Russian knapweed, Rhapontocum repens (L.)
Also known as Turkestan thistle, creeping knapweed, mountain bluet, Russian cornflower, or hardheads.
Weed identification:
Russian knapweed is a clonal perennial forb in the Asteraceae (Sunflower) family. It typically forms dense monocultures in degraded or disturbed areas, displacing native vegetation.
- Stems can grow up to 3 feet tall, are cormybosely branched, and are initially covered in soft gray hairs. Leaves are alternately arranged on the stem.
- Leaves: Lower-stem leaves are broad, oblong to lance-shaped, irregularly notched, and deeply lobed. Upper-stem leaves are smaller with smooth (entire) edges and a lance-shaped appearance.
- Seeds are approximately 1/16-inch, oblong, and gray-brown with fine white hairs and a ring of bristles at the apex.
- Seedlings emerge with broad, lance-shaped cotyledons (seed leaves) and finely lobed hairy true leaves. As they mature, the leaves take on a bluish-green hue.
- Flowers: Russian knapweed blooms from June through August. The flowers are solitary, urn-shaped, and range in color from pink to purple, turning a straw color at maturity. Flower heads are ¼ to ½ inch in diameter and appear at the ends of leafy upper branches. A distinguishing feature is the smooth, papery, rounded bracts encircling the flowers, which differentiates it from other knapweed species.
- Roots are dark brown or black, and scaly. Their expansive root system produces buds capable of developing into leafy shoots. Roots can grow to depths of eight feet during their first year of establishment. Russian knapweed’s aggressive roots facilitate its ability to form dense colonies and outcompete other vegetation.
Russian knapweed
Image credit: Steve Dewey, Utah State University, Bugwood.org
Russian knapweed plants grouped
Image credit: Steve Dewey, Utah State University, Bugwood.org
Russian knapweed flowers
Image credit: Steve Dewey, Utah State University, Bugwood.org
Biology and life cycle:
Russian knapweed is native to southern Ukraine, southeast Russian, Iran, Kazakhstan, and Mongolia. It grows on prairies and sunny meadows; on saline soils; or shores of lakes and rivers; and on rocky slopes of hills and bottomlands. Cultivated degraded land and dry pastures support growth in its native land. Although Russian knapweed is more common in North America’s drier regions, it can persist as a near monoculture in a range of habitat types. Russian knapweed occurs in most western states, in Colorado, this weed occurs in many environmental conditions and stands may survive 75 years of longer. This weed can compete with almost any crop in tillable soil and is most competitive in pasture and rangeland.
- Growth habit: Russian knapweed is an invasive, deep-rooted perennial that spreads through both seeds and aggressive, creeping horizontal roots. Roots produce buds that can develop into leafy shoots. Seedlings allocate most of their energy to root growth in the early stages of development. Roots of the plant can grow 6-8 feet deep during the first growing season and 16-23 feet deep in the second season. Russian knapweed’s extensive root system allows populations to persist through vegetative reproduction methods. Root fragments larger than 1 inch and buried in the top 6 inches of soil are able to produce new plants. Their root system also enables exclusion of other plants through competition for soil water and nutrients, and through the exudation of allelopathic chemicals. Three sesquiterpene lactones, Repin, acroptilin, and hyrcanin have been found in Russian knapweeds. Much of their successful infestation is attributed to these compounds, in addition to the release of root exudates that is found to be phytotoxic to native plant species. This ability to create harmful allelopathic chemicals hinders management unless tools are used to address soil conditions. Russian knapweed is also able to increase soil zinc condition as much as eight- fold in surrounding soil surface layers.
- Emergence dynamics: After soil temperatures remain above freezing, Russian knapweed shoots emerge from root buds in early spring. Floral stems bolt in late May to mid-June. Flowering usually begins in early June and continues into September and seeds are produced from August through September. Research has shown that this plant has a high rate of ovule abortion. Russian knapweed seed production is highly variable, but generally lower than other knapweeds. One plant may produce an average of 100 to 300 seeds but may increase up to 1,200 under optimal environmental conditions.
- Seed production: Seeds continue to be viable for two to three years. Germination is limited to areas where wetter soil conditions are maintained for extended periods of time (minimum ~7 days) and where seeds are covered by litter or soil. It is widely believed that Russian knapweed does not reproduce extensively by seed. It is probable that seed dispersal is passive as the bracts of the head generally remain closed and the pappus is too small to facilitate wind dispersal. Although, the urn-shaped flower head is able to float in river currents while keeping the seeds intact inside the head. The most common mechanism for seed dispersal is thought to be transmitted through infested hay. Seed and root fragments may also be transported to new areas by waterways such as irrigation ditches or rivers.
Resistance
Russian knapweed has infested approximately 1,400,000 acres in the western United States, with about 30,000 of those acres located in Colorado. On the Colorado Noxious Weed Act, Russian knapweed is designated as a “List B” species. Depending on the severity of local infestations, management may require eradication, containment, or suppression to prevent its spread.
