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Canada Thistle Cirsium arvense also called Creeping Thistle, is a noxious weed found throughout the United States and Canada.
It spreads throughout lawns and landscape beds by an expanding network of roots. These roots sprout new plants. A very persistent and hard to control weed once established. Featuring sharp spines that make it very unpleasant in lawns.
This noxious weed will grow and thrive in many inhospitable areas - however it prefers moist, loam soils and full sun.
Flowers - Small lavender, rose-purple, sometimes white flowers are 1/4 to 3/4 inch wide, in groups of 2 to 5 at the end of stems. Flowers appear in June through October.
Roots - The vigorous roots will grow 3 feet wide and 3 feet deep. The roots survive the winter (overwinter) and produce new plants in the spring.
Stems - Branching stems grow 2 to 4 feet tall and are spineless. Stems are slightly hairy and ridged.
Leaves - are attached alternate along the stem. They are oblong to lance-shaped with edges that are irregularly lobed with spiny margins. Dark green on top, light green and hairy beneath.
Fruits and seeds - Mature seeds are brown, 3/16 inch long, and similar in shape to a chile pepper. A plume of tannish silky hairs is attached to one end.
Since Canada thistle reproduces by seeds and spreads through its creeping roots - the best way to prevent an infestation is to keep your lawn healthy, dense and vigorous.
Hand pulling or mowing does not work because it will grow again from it roots.
Chemical control - The most effective way to kill perennial thistle is with a systemic herbicide that kills both roots and shoots. Herbicides are most effective when weeds are actively growing and air temperatures are between 60° F and 85° F.
The best time to spray with a systemic herbicide - According to Purdue University Department of Botany and Plant Pathology - is in early June after the first flower buds are formed and before the first flowers open and/or on fall regrowth during September and early October.
Glyphosate (Roundup) is the best chemical to use on Canada thistle, however it is not selective and will kill grass plants as well as weeds. Glyphosate can be used in landscape beds and will not harm nearby plants as long as the spray does not contact the plants you don't want to kill.
To make Glyphosate even more effective, use a spreader/sticker in your spray mix. Crushing or mowing leaves also helps break the waxy leaf layer allowing the chemical to penetrate into the leaves. Another option, use a paint brush to "paint" straight Roundup - not mixed with water - directly onto the thistle leaves.
Broadleaf herbicides are most effective when plants are young and actively growing. Mowing to generate growth before spraying works well. Herbicides containing triclopyr or dicamba (Trimec products) are more effective than those commonly used containing 2,4-D or MCPP.