Russian Knapweed

Click on image to see mature plant

knapweed

Jude Sirota, Mesa County Horticulture Pest & Weed Inspector. Photos by John Jenkins, Jude Sirota and Karen Eslinger

Russian knapweed [Acroptilon repens (synonym = Centaurea repens)] is a creeping perennial weed native to Eurasia. This species has properties making it a difficult weed to control: it spreads by underground roots that may go to a depth of 8 feet or more and it puts out a chemical that inhibits other species from growing near it (allelopathy). It grows in pastures, rangeland, roadsides, waste areas, and on neglected agricultural land. Russian knapweed is toxic to some animals and must be handled carefully. Horses may become addicted to Russian knapweed if it is the primary food source in their pasture. In severe cases the animal may die. However, cattle and sheep are not affected.

In lower elevations in western Colorado Russian knapweed emerges in late April. Young plants can be identified by their silvery green color, hairy leaves and shoots, wavy edged leaves growing in a "rosette", and black scaly root. The pink to lavender cone-shaped flowers bloom from late May to September. Vegetative growth is minimal during the summer, however the plants do translocate nutrients (carbohydrates) to the root system after bloom and into the fall. These nutrients help with the formation of the root buds that will produce new shoots in the spring.

If left uncontrolled, Russian knapweed forms dense stands, displacing native plants and reducing forage for animals. Control measures should be aimed at stressing the plant over a period of time to deplete the stores of nutrients in the extensive root system. Getting Russian knapweed under control may take several years. large stands may only be reduced to manageable levels. No single control strategy will work for Russian knapweed; a combination of cultural, mechanical and chemical controls are necessary.

Repeated mowing will stunt the growth of the plants but they will continue to flower at a shortened height. Mowing followed by spraying of regrowth is not as effective as spraying an herbicide on unmown plants.

Tillage will cut up the root pieces and spread them to new areas. It only takes a small piece of root to start a new plant. However, tillage is important several weeks or months AFTER an herbicide treatment and before reseeding to allow the knapweed's own herbicidal chemical to dissipate.

Most herbicides available at gardening centers are not concentrated enough or do not contain the right herbicide for effective control. Spraying any herbicide during early spring growth only kills the tops of the plants, and causes the root systems to send up new shoots. Repeated spraying will be necessary. This is a waste of time, chemicals, and money.

Cultural control (e.g., reseeding with competitive plants) and mechanical control (e.g., tillage) are most effective once the knapweed has been stressed with herbicides.

Timing: There are two times when herbicides are effective: just before flowering (the "bud" stage, early to mid June) and in the fall before the plants dry down (late August or early September). Fall applications are more effective as the plants are drawing nutrients into the root system for winter storage, and the herbicide will move more efficiently into the root system. Bud stage applications prevent flowering (and thus seed set) and are suggested for areas such as ditch banks and other waterways where seed may be transported downstream.

Pasture and range: Recent experiments in Mesa County have found that a product called Curtail (clopyralid), sprayed in the fall before the plants dry down, reduced the stand by 90% for up to 3 years (so far). Some spot spraying is necessary to keep newly sprouted plants from maturing. Use 3 quarts per acre for broadcast spraying or a 2% solution for spot spraying. Curtail can be used close to desirable trees and shrubs but spray drift must be avoided to prevent injury to the plants. Care must be taken when applying Curtail in areas where there is a high water table or very permeable soils.

Ditch banks, irrigation canals, and other waterways: Rodeo (glyphosate) is the only effective chemical for Russian knapweed that is labeled for use around aquatic systems. Apply in the bud stage or in the fall. Use a 2% solution and hand spray, preferably when the ditch is dry. Follow all instructions on the label.

Lawns/Turf: Confront (triclopyr & clopyralid) is an herbicide similar to Curtail that is labeled for use in turf. Spot spraying with Roundup (glyphosate) can be done, but some grass damage must be tolerated.

Non-crop areas (industrial sites, roadways, driveways, parking lots):
Curtail is labeled for use on non-crop areas as is Telar (chlorsulfuron). Curtail may be applied at 3 quarts per acre or a 2% solution during bud to bloom stage or in the fall. Telar should be applied during bloom to post-bloom stages at 1 ounce per acre with a surfactant at 0.25%-0.5% solution. Application of Telar in the fall may damage brome grasses so care must be taken.

Tordon (picloram) is an effective herbicide for controlling Russian knapweed, but it is a restricted use pesticide and can only be purchased and applied by a certified applicator or someone under their supervision. It is labeled for use on range, permanent pasture, fallow cropland, non-crop areas, and Conservation Reserve Program acreage. Apply at bud to bloom stage or in the fall. Follow all label instructions and precautions.

Always read the label before buying and again before applying any pesticide.

RESEEDING: Russian knapweed is successful because it out competes most other broadleaf plants and many grasses. It is often found in a dense, single species patch because it releases a chemical into the soil that prevents other species from germinating. An important part of any control program is to get rid of this chemical by tilling the soil. Following tillage, seeding with a competitive grass seed is recommended. Grasses are better than clovers or alfalfa because a broadleaf herbicide can be sprayed if the knapweed reappears. CSU recommends smooth brome, streambank wheatgrass, thickspike wheatgrass, crested wheatgrass, and Russian wildrye. For local recommendations for specific sites, call the Natural Resources Conservation Service in Grand Junction at 244-5069.

These photographs show how persistent this species can be. Knapweed in Crawl Space These Russian knapweed stems are coming up in the crawl space of a recently built house. The crawl space was excavated to a depth of three feet before construction. The lot must have been heavily infested with Russian knapweed before building.Knapweed crawling up the insulation on the stem wall in the crawlspace. Knapweed is also coming up on the outside of the foundation, in the lawn, and under landscape plastic. Builders, developers and home buyers need to be aware of noxious weeds. If the weeds had been controlled before building, this situation probably would not have developed or would at least not have been as severe.


Placed on the Internet May 21, 1998

Comments on this page should be addressed to Dr. Curtis E. Swift, Area Extension Agent, Horticulture
Colorado State Cooperative Extension
2775 US Hwy 50, Grand Junction, CO. 81503
voice: 970-244-1834
fax: 970-244-1700