Northwest Management, Inc.
Spotted Knapweed was introduced from Europe and is widely dispersed in the western United States. Each plant produces up to 25,000 seeds that are dispersed by wind, animals, and people. Seeds can remain viable for up to 8 years.
Spotted knapweed generally is a short-lived perennial, reproducing solely by seeds. The seeds may germinate from spring through early fall. Seedlings emerging in the fall often over-winter as a rosette of leaves, resuming growth again in the spring. The plant grows 2 to 4 feet tall and bears alternate, pale green leaves. The upper leaves are linear in shape. Stems are erect and rough, with slender branches. Numerous flowers are produced from early July through August. Flowers are pink to light purple.
Several biological control agents are available including root boring beetles and moths, seed head gall flies, and seed head weevils. The success to these control agents area generally good; however, re-application is often recommended due to the viability of the seeds remaining in the soil.
The best spotted knapweed control is prevention. The plant generally is easy to control with herbicides but an area must be monitored for several years and retreated.
Picloram (Tordon) at 0.25 to 0.5 pounds (1 to 2 pints) per acre will control spotted knapweed plants and seedlings for two to three years. The residual control period may be shorter on gravelly soils, in wet areas, and where soil organic matter is high. The optimum application time is during the rosette growth stage in the fall or in the bud to bloom stage in the spring. Picloram should not be used near water or where a sandy porous surface and substrata overlie ground water 10 feet or less below the surface.
Clopyralid + 2,4-D (Curtail) will provide good control of spotted knapweed with less soil residual than picloram or dicamba. Control is greatest when fall. A follow-up treatment the following year may be necessary to control seedlings.
For more information contact your County Weed Control Agent or Chemical Company.