Invasive Plant Species Control
Spotted Knapweed
(Centaurea maculosa)
Native Substitutes
- Blue giant hyssop (Agastache foeniculum)
- Aromatic aster (Aster oblongifolius)
- Canada tick trefoil (Desmodium canadense)
- Wild bergamot (Monarda fistulosa)
- Purple prairie clover (Dalea purpurea)
Description
Appearance: Biennial or short-lived perennial forb, 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 lanceolate lobes, decreasing in size at the top.
Flower: Thistle-like pink to purple flowers sit at the tips of terminal and axillary stems; bloom from July through September.
Fruit: Clusters of black 1/4" fruit ripen on female plants in August and September.
Seeds: Brownish, 1/4" long with small tuft of bristles; dispersal mostly stationary, over distance by rodents, livestock and commercial hay. Viable in the soil for 7 years.
Root: Stout taproot.
Ecological Threat
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 rangelands of the western states.
Control Methods
| Mechanical | Chemical | Biological |
|---|---|---|
| Mowing may help control; best to mow at peak flowering, and remove flowerheads from site or seed will develop from discarded flowers | Effective means of control with selective herbicides, such as clopyralid, but caution in quality natural areas as it targets native forbs of the daisy and pea families as well | Thirteen insects identified |
| Early detection and pulling; wear gloves as can be irritating to some and scratches may fester | Two seedhead flies are most promising | |
| Prescribed burning; only very hot burns are effective which may also damage native plants |
Sources
Wisconsin Manual of Control Recommendations for Ecologically Invasive Plants, 1997
Minnesota Department of Natural Resources, Restore Your Shore CD, 2001
