Russian knapweed [Acroptilon repens (L.) DC.][CENRE][CDFA list: A] Photographs Map of Distribution
SYNONYMS:Turkestan thistle; Centaurea repens L.
GENERAL DESCRIPTION:Noxious perennial to 1 m tall, with dark, creeping rhizomes. Plants exhibit allelopathic effects and are aggressively competitive, facilitating rapid colonization and development of dense stands. Infestations can be extremely long-lived due to extensive root and rhizome systems. Stems dieback after flowering in summer, and new shoots are generated in spring. Introduced from Central Asia. Like yellow starthistle [Centaurea solstitialis L.], Russian knapweed is toxic to horses, causing nigropallidal encephalomalacia or "chewing disease" when sufficient quantities are consumed. Under most circumstances livestock will avoid grazing Russian knapweed because of its bitter taste. back to top of page
SEEDLINGS:Uncommon in the field. Cotyledons ovate to spatulate and scurfy on the lower surface. First several rosette leaves elliptic to oblanceolate and covered with a white, powdery bloom. Margins entire. Subsequent rosette leaves irregularly 1-pinnately lobed with pronounced wavy margins.
MATURE PLANT:Stems erect, openly branched, leafy, and mostly covered with cobwebby gray hairs. Stem leaves alternate and not extending down the stem (winged). Basal and lower stem leaves mostly oblong, 4-10 cm long, and 1-pinnately lobed with pronounced wavy margins or 2-pinnately lobed. Upper stem leaves narrowly lanceolate to linear, entire or toothed, and 1-3 cm long. Leaves lack hairs or are covered with short to medium interwoven hairs.
ROOTS and UNDERGROUND STRUCTURES:Slender creeping rhizomes branch frequently at various depths forming an extensive vertical and horizontal root system. Rhizomes covered with alternate, small, narrow, appressed, clasping scale leaves. Each scale leaf has a bud in its axil capable of producing a new shoot. Mature rhizomes dark brown to black. Young rhizomes paler, with longer, less appressed scale leaves. Roots and rhizomes can penetrate the soil 1 to several meters deep. New shoots and roots are produced at various intervals along the rhizomes from depths to 120 cm or more. Severed root pieces as small as 2.5 cm can generate new shoots from depths to 15 cm. back to top of page
FLOWERS:May-September. Hemispheric flower heads arranged in panicle-like or flat-topped clusters and consist of about 30 white, pink, or lavender-blue disk flowers interspersed with bristles on the receptacle. Corollas about 15 mm long. Phyllaries (flower head bracts) arranged in several overlapping rows. Each phyllary is ovate, with a green base and a broad, papery margin at the tip. Plants primarily outcross.
FRUITS and SEEDS:Achenes white or pale gray, obovoid, lacking a lateral notch at the base, and 3-4 mm long. Pappus consists of many unequal, early deciduous, white bristles, about 6-10 mm long. Each bristle is minutely barbed on the lower part and plumose on the upper part.
POSTSENESCENCE CHARACTERISTICS:Old flower stems can persist for an extended period after senescence. Phyllaries and achenes remaining on old stems can aid with species identification.
HABITAT:Fields, cultivated sites, orchards, vineyards, roadsides, ditchbanks, and waste places. Inhabits many soil types.
back to top of page
DISTRIBUTION:Throughout California, except for the wettest areas of the northwest and driest regions of the Great Basin and Mojave and Sonoran Deserts. To 6250 ft (1900 m). Known populations in all counties except Calaveras, Del Norte, El Dorado, Humboldt, Mariposa, Mendocino, Sierra, and Trinity.
PROPAGATION/PHENOLOGY:Reproduces primarily by vegetative shoots from rhizomes. Plants usually produce small quantities of viable seed. Seed heads mostly remain closed. Seeds disperse passively near the parent plant or with the seed head. Seeds germinate over a broad temperature range (0.5-35º C; optimal 20-30º C), and light is not required. Scarification, fluctuating temperatures, and alternating light and dark periods increase germination. Seed can remain viable about 2-3 years.
MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Above ground removal of plants encourages new shoots to sprout from rhizomes. Shallow cultivation can severe roots and encourages spread.
SIMILAR SPECIES:Other white, pink, and purple-flowered knapweeds in the genus Centaurea and bearded creeper [Crupina vulgaris Cass.] are most easily distinguished by their lack of dark, spreading rhizomes and by phyllary and achene characteristics. Refer to the Flower Head Comparison of White, Pink, or Purple-flowered Centaurea and Related Species Table for comparison. In addition, only bearded creeper has leaf margins with stiff hairs barbed at the tips (glochidiate hairs) and flower receptacles with flattened, scale-like, chaffy bracts.
