The issue: Spotted knapweed

Spotted knapweed (Centaurea stoebe ssp.micranthos) is a native of Eurasia. It is a short-lived perennial, with a mature size of 12 to 48 inches tall. Its deep taproot makes it drought tolerant and helps to survive fires. Rosette leaves are deeply lobed. The leaves and stems are gray-green in color. The pink to purple flowers have dark-tipped, spineless bracts and appear June – October.

Spotted knapweed rapidly colonizes disturbed areas by seeds. It releases allelopathic chemicals (inhibits the growth of other plants), allowing it to form dense, monoculture stands. It is a serious weed pest of Idaho rangeland, recreation sites, rights-of-way, and pastures.

Integrated pest management options:

· Prevention: Keep ditch banks free of seed-producing plants; clean all equipment and vehicles before moving to a new area; feed weed-free hay; manage pastures for good competition; establish desirable plants in disturbance areas.

· Mechanical: Hand pull or shovel plants below the crown; use a propane torch on seedlings.

· Cultural: Establish and encourage desirable vegetation after herbicide treatment; use plastic or landscape cloth mulch in yards and gardens.

· Biological: Biological controls should be combined with other efforts. There are 13 approved insects that will attack spotted knapweed. Grazing by sheep in spring and fall can help reduce, but not stop, seed production.

· Chemical: Spring and fall application on rosettes is usually the best timing — Transline, Curtail, Perspective, Milestone, Telar, Plateau, Tordon 22K (restricted use herbicide) are effective. Be sure the target weed and crop or landscape situations are listed on the product label. Always read and follow herbicide label directions.

Combine them:

Spotted knapweed seeds can stay dormant up to eight years so a long-term program is needed. Combine different IPM options over a period of years to help improve the effectiveness of your efforts.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County.