Appearance: Biennial or short-lived perennial herbaceous plant, 2 - 3' high. Basal leaves form a rosette the first year from which grow 1- 20 wiry, hoary, branched stems during the second year.
Leaves: Alternate, grayish, hoary, and divided into lance-shaped lobes decreasing in size at the top.
Flowers: Thistle-like pink to purple flowers sit at the tips of terminal and axillary stems, bloom from July through September.
Seeds: Brownish, 1/4" long with small tuft of bristles, dispersed by rodents, livestock and commercial hay. Seed viable in the soil for 7 years.
Roots: Stout taproot. Lateral shoots form new rosettes near the parent plant.
Caution: Wear long sleeves and gloves, can be a skin irritant to some people.
- Especially threatens dry prairie, oak and pine barrens, dunes and sandy ridges.
- Spotted knapweed is poisonous to other plants (phytotoxic).
- Spreads rapidly in artificial corridors, gravel pits, agricultural field margins and overgrazed pastures.
- A native of Europe and Asia it has become a serious problem in pastures and rangeland of the western states.
- Spotted knapweed is a MDA Prohibited noxious weed (Control List) in Minnesota.
- Early detection and pulling
- Mowing as needed so plants cannot go to seed
- Prescribed burning, only very hot burns are effective which may also damage native plants
- Apply selective herbicide clopyralid during bud growth in early June for best results (48 oz per 100 gal water). Use caution in quality natural areas herbicide affects native plants of the sunflower and pea family as well.
- Seedhead weevils, root-boring weevils, and seedhead flies are commonly used
- Biological Control of Invasive Plants in Minnesota brochure
- Minnesota Noxious Weeds (MN DOT)
- Meet the Knapweeds brochure
- US Forest Service Fact Sheet
- Video: Spotted Knapweed Identification (University of Wisconsin)
- Video: Little things, big problems: Spotted Knapweed (National Park Service)
- MISIN spotted knapweed identification training module
- Watch Out for Knapweed (Montana State University)
- NRCS Management of spotted knapweed