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Canada thistle flowers. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org -
Canada thistle flower. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org -
Canada thistle leaves. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org -
Canada thistle infestation. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org
Also known as: Californian thistle, Canadian thistle, creeping thistle, corn thistle, field thistle
Chances are you’ve seen Canada thistle. This tall, prickly perennial is the most common thistle in the United States. Its extensive root system spreads quickly and can overtake large areas of pastures or fields in just one growing season.
Description:
| Life Cycle: | Perennial (life cycle lasts more than one year) |
| Early Detection and Rapid Response species: | No |
| Height: | 5 feet (1.5 meters) |
| Leaf Description: | Lanced-shaped leaves grow alternately along the stem. The edges of the leaves are lobed and have yellow spines at their tips. |
| Fruit & Flower Description: | Purple or white flowers grow in clusters at the ends of each stem. There are tiny bractsSmall leaf-like structures that are just below a plant’s flower. under each flowerhead that have weakly spined tips. |
| Bloom Time: | June to October |
Habitat:
- Canada thistle is common in pastures and croplands. It also can be found in natural areas such as prairies and along stream banks.
- It typically grows in disturbed areas with abundance sun and moist but not wet soils.
Impact:
- Canada thistle spreads quickly by its extensive root system. In one season it can spread across a 10 – 12 foot (3 – 3.5 meter) area.
- It poses an economic threat to the agriculture industry by reducing crop yields.
What you can do about it:
- Canada thistle is difficult to control once it is established on a site. A combination of treatments may be needed to control dense stands.
- Repeated mowing to weaken the stems and prevent seed from spreading is effective in smaller infestations.
Look-alikes:
Canada thistle looks similar to other invasive thistles, including bull thistle (Cirsium arvense). Canada thistle’s flowerheads are smaller and the spines at the tip of its leaves are less pronounced than bull thistle’s.
Noxious Weed Listing:
| State of Oregon: | Class B |
| State of Washington: | Class C |
More Resources:
- Pacific Northwest Weed Management Handbook: Canada thistle
- University of California, Weed Research & Information Center: Canada thistle