Russian knapweed field infestation
Image credit: Steve Dewey, Utah State University, Bugwood.org
Russian knapweed seedling
Image credit: L. L. Berry, Bugwood.org
Russian knapweed seeds
Image credit: Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org
Integrated Weed Management Strategies based on Colorado cropping systems:
Prevention
Prevention, early detection, and eradication are the most important management tools for controlling Russian knapweed. Prevention techniques include maintaining healthy plant communities and ground litter. This may be done by removing excessive grazing and reseeding desirables after disturbance.
Cultural
Land managers, road crews, and the local public should be educated on knapweed species identification so that all suspected infestations can be better reported. Use certified weed-free seed, hay, and mulch. Vehicles, humans, and livestock should be discouraged from moving through infested areas. If necessary, a program should be implemented to check and remove seed from vehicles and livestock to stop dispersal.
Mechanical
Most mechanical control methods for Russian knapweed are limited but may be useful to combine with chemical spraying. Manual methods, like hand pulling or hoeing can be effective for small, less established infestations of Russian knapweed if repeated annually over multiple years. Other mechanical methods like tillage is not recommended since cut roots promote further spread of knapweed. Although mowing alone will does not reduce Russian knapweed population, it can suppress shoots and flowers if repeated throughout the growing season.
Biological Control
Biological control is an effective tool for management infestations of Russian knapweed, although it is not intended as an eradication method. Biological agents may be helpful to use in combination with herbicide application for improved results. Currently, there are three biological control agents approved for use on Russian knapweed.
Gall Wasp (Aulacidea acroptilonica)
- Lifecycle: Adult female wasps lay their eggs in the main and lateral shoots of Russian knapweed. As larvae hatch, they feed on the plant’s nutrients, causing galls (swelling) in the stems. Larvae overwinter in these galls and pupate in early spring. Gall wasps complete one generation per year, emerging from stem galls in mid-to-late spring.
- Impact: Galls reduce plant vigor by decreasing shoot length, biomass, and flowering/seed potential.
- Release recommendations: On a minimum plot size of 1/8 acre of undisturbed land and an optimum plant height of 4-6 inches.
Gall midge (Jaapiella ivannikovi)
- Lifecycle: This midge can complete up to five generations per growing season, with a 28-day cycle. Females lay eggs on young bud tips, and larvae feed on the plant, forming rosette galls of fused leaves. Up to 14 larvae can develop within a single gall, hibernating until spring.
- Impact: Larvae-induced galls stunt stem growth, reduce plant vigor, and halt flowering/seed production. This damage allows native plants to compete effectively.
- Release recommendations: Releases should be with lab colonies or field redistributions by moving infested plants or rearing adults.
Nematode (subanguina picridis)
- Mechanism: Forms galls on stems, leaves, and root crowns, effectively reducing seed production and stem, leaf, and root crown biomass.
- Considerations: Nematode impacts are limited to plants on moist sites and significant reductions in plant biomass have not been observed where it is established. This agent’s low impacts and inability to spread unaided prevent it from being a cost-effective management strategy. The USDA does not recommend nematodes for new releases.
Chemical control
A selective, post-emergent herbicide best controls Russian knapweed. Herbicides that effectively control Russian knapweed include Aminopyralid, Picloram, and Chlorsulfuron. Aminopyralid (Milestone) is most effective when applied at a rate of 4-6 ounces/acre. Application should be done in the spring and summer to plants in the bud and flowering stage and to dormant plants in the fall. Picloram (Tordon 22K) should be applied at a rate of 1 qt./acre. This herbicide is most effective when applied in spring to the bud or early flower stage. Chlorsulfuron (Telar) should be applied at a rate of 1-3 oz/acre and during spring bloom and fall rosettes.
References
References and resources for more information:
Resources:
Colorado Department of Agriculture- Yellow and hybrid toadflax
USDA (NISIC)- Yellow Toadflax | National Invasive Species Information Center
USDA Weed Field Guide- Managing Yellow Toadflax
EPA Herbicide online search- Pesticide Product and Label System | US EPA
References:
Alaska Natural Heritage Program. (2006). Environment and Natural Resources Institute, University of Alaska Anchorage.
Crocker, W. L. (1977). Wildly successful plants: a handbook of North American weeds. New York, NY: Collier Books.
Early Detection & Distribution Mapping System (EDDMapS). (2025). Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Retrieved February 4, 2025, from yellow toadflax (Linaria vulgaris P. Mill.) – EDDMapS
Ņečajeva, J., & Probert, R. (2011). Effect of cold stratification and germination temperature on seed germination of two ecologically distinct species, Linaria loeselii and L. vulgaris (Scrophulariaceae). Polish Botanical Journal, 56.
Ogden, J. A. O., & Renz, M. J. (2005, November 6). Yellow toadflax (Linaria vulgaris). Weed Fact Sheet. New Mexico State University.
Ontario Ministry of Agriculture and Food. (1993). Guide to weed control. Queens Printer, ON. Publication 75, 216p.
Royer, F., & Dickinson, R. (1999). Weeds of the Northern U.S. and Canada. Edmonton, AB: The University of Alberta Press.
Warren, R. (1957). Toadflax. Oregon Agricultural Extension Service, Extension Bulletin 763. Oregon State College, Corvallis, OR.
Zouhar, K. (2003). Fire Effects Information System (FEIS). US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. Retrieved February 4, 2025, from http://www.feis-crs.org/