Spotted Knapweed
Description: Spotted knapweed is a biennial or short-lived perennial plant that grows from 1 to 4 feet tall. It reproduces by seed and has a thick taproot. Seedlings develop the first year into rosettes of narrow, deeply lobed leaves that are up to 6 inches long. The upper leaf surface is rough. Flowering plants produce one to many stems with numerous branches. Stem leaves are smaller and linear, arranged alternately along the stem. A single flowerhead is produced at the end of each branch. Bracts at the base of the flowerhead are black-tipped, which gives them a spotted appearance when viewed from a distance. The flowers are pink to light purple in color and mature into brown seeds tipped with a plume of soft tawny bristles.
Management Guidelines:
Type and Class of Livestock – Sheep and goats.
Grazing Objective – Graze to prevent seed production and reduce biomass.
Growth Stage for Treatment – Graze spotted knapweed heavily during the rosette or bolting stage. Livestock prefer young, smaller plants, but will usually readily consume it at all growth stages. Two grazing periods per year, once during rosette to bolting stage and again in the bud stage, provide the best control. Stem reductions, smaller plants, and lower seed production can occur after three to six consecutive years of grazing.
Potential Effectiveness – Sheep and goats readily graze spotted knapweed, considered to be moderately good forage for livestock. Sheep tend to strip leaves and avoid the fibrous stems of mature plants. Grazing can reduce plant vigor, density, size, flower stems, and seed production. It may be necessary to manage grazing based on degree of utilization of desirable species. Palatability may be reduced as the plant ages because of reduced forage value and the presence of a bitter-tasting compound called cnicin. Sheep digestive systems may suffer if diets are composed of more than 70% spotted knapweed. Grazing is most effective when combined with herbicide treatments.
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