Spotted knapweed reaches a height of 2 to 4 feet when fully mature. Rough, pale green leaves of 1 to 3 inches long alternate along the stem. The stems are erect and rough and have slender branches. Flowers are pink to light purple and the petals are surrounded by stiff, black-tipped bracts. Flowers produce brownish seeds of less than 1/4 inch long. Seeds are notched on one side of the base and have a short tuft of bristles at the tip.
Spotted knapweed reproduces mainly by seed. Each plant produces an average of about 1,000 seeds. Seeds can survive for up to seven years and will germinate throughout the growing season. Seedlings will develop into a rosette of leaves in the fall and resume growth in the spring.
Spotted knapweed was brought to the United States from Europe and Asia. It is an aggressive weed that was thought to only inhabit disturbed areas such as road ditches and railroad beds. Now spotted knapweed can be found in dry prairie sites, oak and pine barrens, and on lake dunes and sandy ridges. Several million acres of grazing land in the northwestern United States and Canada are infested.
Blooms: June through October
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Last updated: March 22, 2018