The issue: Russian Knapweed

Russian knapweed (Acroptilon repens) is a native of Eurasia. It is a creeping, spreading perennial, with a mature size of 18 to 36 inches tall. Its deep roots make it drought tolerant, once established. Lower leaves are 2 to 4 inches long and deeply lobed. The gray-green color is due to the hairs on the leaves and stems. The white to lavender, urn-shaped flowers look like small Canada thistle flowers and appear from June through September.

Russian knapweed spreads mostly by rhizomes (underground stems). 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. Seeds transported to new sites results in new infestations. It is toxic to horses.

Integrated pest management options:

· Prevention: Keep ditch banks free of seed-producing plants; clean farming, recreational and construction equipment and vehicles before moving to a new area; feed weed-free hay.

· Mechanical: Two mowings (the first at bud stage), eight weeks apart, after herbicide treatment will help reduce seed production but won’t kill plants.

· Cultural: Establish and encourage desirable vegetation after herbicide treatment. Sod-forming grasses compete better than bunch grasses.

· Biological: No approved insects. Sheep and goat grazing of young plants can be combined with other efforts.

· Chemical: Fall is usually the best time to spray Russian knapweed —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: Seeds can stay dormant for up to three 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.