Invasive Species: Canada Thistle

Cirsium arvense

This thistle has been accidentally introduced many times all the way back to the 1600s. It is the most widespread thistle in the United States.

Why is it important to control?

Canada thistle is a perennial with a large, deep root system. Its seeds spread easily. This weed degrades fields and pastures and can dominate areas.

Green vigorously growing Canada thistle

How can you identify it?

A close up photo of a Canada thistle flower with two bees.Canada thistle seedheads
Habitat: Pastures, cropland, meadow, roadsides
Life Cycle: Perennial
Height: Up to 5 feet
Leaf Description: Lance-shaped with yellow-spined lobes
Fruit & Flower Description: Pink to purple flowers; tiny bracts on flower bulb
Bloom Time: June to October

How do you control it?

  • Hand-pulling works for small infestations. Larger infestations will have established rhizomes.
  • Dispose of all plant material in a tied bag and throw away in trash.
  • Herbicide control:  Oregon State University Extension recommends a triclopyr or glyphosate based herbicide. Take care to avoid drift and be cautious near crops. FOLLOW ALL LABEL DIRECTIONS!

  • The Oregon Department of Agriculture has approved and implemented four biocontrols, a seed head weevil, a stem weevil, a crown weevil, and a stem gall fly. This has had a significant impact on populations, but it needs to be kept in mind that biocontrols are not focused on eradication.

How to Report it?

There are various ways you can report this noxious weed.

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