Canada thistle is a native of Eurasia but was introduced to North America in the late 18th century. A perennial with extensive underground creeping roots – which makes this weed a challenge to control. Widespread in many landscapes and the infestations often form dense patches.
Did you know?
1. It is not a native to Canada.
2. One of the worst perennial weeds across North America.
3. Plants are either male or female, but only female flowers can produce seed.
Identification
Rosette Leaves:Deep horizontal and horizontal creeping rootstocks which are the primary method of local spread.
Cotyledon: Only true seedlings from seed will have cotyledons; oblong to oval; rough; distinctly veined, fleshy, with a rounded top. Re-emerging seedling from rootstocks will not have cotyledons.
Leaves: Generally dark green, alternate, smooth, and irregularly lobed with no petiole. The leaf margins have spiny teeth. Leaves decrease in size moving upwards on stem.
Stem: Erect, hairless, and hollow in cross-section. Plants can grow up to 1.2 m. in height.
Flowers: Purple, pinkish or white flowers about 1 cm in diameter, arranged in clusters and smaller than others in the thistle family.
Seeds: An achene dull yellow to brown 2.5 - 4.0 mm in length by 1.0 mm width. Each seed has a feathery pappus to disperse via wind. Dispersed primarily by wind, seeds can also be dispersed by water, animals, clothing, equipment, and vehicles.
Distinguishing Features
A highly variable species, especially in its leaf characteristics with four distinct groups: 1) Spiny Canada thistle, 2) Entire-leaved Canada thistle, 3) Canada thistle, and 4) Woolly Canada thistle.
Legislation
It is considered a noxious weed in Ontario, Quebec, British Columbia, Saskatchewan and Alberta while, in Manitoba it is a designated tier 3 noxious weed. The seed is a Primary Noxious, Class 2 in the Canadian Weed Seeds Order, 2016 under the Seeds Act.
Want to learn more about other problem weeds?
Check out our Nasty Weeds (Part 1) course online.
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