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Canada Thistle is a perennial that grows up to four feet tall. The leaves are alternate with irregular lobes and spins. Stems are slightly hairy and ridged. The small flower heads are clustered, having up to five purple to white blooms in early June.

Native to temperate regions of Eurasia, thistle-contaminated seed originated in Canada and spread quickly. The plant thrives in upland disturbed areas but commonly can be found along roadsides, forest edges, fields, pastures, meadows, and wetlands.

Canada Thistle can reproduce either by seeds or through its extensive, creeping root system. Seeds, produced in late summer, can be dormant for up to 20 years. Due to its root system, the plant is highly invasive and can crowd out native plant life. It also competes with crops and may have chemical toxins that kill other plants.

Both mechanical and chemical methods of control can be effective. Cutting should be done in the early season to prevent the development of seeds. Since the whole plant needs to be killed to prevent new growth from the roots, chemical control should be used in the fall so the plant takes the herbicide into the root system. Sites must be monitored for many years because of the dormant seeds.
More Resources:

Element Stewardship Abstract
Forest Pests
U.S. Forest Service


Biocontrol of Invasive Plants
Plant Conservation Alliance
Plant Invaders of Mid-Atlantic Natural Areas


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