CONTROL METHODS:
Prevention: Russian knapweed and squarrose knapweed are both deep rooted perennials and established stands are more difficult to control than either spotted or diffuse knapweed. Russian knapweed can invade and persist in numerous ecosystems, including rangeland, pastures, agricultural fields, riparian areas, and wildlands. It has been found in saline, alkaline, low lying areas, but prefers deeper soils with more available moisture. Russian knapweed primarily regenerates new shoots from dark, scaly, rhizomes several inches below the soil surface. Seed production is to a much lesser extent than diffuse or spotted knapweed. Therefore, rate of spread is less rapid than the prolific knapweeds, but very persistent patches develop. Seed production and dispersal are still extremely important for colonization of new areas. Proper identification of Russian knapweed is important because it may be found in the same areas as other knapweeds, and different control strategies are needed. Since Russian knapweed is found in many production agricultural areas, growers should use certified weed-free crop seed to prevent spread. Certified weed-free hay should also be used to winter feed cattle and horses and pelleted feed can be used when using horses in backcountry areas. Clean agricultural tillage implements after working in infested fields. Also, avoid driving vehicles or equipment through mature patches as seedheads can become attached and spread over long distances. Avoid overgrazing of pastures and rangeland and maintain litter cover to prevent seed germination and establishment. Squarrose knapweed is a tough, taproot forming perennial which is adapted to colder, drier, environmental conditions than most other knapweeds. Establishment is slow, as new rosettes remain for several years from a very persistent crown before seed are produced. Seed production is limited, but effective dispersal of closed, entire seed heads occurs by curved spines, which readily attach to hair, fur, or clothing of animals and humans. Squarrose knapweed is found in dry, rocky, degraded juniper-shrub savanna, but may establish in better environments. The spread of squarrose knapweed has been reported to follow animal routes, especially sheep. Avoid overgrazing of poor rangeland environments, where squarrose knapweed may initially establish. Remove livestock, especially sheep, from areas with mature infestations to prevent seed dispersal.
Mechanical: Hand pulling of either species is very difficult, and has limited effectiveness. Since Russian knapweed primarily reproduces from rootstocks, removing a majority of the root system is required. Squarrose knapweed has a deep taproot, which is also very difficult to remove. It must be severed below a depth of eight inches to prevent regrowth. Intensive cultivation of Russian knapweed will sever and spread root fragments, which will quickly reestablish. Mowing will reduce, but not eliminate seed production and should be conducted prior to seed set. Squarrose knapweed seed may remain in closed seed heads on the plant into the next growing season. Care should be taken to avoid dispersing persistent seed heads from older plant skeletons.
Fire: Burning will not provide long term control of either species as rootstocks will rapidly regenerate. However, integrated strategies using fire the first year followed by herbicides the second year have provided increased short term control of squarrose knapweed compared to burning alone.
Biological: There are currently no insects approved for introduction into the United States for control of Russian knapweed. However, the Russian knapweed gall nematode (Subanguina picridis) has been released in the U.S., but is not currently established in California. This nematode is destructive in its larval and adult stages, and forms galls in the stems, leaves and root crowns of Russian and diffuse knapweed. This is the first nematode species to be released in North America for biological weed control. Two insects currently established for control of other knapweeds in the Northwestern U.S., are also active on squarrose knapweed. The banded gall fly (Urophora affinis) and the UV knapweed seed head fly (Urophora quadrifasciata) feed on the seed heads of squarrose knapweed plants. The banded gall fly is currently established in California and is available for redistribution.
Chemical: Herbicides can be used to manage existing stands of Russian knapweed, but repeat treatments are required and control may be variable over years. There is little information on herbicide use for squarrose knapweed. It may respond to herbicides in a manner similar to diffuse knapweed, but this is uncertain. While seed longevity is unknown for squarrose knapweed, reductions in the soil seed bank is essential for long term management. In California, there are two herbicides useful for Russian knapweed management: dicamba, and clopyralid. Dicamba should be applied at 2-4 lb ae/A or clopyralid at 0.38-0.50 lb ae/A. Dicamba should be applied when plants are in the early bud stage and clopyralid should be applied in the late bolting to early bud stage. Both herbicides will provide some residual control, particularly clopyralid, but yearly retreatments may be necessary for several years. Dicamba will injure or kill any other broadleaf plants it contacts, including desirable forage and native broadleaf species. Clopyralid is more selective, but will injure legumes such as clovers. The most effective knapweed treatment is picloram applied at 1.0 lb ae/A. However, picloram is not labeled for use in California. Picloram will provide long term residual control of Russian knapweed for several years after treatment. This herbicide will kill or severely injure most broadleaf species it contacts in the soil or on the foliage, including trees, forbs, and shrubs.
Integrated control strategies: When irrigation is possible, Russian knapweed may be effectively managed with a combination of herbicides and crops that provide dense shade. It is extremely difficult to establish perennial grasses in dense stands of Russian knapweed due to allelopathic chemicals produced by the knapweed. Use of herbicides followed by reseeding of perennial grasses has resulted in increases in grass cover by >50%. Reseeding of perennial grasses in combination with the knapweed gall nematode has not been examined. Long term control will inevitably require yearly examinations of infestations and subsequent management for success.