A long lived deep rooted perennial can grow to 5 feet but more commonly under 3 foot locally. Seedlings quickly produce an extensive root system and will form very dense clusters radiating from an original seedling. Spreads mainly from creeping root shoots however also produces wind borne seed. Seedlings are not particularly competitive. Leaves are wavy margined to lobed, up to 6 inches long, and armed with yellowish spines. Flower heads are urn shaped typical of the aster family and with lavender to pink flowers. Seeds are slender, tan, about 1/8 inch long, attached to a fine plume. One plant will easily produce over 5000 seeds per season with a high percentage of them viable.

Canada thistle is extremely difficult to control. There are several approved insect controls, however none have been very effective yet. Mechanical or hand pulling has not proven an effective control due to regeneration from root fragments. Also, due to the extensive root system herbicide control is difficult and usually works best in conjunction with other control measures. Fall applications of certain chemicals can be the most effective herbicide treatments, generally after a killing frost. Any herbicides that contain Clopyralid, Aminopyralid or Picloram are highly effective but should be used cautiously. Caution, these chemistries can have a very long residual effect.
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Modified from Selected Noxious Weeds of Eastern Oregon, Gary Page, Malheur County Weed Inspector.