Canada thistle (Cirsium arvense) sometimes called field thistle or creeping thistle, is an erect perennial weed that is typically found in open areas, pastures, roadsides, field borders, and low-lying areas. It is an aggressive perennial weed capable of growing to 2 to 5 feet in height. It can spread quickly due to its fibrous creeping, taproot system that frequently produces new shoots, enabling it to invade new territory and helping it outcompete other vegetation.
Identification of Canada thistle is important for management. It has alternate, long, wide, lobed leaves with crinkled margins and spines on the outer lobe points (Figure 1). It is most easily recognized by its inflorescences, which appear as numerous heads on the plants, with male and female inflorescences on separate plants. They are typically round and compact with terminal clusters of lavender or rose-purple disk flowers (Figure 2). In mid-July, the flowers change to seedheads with obvious white fluffy tops, which help to disperse the seeds.
Control of this weed can be difficult and requires a sound management plan. With this plant reproducing via horizontal roots and through prolific seed production, efforts must center on eliminating root growth and stopping the spread of seed. Fall and early-spring herbicide applications, combined with in-season mowing, are the most effective ways to manage this troublesome weed.
While mowing in-season can be effective, mowing alone requires frequent re-treatment, as Canada thistle will continue to reflower after mowing. Repeated mowing must be made to gain effective control.
Herbicide selection and application timing is of critical importance when managing Canada thistle. Using a systemic herbicide that can be translocated in the phloem of the plant is critical when trying to control perennial weed species. Herbicides such as dicamba, 2,4-D, clopyralid, and aminopyralid are optimal choices, as they carry activity only on broadleaf weeds and will not affect desirable grass species. These herbicides are readily translocated throughout the plant and provide systemic control.
Management with herbicide is centered around spray applications in both the early-spring and in the fall. Application timing is critical as each application helps control a different part of the plant. With a spring herbicide application, the herbicide is translocated to the new growth and to the flowering tissue, as the plant is prioritizing flowering and vegetative growth at this stage. This application can help reduce top-growth. With a fall herbicide application the herbicide is translocated to the root system, where it can cause the most damage to Canada thistle, as it focuses on replenishing root reserves prior to winter so it can resume upward growth again in the spring.
There is no simple method to effectively control Canada thistle, and acknowledging that a multi-step approach to control this weed can help achieve success from the start.
Canada thistle. Minnesota Department of Agriculture. https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/canadathistle
Bryson, C.T. and DeFelice, M.S. 2010. Weeds of the midwestern United States and central Canada The University of Georgia Press, Athens, Georgia, 30602.
Web sources verified 7/27/22
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