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Pacific Northwest Research Station
Blue Mountains National Resources Institute

Forestry and Range Sciences Laboratory
1401 Gekeler Lane
La Grande, OR 97850

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BMNRI Home > Publications > Weeds > Canada Thistle

Publications: Noxious Weeds

Explosion in Slow Motion: A talk by Jerry Asher about noxious weeds in the Blue Mountains

Click on a weed to learn about how to find it and kill it!

Common Crupina
Diffuse Knapweed
Russian Knapweed
Spotted Knapweed
Purple Loosestrife
Perennial Pepperweed
Tansy Ragwort
Medusahead Rye
Rush Skeletonweed
Yellow Starthistle
Canada Thistle
Musk Thistle
Scotch Thistle
Dalmatian Toadflax
Mediterranean Sage
St. Johnswort
Leafy Spurge
Dyers Woad

Wanted: DEAD!

Canada Thistle

Alias: Creeping thistle

  • Stems: 1 to 4 feet tall, erect, rigid, branching.
  • Leaves: Sessile, deeply lobed, edged with stiff yellow spines.
  • Flowers: Many 1/2-inch flowering heads; male and female flowers borne on different plants. Flowers purple, lavender, or white, with overlapping, weakly spine-tipped bracts. Flowers in July and August.
  • Seeds: 1/8-inch long, somewhat flattened, tan, with an apical circle of long hairs.

Canada thistle is a perennial that grows from a deep, complex root system. The roots spread horizontally and send up new stems, forming dense colonies. It grows in a wide range of soils and environmental conditions. It is aggressive, spreading by root and seed. The deep root system makes it difficult to control.

Control: An integrated system must be used for many years to control this pest.

  • Biological: Competition with grasses and alfalfa is effective, as are seedhead weevils, and the painted lady butterfly. A 3-year intensive cattle grazing program has proved effective in southwestern Colorado.
  • Chemical: Fall treatments, combined with mowing, are effective. Get in touch with you local weed control office for the latest information on controlling this pest.
  • Mechanical: Mowing can be effective if combined with herbicide application. Plowing increases plant numbers and should be avoided.

Report all sightings to your local Weed Board

US Forest Service - Pacific Northwest Research Station, Blue Mountains National Resources Institute
Last Modified: Monday, 16 December 2013 at 14:18:43 CST

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