Spotted Knapweed
Scientific Name: Centaurea maculosa

History/Origin
          Spotted knapweed was first introduced into America from Eurasia (central Europe and east to central Russia) as a contaminant in alfalfa and clover seeds in the late 1800's. Until 1920, it was limited to San Juan Island, Washington. By 1997, it could be found in 14 western states and in every county in Washington, Idaho, Montana, and Wyoming.

Biology/Ecology
           This aggressive plant is very competitive and can form a monoculture. It releases allelopathic chemicals from its roots that stunt the growth of many of the surrounding native plants. Early spring growth and lateral shoot development allow it to effectively compete for water and nutrients with desirable native plants.

          This knapweed is capable of invading both well-managed pastures and disturbed areas, although a higher soil disturbance creates a greater knapweed density. Each plant starts out as a rosette the first year and produces one to six flowering stems the following years. Spotted knapweed can send lateral shoots and form new rosettes less than a inch from the parent plant. It is not uncommon to see multiple rosettes on a single root crown.

           Seed production is extensive, ranging from 5,000-40,000 seeds/m2/year (greater number during wet years). Germination occurs under a wide variety of conditions. Seeds are generally dispersed by water and animals, and with crop-seed, or hay.

Known Impacts
Detrimental:
           Spotted knapweed suppresses the growth of other plants by releasing inhibiting chemicals from the roots and by aggressive competition. Loss of native plants has reduced available forage for wildlife and livestock. In areas of Montana, elk use on winter range has been reduced by 98% due to spotted knapweed invasions. Loss of vegetation has also produced an increase in surface water runoff and stream sediment yields, and a decrease in soil infiltration.

Beneficial:
           The flower produces a great nectar for domestic bees.

Potential Locations
           Spotted knapweed's natural location is the forest grassland interface in Europe, but it can easily invade and dominate rangelands that receive less than eight inches of annual precipitation. Areas without disturbance are not safe from invasion by this plant. It occupies a wide range of sites, including elevations from 1,900 to more than 10,000 feet, and precipitation zones of 8-79 inches.

Control/Management
            On a large scale, simply going out and killing the existing plants is not effective. Small patches may be eradicated, but with existing technology and money, larger populations can only be slowed and hopefully contained. Once again a combination of the following tools provides the best control:
• Herbicides are the most effective (example herbicides: Tordon 22K, 2-4,D, Clarity, Curtail [look at herbicide label: free search]). A combination of plowing or herbicide and seeding has been effective.
• Biological control agents have not been able to completely control knapweed, but they have reduced the competitive edge and reduced seed production in some areas.
• Persistent and thorough hand-pulling can be effective.
• Don't burn. Burning has been known to increase knapweed densities by six-fold.

NAPIS : biological control (maps)
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