The issue: Canada thistle

Canada thistle (Cirsium arvense) is a creeping-spreading perennial from Eurasia that grows 3 to 5 feet tall. It is most often found along ditch banks, crop fields, pastures, rights-of-way, home landscapes and open range. It competes poorly in shady environments. While moist soils are preferred, semi-droughty conditions are tolerated.

Canada thistle is dioecious — male flowers and female flowers that are on separate plants. The purple to almost white flowers are borne in clusters at the end of the stems. Some seeds stay attached to a fuzzy pappus, which help with wind dispersion, most drop to the ground. Seedlings become perennial plants six to eight weeks after germination.

The glossy leaves have stiff spines and no petiole. There are few spines along the stem.

Canada thistle has deep roots and rhizomes (underground stems). New plants will often rise from the rhizomes near the surface. Rhizome fragments can generate new plants.

It is important to keep Canada thistle from going to seed while employing practices to kill the roots.

Integrated pest management options:

· Prevention: Feed weed-free hay; keep ditch banks free of seed-producing plants.

· Mechanical: Repeated tillage 21 days after leaf emergence throughout the growing season for three to five years; hand-pull or cultivate seedlings within six weeks of emergence; plastic mulch for three to five years; solarization for seedlings.

· Cultural: Crops that provide early spring shade, or recover quickly from mowing help reduce Canada thistle stands; manage pastures for good grass health.

· Biological: Some are available.

· Chemical: Several broadleaf herbicides are effective. Be sure the target weed and crop, or landscape situations are listed on the product label. Always read and follow herbicide label directions.

Combine them:

Canada thistle seed can remain viable in field conditions up to 22 years. A longterm monitoring program is essential.

For more information, contact Ron Patterson, University of Idaho Extension Horticulture/Agriculture Educator in Bonneville County, at 208-529-1390 or