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Canada thistle (Cirsium arvense) was introduced from Europe as a contaminant in crop seed. It spreads primarily through its creeping root system, which can spread up to 4.5 meters horizontally and up to 6 meters vertically!
Stems are hollow and woody, covered in sharp spines and grow 50 to 150 cm tall.
Leaves are alternately arranged, lance-shaped and dark green. Depending on the plant, leaves vary from smooth-edged and spineless to the more common irregularly lobed studded with sharp spines.
Canada thistle flowers form in clusters at the end of stems and vary from light lavender to dark purple in color. Seeds are borne only on female plants and have a 2% successful germination rate. Buried seeds may stay dormant for up to 20 years.
Due to its extensive root system, Canada thistle can thrive in a wide variety of soil types, and does particularly well under drought conditions that would stress turfgrass and other desirable plants. It thrives in disturbed areas (recently landscaped yards) or short lawns where there is little competing vegetation present. However, it does not tolerate waterlogged soil or complete shade.
Canada thistle is listed on the Weed Control Act as a noxious weed that must be controlled wherever found. Most livestock find Canada thistle unpalatable due to its sharp spines. It can spread rapidly throughout pasture and hay land, rendering it unusable for forage. Riparian infestations reduce food, cover and nesting sites for waterfowl. In urban areas, it thrives in disturbed areas and quickly invades lawns and park areas, giving children and pets an unpleasant surprise should they come into contact with it.
The best measure to prevent Canada thistle invasion is to develop and maintain healthy desirable plant cover, such as thick turfgrass in yards and parks. Canada thistle seedlings are very shade intolerant and will have difficulty establishing in low light conditions. Homeowners should leave their lawns 4 inches (10 cm) tall to help grass compete against weeds.
Repeated mowing will gradually deplete food reserves in the roots, resulting in eventual starvation and death of the plant. In home yard situations, it is best to hand pull seedlings as they sprout to allow the surrounding grass to thrive and outcompete the weeds.
A variety of herbicides are available to help suppress Canada thistle in lawns. Consult a local landscape company or garden centre for recommendations. The best time to control thistle chemically is in the fall, when the plant will pull the herbicide deep into its roots along with its winter food reserves to maximize kill.
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