavier than usual rates will give thistles greater competition. Planting contaminated geed has spread this and other weeds to a high degree in Colorado. Always use seeds free of noxious weeds. One season of intensive cultivation from spring until freeze-up will usually eliminate over 90% of Canada thistle. Cultivate every 14 to 21 days; use a. snarp duckfoot cultivator with at least 3" overlap of shovels or a blade-type implement. Cut thistle plants no more than 4" below the surface. Persistence and proper tim- ing are important. Three-fourths to 1 pound of 2,4-D per acre should be used to control this weed in small grains. Spray when thistles are at the early bud stage of growth. In corn, use 1/2 to 3/4 pound of 2,4-D amine per acre. Treat before corn is 12" tall. Plow and cultivate 2 or 3 times after harvest. This process repeated for three years will kill a high percentage of thistles. Plowing after harvest and spraying thistles that emerge after plowing is about as effective as cultivation. In small grains underseeded to alfalfa, the goal is to prevent seed forma- tion and establish a dense legume stand. Periodic cutting after the alfalfa is established will control Canada thistle. If the field is heavily infested, one year of cultivation prior to seeding the legume will help to insure a stand. In fields of lighter infestation, mow prior to thistle bloom and use as a hay crop. If the infestation is in spots, burn now, or spot spray. One spraying, either during the summer or after harvest, will usually not kill thistles - they will come back. Eradication depends largely on follow-up and persistence. Fence rows, ditches and other waste areas should be sprayed with 2,4-D at 2 to 3 lbs a. i. per acre, in the spring, or early summer when thistles are in an early bud stage of growth, and again in the fall when regrowth appears. (3/76)