HPIPM:Russian knapweed

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Asteranae
Order	Asterales
Family	Asteraceae
Subfamily	Carduoideae
Tribe	Cardueae
Genus	Rhaponticum

Scientific Name

Rhaponticum repens

Scientific Name Synonyms

Centaurea repens

Centaurea picris

Acroptilon repens

Common Name

Russian knapweed

Russian knapweed (Acroptilon repens)

Author: George Beck, Colorado State University Cooperative Extension

Adapted from the Colorado State University Extension Natural Resources Fact Sheet " Russian Knapweed"

Identification and Life Cycle

Russian knapweed (Acroptilon repens) is an introduced perennial forb in the sunflower family (Asteraceae). Flowers can be pink, lavender, or white. Lower leaves are 2 to 4 inches long and deeply lobed. Seeds are grayish or ivory and 0.08 to 0.1 inches long with long white bristles (pappus). Upper leaves are smaller, generally with smooth margins, but can be slightly lobed. Shoots are erect, 18 to 36 inches tall, with many branches. Shoots and leaves are covered with dense gray hairs. The solitary, urn-shaped flower heads occur on shoot tips and generally are 1/4 to 1/2 of an inch in diameter with smooth papery bracts. Russian knapweed has vertical and horizontal roots that have a brown to black, scaly appearance, especially apparent near the crown. Newly emerged leaves are toothed and covered with fine hairs.

Habitats

Russian knapweed grows on clay, sandy, or rocky prairies and sunny meadows; on saline soils; or clay, rocky or sandy shores of lakes and rivers; and on rocky and clay slopes of hills and bottomlands. It occurs in pastures, agronomic crops, roadsides, waste places, and rangeland.

Impacts

Russian knapweed forms dense, single species stands over time due to competition and allelopathy (biochemicals it produces that inhibit the growth of other plants). On rangeland, invasion by Russian knapweed can reduce forage availability. On agricultural land, Russian knapweed can reduce crop yields. Russian knapweed is toxic to horses, causing a neurological disorder called "chewing disease". It is palatable to livestock in the rosette and bolting stages, but is avoided naïve animals and all livestock after flowering occurs.

Biology and Ecology

Russian knapweed emerges in early spring, bolts in May to June (elevation dependent), and flowers through the summer into fall. It produces seeds sparingly, approximately 50 to 500 per shoot. Seeds are viable for two to three years in soil. Its primary method of reproduction is from vegetative propagation, with seed of secondary importance. Roots from a recently established plant expand rapidly and may cover up to 12 square yards in two growing seasons.

Management Approaches

An integrated management plan should be developed that places continual stress on the weed. Currently, the best management plan includes cultural control combined with mechanical and/or chemical control techniques. A single control strategy, such as mowing or herbicide application, usually is not sufficient.

Biological Control

Only the gall-inducing nematode, Mesoanguina picridis, has been introduced and established in North America with limited success. A gall wasp, Aulacidea acroptilonica, is under review as a potential biological control agent.

Mechanical and Cultural Control

Russian knapweed tends to form monocultures and usually eliminates other plants. Therefore, sowing desirable plant species is necessary after the weed is controlled. Smooth brome (Bromus inermis) will compete with Russian knapweed. Research shows that streambank wheatgrass (Elymus riparius), thickspike wheatgrass (E. lanceolatus), crested wheatgrass (Agropyron cristatum) and Russian wildrye (Psathyrostachys juncea) could become established after Russian knapweed was suppressed with herbicides. Sod-forming perennial grasses, like streambank or thickspike wheatgrasses, help prevent reinvasion better than bunchgrasses like crested wheatgrass. If the Russian knapweed stand is not too old and grasses are still present, stimulating grass growth by irrigation (where possible) should increase grass competition with knapweed and keep the weed under continual stress. In addition, tilling the soil may reduce allelopathic chemicals in the soil. A treatment to reduce built-up plant litter may also be needed before seeding into Russian knapweed infestations.

Chemical Control

Herbicidal suppression of Russian knapweed will increase grass establishment in a re-vegetation application. In most cases, herbicidal control of Russian knapweed will be most cost-effective using a chemical that translocates into the roots, and for most chemicals, if applied during the bloom or seed stage (Fall) rather than the rosette or early growth stage. In most circumstances, an herbicide alone will not effectively manage Russian knapweed. However, there may be situations where desirable plants within a Russian knapweed infestation may compete effectively with the weed if it is stressed with a single weed management technique.

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=Russian%20knapweed