Spotted Knapweed (Centurea biebersteinii syn. Centurea maculosa)

DESCRIPTION: Spotted knapweed is a biennial or short-lived perennial forb of the composite family. It commonly grows to 3-4 feet in height. The leaf margins on lower leaves are indented or divided about halfway to the midrib and the leaf surface is rough; upper leaves are more linear in shape. The alternate, pale leaves grow from 1-3 inches in length. Stem height varies from two feet on upland sites to four feet on wetter sites. The majority of stem growth occurs in June. The slender, hairy stems grow in an erect and branched arrangement. Single thistle-like, pinkish-purple flower heads reach 3/4 inch in diameter and occur at the tips of terminal or axillary stems from late June through August. Each flower head has stiff bracts marked with fine, vertical streaks and tipped with dark, comb-like fringes that give the flower head a spotted appearance. Seeds are brownish in color, under 1/4 inch in length, notched on one side of the base, and have a short tuft of bristles at the tip, enabling wind dispersal. Knapweed has a stout, elongated taproot.

Spotted Knapweed (c) John M. Randall/The Nature Conservancy

Species similar to spotted knapweed include the following:

• Russian knapweed (Centaurea repens): found in disturbed areas
• Bachelor's button (C. Cyanus): currently in cultivation and common in "wildflower" mixes; with blue flowers generally larger than C. maculosa.
• White-flowered knapweed (C. diffusa): found in disturbed areas; generally with white flowers, but easily confused with C. maculosa.

DISTRIBUTION AND HABITAT: This plant was probably introduced in the 1890's as a contaminant in alfalfa or hay seed from Europe and Asia. Spotted knapweed has become a serious problem in the rangelands of the northwest United States. In recent years, the species has invaded relatively undisturbed natural areas in Wisconsin as well as heavily disturbed sites. The extent of the invasion and the communities potentially affected are not well known. Until recently, spotted knapweed was presumed to inhabit only heavily disturbed areas such as road ditches, agricultural field margins, railroad beds, pipelines, and recently installed utility lines; the plant has now been found in dry prairie sites, oak and pine barrens, and on lake dunes and sandy ridges. It seems to be especially problematic in the central sands, northern Wisconsin, and near the Great Lakes.

LIFE HISTORY AND EFFECTS OF INVASION: Spotted knapweed reproduces solely by seed. Individual flower heads bloom from late June through August for 2-6 days each. The bracts reopen after about 20 days and scatter seeds. Plants average about 1,000 seeds per plant. Seeds are viable for seven years, and germinate throughout the growing season. Seedlings emerging in fall develop into a rosette of leaves that resume growth in spring.

Spotted knapweed often attains high densities on sunny wild lands--even ones undisturbed by human or livestock activity. Knapweed tends to dominate sites at the expense of community diversity or forage production. Knapweed infestation can also increase surface run-off and sedimentation.

Spotted Knapweed (c) John M. Randall/The Nature Conservancy

CONTROLLING SPOTTED KNAPWEED
Prevention is extremely important--spotted knapweed spreads readily in hay and on vehicle undercarriages. Caution is also necessary when using hay from the road ditches of primary roadways, or hay purchased from known infested areas. Outlying plants should be controlled before main populations.

Mechanical Control: The most effective control is early detection and removal of pioneering plants. Small populations can be removed by digging or pulling. This is best done where the soil is moist. The entire root should be removed. Mowing has not been successful--plants merely reflower at a lower height.

Once established, knapweed may be reduced by hot prescribed burns. These can be followed by selective pulling and digging once the population has been decreased. Annual burns have reduced populations anywhere from five to ninety percent. Reductions seem to correlate to the intensity of the burn administered; burns that remove nearly all the duff are most effective. Following a burn, reseed with native species. The potential effects of intense burning on native species must be taken into consideration when planning a burn.

Biological Control: Several biological controls exist, including two root mining moths, a flower moth, and a root mining beetle. These have met with varying degrees of success. Most promising are the two seed-head attacking flies Urophora affinis and U. quadrifasciata. Congruently, these two flies have reduced seed production 95% in experimental populations. While both flies are being released experimentally in Wisconsin, their effectiveness here is still unknown. These insects were not available for general use at the time of printing. The USDA should be consulted to determine the current status of these controls.

Spotted Knapweed (c) John M. Randall/The Nature Conservancy

Chemical Control: Chemical controls are an effective means of eliminating spotted knapweed, but they may also have the most adverse consequences. The recommendations from western rangelands involve the use of some very potent chemicals. These restricted chemicals are not recommended for use on quality natural areas, but may be appropriate on roadsides and other highly disturbed areas. Experimentation testing the effectiveness of less toxic broad-leaf herbicides such as triclopyr or glyphosate is encouraged.

A 3% active ingredient (a.i.) solution of triclopyr in the water-soluble formulation with dye can be sprayed on the entire plant except the flower, which should be spared for native fauna. This application should be repeated 3-4 times per year for two years. Triclopyr will not affect grasses.

Picloram at 0.25 to 0.5 pounds per acre will control spotted knapweed plants and seedlings for 2-3 years, although the residual control period may be shorter on gravel soil, where soil organic matter is high, or in wet areas. Picloram should be applied either in fall when the plant is in the rosette growth stage, or in spring during the bud to bloom stage. Picloram should not be used near water or on sandy soils with ground water ten feet or less below the surface.

Dicamba at 1-2 pounds (acid equivalent--see label) per acre also controls spotted knapweed, but may require annual follow-up treatment for a minimum of two years.

Clopyralid is more selective, affecting only legumes and composites. It can be applied at 0.25 pound acid equivalent per acre. A mixture of 0.19 pounds of clopyralid per acre plus 2,4-D at 1 pound acid equivalent per acre is also an option; both herbicides provide good control of spotted knapweed with less soil residual than picloram or dicamba. Spotted knapweed that is still in the rosette stage can be controlled by applying a 2,4-D low volatile ester, oil soluble amine, or water soluble amine formulations at 2 pounds per acre. Annual spraying for several years may be required to deplete the seed bank.

Picloram and clopyralid are the most commonly used and effective herbicides for spotted knapweed. Picloram causes the largest initial decrease in native forb cover, but the experiment that produced this data also concluded that most non-target forb populations were tolerant of herbicidal treatments and benefited from being released from knapweed competition after three years of selective herbicidal application. Application of these herbicides has not been found to decrease the frequency of occurrence of grasses, sedges, shrubs, or trees in the treated site. Herbicides were applied manually using a carbon dioxide pressure-regulated backpack sprayer at the concentrations listed in previous paragraphs.

More Information on the Web (links exit DNR)
Spotted Knapweed - Wisconsin State Herbarium
Spotted Knapweed - Invasives on the Web

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