Scientific name: Cirsium arvense

Common name: Canada Thistle Updated 07/2002

A. PRIORITY #4

B. DESCRIPTION Canada thistle does best in disturbed areas. It sometimes occurs in wet areas where water levels fluctuate. It can invade sedge meadows and wet prairies from adjacent disturbed sites. This thistle does not do well in undisturbed prairies, good to excellent pastures, or in woodland. Plants are tall and lax, with few flowers, on sites that are shaded most of the day. This dioecious, weedy perennial occurs in patches. Introduction to new areas occurs mostly by windborn seed or sometimes by run-off in ditches. It spreads rapidly by rhizomes or root segments. Lateral roots 3 or more feet deep spread from a fibrous taproot. Aerial shoots are sent up at 2 to 6 inch intervals. Basal leaves are produced the first year, flowering stems the next. Pollination is mostly by honeybees, and wind pollination is limited. Most seeds germinate within one year. Some seeds immediately produce rosettes before winter and emerge to flower the next spring. Seeds remain viable in soil up to 20 years in some cases. Emergence occurs in early May, with bolting in mid-to-late June. As frequency of Canada thistle increases at a site, species diversity decreases, possibly due to allelopathic substances.

C. CURRENT DISTRIBUTION ON THE SITE SEE APPENDIX 2, MAP #4

D. DAMAGE & THREATS Canada thistle is an alien species capable of crowding out and replacing native grasses and forbs. It is detrimental to natural areas where it occurs, particularly non-forested communities, and it can change the natural structure and species composition where it becomes well-established. Prairies, barrens, savannas, and glades are susceptible, particularly those sites that have been disturbed and are reverting naturally to native species, as well as those undergoing manipulative restoration management. E. GOALS The major goal the MNWAG wants to accomplish is to lessen the economic impact caused by canada thistle by eradicating small stands and reducing, then controlling, larger stands.

F. OBJECTIVES (Measurable) (Establish measurable objectives for the planned control activities. Include:

1. the impact on numbers, density, cover, etc. that you want to achieve;

2. the size of the area in which you hope to achieve this;

3. the period in which you hope to achieve it. For example you may state your objectives in terms of reducing percent cover of the species by 50% over an area of 5 acres within 3 years. Another possible objective would be eliminating the species from the site within 2 years.)

G. MANAGEMENT OPTIONS Viable control options are:

(1) CHEMICAL – Spot application of the amine formulation of 2,4-D according to label instructions can control this plant. Individual plants of Canada thistle should be treated with a wick applicator or hand sprayer. The herbicide 2,4-D amine is selective for broadleaf plants. To reduce vapor drift, use an amine formulation of 2,4-D rather than an ester formulation. Precautions should be taken to avoid contacting non-target plants with the solution. Do not spray so heavily that herbicide drips off the target species. (Recommended in natural communities of high quality) A foliar application of a 1-2% solution of Roundup applied in spring when plants are 6-10 inches tall is an effective herbicide treatment. Individual plants should be spot-treated with a wick applicator. Roundup normally kills the entire plant, including the roots, when applied in this manner. Roundup is a nonselective herbicide and precautions should be taken to avoid contacting non-target plants with the solution. (Recommended on lands other than high quality natural areas)

(2)PRESCRIBED BURNING AND HERBICIDING – Prescribed fire can be effective in controlling this species and is a preferred treatment. Late spring burns, between May and June, are most detrimental to this noxious weed and should be used when possible. Prescribed burns to control this plant should not be conducted early in the spring, as early spring burns can increase sprouting and reproduction of this species. During the first 3 years of control efforts, burns should be conducted annually.

(3) MECHANICAL – Repeated and frequent pulling or hand-cutting of individual plants will eventually starve underground stems. Cutting or pulling should be at least 3 times each season, in June, August, and September. This treatment is feasible for light and moderate infestations, but may be relatively time consuming in heavy infestations. On large sites with heavy infestations, thistles should be mowed when in full bloom, and as close to the ground as possible. Cut flower-heads should be removed to prevent scattering seeds on site. Repeated mowing may be needed for several years to obtain adequate control. On severely disturbed sites with heavy infestations, such as cropland or abandoned cropland, the site could be plowed and sowed to a cover crop (wheat, alfalfa, rye), if practical and desirable. The following May, the cover crop should be plowed under and desired native species should be seeded.

H. ACTIONS PLANNED (Treatments and monitoring) (Briefly describe the locations to be treated, materials and methods to be used, and an approximate schedule for control and monitoring activities. If several methods are to be tested, outline the design of the planned experiment or demonstration.)

I. HOW ACTIONS WILL BE EVALUATED (Criteria for success) SEE APPENDIX 4: Examples of data collection forms.

J. RESOURCE NEEDS SEE APPENDIX 3: Weed Management Plan Tables

K. RESULTS OF EVALUATION (This section is to be filled in later, preferably within 1 year, when monitoring data has been taken and evaluated, at least preliminarily. The evaluation should be used to determine whether any of the sections B-K above should be modified.)