With its dense, creeping roots that enable rapid spread and spiny thistles that discourage grazing, Canada thistle (Cirsium arvensis) can significantly decrease pasture productivity.
This troublesome perennial weed grows in distinct patches that emerge in early spring. Its underground roots form multiple new shoots each spring and summer, effectively spreading one plant over a large area. Further enhancing the weed’s spread is its prolific seed production, with one female plant having the ability to produce 50 to 100 flower heads, each with 80 to 90 seeds. These light, fluffy seeds easily float in the breeze or attach to wildlife. Cropland losses to Canada thistle can decimate yield. Controlling the weed on rangeland, pastures and other noncrop areas helps reduce the seed source.
Canada thistle is easily identified by its erect stems that reach up to 4 feet tall and its pinkish-purple colored flowers. With tremendous leaf variability, leaves range from light to dark green, are oblong or lance-shaped and often have spiny-toothed margins. The weed blooms in late spring with bristly clusters of flowers that are generally less than 1 inch in diameter. The plant’s smooth stems between the leaves distinguishes it from other common types of thistles.
Native to Europe, Canada thistle was first documented in North America in the late 1700s. Today, it is common throughout the Midwest and northern Plains, and it has been called one of the most troublesome perennial weeds in the United States.
For effective control of Canada thistle, apply 16 to 20 fluid ounces per acre of DuraCor® herbicide after the first buds form in late spring. This timing provides the best compromise between Canada thistle emergence and the stage of growth for older plants. Because root kill is critical to success against Canada thistle, fall to early winter applications of DuraCor made prior to the first hard frost can be especially effective. This treatment timing moves the herbicide to the plant’s massive root system right along with its winter food reserves.
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