Canada Thistle

Cirsium arvense

Class C Noxious Weed

General

Native to southeastern Europe and the eastern Mediterranean.
Once established it spreads quickly replacing native plants and crops.
Poses an economic threat to the agriculture industry; the weed is an effective competitor for light, moisture and nutrients and is able to reduce crop yields.
May be confused with bull thistle (Cirsium vulgare), a class C noxious weed, that is larger in size 3 to 7 feet tall, with larger flowers 1.5 to 2 inches, and grows as a single upright branched stem.

Identification

Rhizomatous, herbaceous perennial that grows 1 to 4 feet tall. Complex system of roots spreading horizontally which give rise to aerial shoots.
Leaves are alternately arranged, lance shaped to narrowly oval. Leaf edges toothed or deeply lobed with stiff yellowish spines. Upper leaf surface varies from hairy to no hairs, generally leaf undersides have hairs.
Numerous, slender, green, freely branched stems; begin as a rosette and grow upright after 2 to 4 weeks. Stems can be slightly hairy or without hairs.
Flower heads many, in clusters at tips of branched stems. Flowers are purple to pink, sometimes white, ½ to ¾ inches in diameter. Plants are dioecious, meaning all flowers on a plant are either male or female. Bloom June to October.
Fruits about 1/8 inch long, somewhat flattened, brownish with an apical circle of long hairs or bristles.

Photos

Habitat

Common in pastures, crop and rangelands. It invades natural areas such as prairies and dunes that have disturbance. Also found along forest margins and stream banks.
Canada thistle grows poorly in shaded conditions producing few flowers; it also does poorly in wet soils without much aeration.

Reproduction and Spread

Reproduces by seed and vegetatively by creeping horizontal roots.
Average seed production is about 1,530 seeds per year.
Seeds are dispersed by wind and water. Seeds can remain viable in the soil for over 20 years. Studies collecting fresh seeds found germination rates of up to 95%.
Plants have a fibrous taproot with deep, wide spreading horizontal roots. As roots travel horizontally they begin to arch towards the water table, at this point another horizontal root develops and continues the horizontal spread. Adventitious buds along the horizontal runners give rise to numerous aerial shoots. Root fragments as small as 8 mm in length can produce new shoots.
Shoots emerge in the spring, after 2-4 weeks the rosettes experience rapid vertical growth for several weeks. Growth slows and flowering commences in early summer and continues for several months. Seeds develop 8-10 days after flowers open.

Control

Difficult to control once established. The key principle is to reduce the root mass by stressing the plant and forcing it to use stored root nutrients.
For new infestations, carefully dig up young plants before they become established. Make sure to remove all mature seed heads so no new seeds are left on site.
Cut plants when they have the least root reserves and have formed tight green buds, usually by June. Cut the regrowth each time buds appear again.
Repeated mowing before flower buds open, can weaken stems, prevent seeding and decrease the number of plants. Mowing once a year is not effective. Combining fall herbicide treatment on regrowth after mowing has been effective.
Planting competitive native species, grasses and crops can be effective in reducing the infestation. Avoid over-grazing and mow thistles after grazing.

Chemical Control

The best way to control this weed is by applying effective, systemic herbicides. Most other methods of management do not give long-term control.
Herbicides containing the following active ingredients are effective against Canada thistle:
- Aminopyralid (Milestone)
  - Selective herbicide with residual soil activity.
  - Foliar application, apply a rate of 5-7 fluid oz per acre.
  - Apply in the spring to plants in the pre-bud stage or in the fall to plant regrowth. Good control up to full flower stage but decreases at the fuzz stage when plants are setting seed.
- Clopyralid (Transline)
  - Selective herbicide with residual soil activity.
  - Consult label. Rate depends on use site.
  - Apply to actively growing thistle before bud stage.
- Glyphosate
  - Non-selective herbicide, no soil activity.
  - Foliar application 2% solution.
  - Apply when thistles are actively growing but during or after the bud growth stage. Fall application must be before the first killing frost.
- Triclopyr (Garlon) or 2,4-D + Triclopyr (Crossbow)
  - Selective herbicide, no soil activity.
  - Foliar application, 1.5 – 2 % solution.
  - Apply at or just before bud stage.
  - Mixed results, timing of application is very important.