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Spotted Knapweed

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Spotted Knapweed Spotted Knapweed

  • Why should we care?
    • This plant is considered a noxious and dangerous plant to agriculture and especially cattle producers because it spreads fast and releases allelopathic toxins into the soil that prevent the growth of neighboring plant species. This gives Spotted Knapweed the advantage to dominate areas and reduce diversity of plant species as well as forage potential for wildlife and livestock. When Spotted Knapweed infestation increases in an area so does surface runoff and sedimentation. Spotted Knapweed has deep taproots that eliminate the network of native root systems resulting in a decrease of the soil’s water holding capacity.  
  • How to Identify
    • This invasive weed thrives in sunny habitats with well-drained or gravelly soils. Spotted Knapweed is a biennial or a short lived perennial that grows 1 to 4 feet tall. A first year plant will grow a basal rosette and in the second year will bolt wiry branched stems. The leaves are alternately arranged and have a pale grayish green color. Deep narrow lobes divide the basal leaves. Stem leaves progressively get smaller from bottom to top of stem. Flowers are pinkish or purplish in color and occur singly on tips of branching stems. The black spotted tips on the bracts is what gives this weed its name. This invasive weed blooms from late June through September with individual flowers blooming for 2 to 6 days.   
  • About the Plant and Where it came from
    • Centaurea maculosa
    • Spotted Knapweed is in the Asteraceae family or the Composite family. This plant is legally considered a noxious weed in fifteen states including Minnesota. This invasive weed, native to Eurasia, was accidentally introduced to North America in the 1890’s in contaminated hay or alfalfa seed. In recent years, Spotted Knapweed has become a wide spread invasive weed in the Midwest
    • Seed production is this plans sole means of reproduction. One plant can produce 500 to 2000 seeds and are mainly dispersed by wind, wildlife, and humans. Seeds germinate throughout the growing seasons and are viable for up to nine years. This makes it hard to control this invasive weed without multiple treatment procedures throughout years until the seed bank is depleted.   
  • How to Control
    • Cultural Control: Early detection is critical. Mowers and Hay equipment should be cleaned before moving to another site. Annual burns, mowing, plowing, and grazing by sheep and goats can be helpful. Burns should be done in the summer during the growing season but are often only effective when the burn is intense which can be damaging to other native plants. Mowing and plowing before seed production is a control method that helps reduce seed dispersal. It is important not to mow or plow after seed production because it will disperse seed.
    • Biological Controls: The most effective insects are the seed-head flies and a moth whose larva feeds on the roots. Infestations need to be at least two acres to support insect populations.
    • Chemical Control: Apply clopyralid (Transline and Pyramid), which is fairly effective and is more selective than other herbicides. Dicamba and clopyralid incorporated with 2, 4-D is also recommended along with triclopyr (Garlon 3A and Triclopyr 3 SL) and clopyralid-monoea salt. Aminopryalid (Milestone) is used in hay fields and grazing areas. It is vital to repeat treatments for several years to deplete the seed bank. When treating rosettes use 2, 3-D water soluble amine formulation in the fall and early spring. Glyphosate (Round-Up) has had mixed results and may not be as effective as other chemicals.
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