Herbaceous, short-lived perennial in Wisconsin, acting as a biennial elsewhere, 2–4’ tall. Persists as a rosette for 1–4 years before bolting. Flowering plants usually have 1–6 stems, but may have up to 20.
CAUTION: Can cause skin irritation in some people. Wear gloves, long sleeves, and pants when handling.
Legal classification in WI: Restricted.
Leaves: Gray-green, covered in rough hairs, and deeply divided. Rosette leaves grow up to 6” long. Stem leaves alternate, with lower stem leaves resembling rosette leaves, becoming small (1–3” long) and linear on the upper stem.
Flowers: Midsummer to fall. Thistle-like, pink to purple flower heads, rarely white. Flower heads are 0.3-0.6” in diameter and have stiff bracts tipped with black, fringed hairs.
Fruits and seeds: Seeds are 0.25”, brownish in color, and have a small tuft of bristles at one end. Roots: Perennial taproot. It can also produce a shallow mat of fibrous roots extending from plant for several feet.
Similar species: There are a number of other knapweeds (all non-native in Wisconsin) that look similar, but spotted knapweed is easily distinguished by the black tips of the bracts surrounding the flower.
• Invades dunes, sandy ridges, prairies, barrens, and roadsides.
• Its roots have been reported to exude allelopathic chemicals that inhibit the growth of other plants.
• It is not palatable as a forage plant and is typically avoided by both livestock and native grazers.
• Infestations cause increased soil erosion on sloped terrain, runoff and stream or lake sedimentation, and decreased water-holding capacity in soil.
For more information visit: http://dnr.wi.gov/topic/Invasives/fact/SpottedKnapweed.html
Photo released under creative commons license.