Authors: A. Carpenter and T. Murray, ed. Russian knapweed was originally, and sometimes still is, classified as Centaurea repens by North American taxonomists. However, it does not share some characteristics common to the genus Centaurea , and has been placed in the genus Acroptilon.
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Authors: A. Carpenter and T. Murray, ed. Russian knapweed was originally, and sometimes still is, classified as Centaurea repens by North American taxonomists. However, it does not share some characteristics common to the genus Centaurea , and has been placed in the genus Acroptilon. The genus name Acroptilon , meaning feathery tip, refers to the plume-like bristle at the tip of the flower head bracts. The specific epithet, repens , refers to the creeping growth of the rootstocks.
Russian knapweed is the most commonly used name in North America. Additional common names include Mountain Bluet, Turkestan thistle, and creeping knapweed. Russian knapweed can commonly be found along roadsides, riverbanks, irrigation ditches,pastures, waste places, clearcuts, and croplands.
Russian knapweed does not establish readily in healthy, natural habitats. It typically invades disturbed areas, forming dense singlespecies stands. Once established, Russian knapweed uses a combination of adventitious shoots and allelopathic chemicals to spread outward into previously undisturbed areas. The most effective method of control for Russian knapweed is to prevent its establishment through proper land management.
The healthier the natural community, the less susceptible it will be to Russian knapweed invasion. Areas should be monitored three times a year spring,summer, and fall and all plants should be destroyed immediately.
Since Russian knapweed is so persistent, it is important to kill all of the plants in the targeted area. Pulling, cutting, and discing can be used to control and reduce an infestation, but used alone will not permanently eliminate a stand of Russian knapweed.
Aggressive monitoring,followed by a combination of mechanical, chemical, and biological control, is needed to remove an infestation. In North America, A. Acroptilon repens is a perennial herbaceous plant of the aster sunflower family Asteraceae.
It is characterized by its extensive root system, low seed production, and persistence. Russian knapweed spreads through creeping horizontal roots and seed.
The following stem and floral description is from The biology of Canadian weeds, Acroptilon repens  unless otherwise noted. The stems of Acroptilon repens are erect, thin, stiff, corymbosely branched, cm 18 to 36 in tall, and when young are covered with soft, short, gray hair.
Lower stem leaves are narrowly oblong to linear-lanceolate, and deeply lobed. The upper leaves are oblong,toothed, and become progressively smaller. Rosette leaves are oblanceolate, irregularly pinnately lobed or almost entire, cm long, and The flower heads of Russian knapweed are urn-shaped, solitary, mm high, and composed of disk flowers only. Flowers are numerous, all tubular. The petals are Anthers are 4.
The stigma is 3. The pollen diameter is mm, spherical, 3-pored, thin-walled, about 2 mm thick and finely granular. Achenes seeds are mm long, oval and compressed, 2 mm broad and 1 mm thick. Acroptilon repens has a well-developed root system, which functions as the major means of propagation and spreading.
The roots of Acroptilon repens can extend more than 7 meters below the soil surface with Acroptilon repens is a strong competitor and can form dense colonies in disturbed areas. Russian knapweed invades many disturbed western grassland and shrubland communities, as well as riparian forests. Once established, Russian knapweed can dominate an area and significantly reduce desirable vegetation e.
Acroptilon repens contains an allelopathic polyacetylene compound which inhibits the growth of competing plants. Acroptilon repens was designated a noxious weed in the Federal Seeds Act of Canada in In the United States, A. On rangeland, the reduction in forage following invasion of Russian knapweed can threaten the stability of ranching operations. Russian knapweed is generally avoided by grazing animals due to its bitter taste. Acroptilon repens is poisonous to horses and can cause a neurological disorder called "chewing disease".
On agricultural land, A. Infestations of Russian knapweed can survive indefinitely through their root system. It is now found on every continent, except Antarctica. Russian knapweed is listed as a serious noxious weed of dryland crops in the southern former Soviet Republics. Russian knapweed was first introduced into Canada around as a contaminant of Turkestan alfalfa seed.
The introduction of Russian knapweed into the United States is also thought to be the result of impure Turkestan alfalfa seed, and possibly sugarbeet seed. Since then, it has become widespread in the United States and is currently found in at least counties in 21 states. In California, 32 counties reported infestations of Russian knapweed, but only two Tulare and Sutter reported heavy infestations over ha.
Russian knapweed has become widespread in the United States, particularly in the semi-arid west. It is more competitive than other weedy species in occupying disturbed areas. Russian knapweed is indifferent to crop association and is able to survive in almost any crop. It is sensitive to decreased amounts of sunlight.
Russian knapweed invades disturbed grassland and shrubland communities, as well as riparian forests. Acroptilon repens is not restricted to any particular soil but does especially well in clay soil.
Watson observed that Russian knapweed infestations increased in dry locations but decreased in moist locations due to competition with perennial grasses. Shoots emerge early in spring shortly after soil temperatures remain above freezing. All shoot development originates from root-borne stem buds. Plants form rosettes and bolt in late May to mid-June.
Acroptilon repens does not appear to reproduce extensively from seed but just one plant may produce 1, seeds per year. Watson  and Maddox et al. However in an experiment, Selleck had successful germination from seeds that were up to 8 years old. Russian knapweed reproduces primarily vegetatively.
The root system consists of the original root taproot , one to many horizontal roots, and their vertical extensions. Buds on the horizontal roots can form adventitious shoots that may grow to be independent plants. Watson  reported that the plant extends radially in all directions and can cover an area of 12 m 2 within 2 years.
Zimmerman  stated similar growth statistics in that the roots of Acroptilon repens can extend horizontally more than 7 meters with Acroptilon repens has been found to have allelopathic effects that inhibit the growth of crops and other plants. Studies have shown at least one of these compounds to be an allelopathic inhibitor.
When polyacetylenes were present in the soil at levels of just 4 parts per million, root-lengths of alfalfa. A controlled greenhouse experiment  indicated light availability affected growth and development of Russian knapweed.
Flower production declined with decreasing light levels. This study suggests that knapweed plants that emerge with other plants are more competitive than plants that emerge under the canopy of growing plants.
Monitoring should be conducted during the spring, summer, and fall of each year. The spring search should be conducted in late May to mid-June when the plants have recently bolted. The summer search should be conducted in July, when any missed plants have flowered and are easily recognizable.
The fall search should be conducted in late August or early September. The fall search should focus on any late blooming plants that might have regrown from the root system of plants that had been pulled during an earlier search.
Any knapweed plants that are found should be destroyed immediately. If an infestation is found, closely search the surrounding area to determine the size and extent of the infestation. Patches of Russian knapweed should be marked, and special attention should be given to these locations during follow-up visits.
There is no single "silver bullet" control method for Russian knapweed. Lasting control requires an integration of mechanical control, chemical control, biological control, proper land management, and vegetative suppression. An effective management program must first control existing infestations, and then promote repopulation by native plants. Continued monitoring and follow-up treatments should be conducted annually to eliminate any reinfestation of knapweed.
The keys to controlling Russian knapweed are to 1 stress the weed and cause it to expend nutrient reserves in its root system, 2 eliminate new seed production, and 3 control its vegetative spread.
If sufficient human resources are available, mechanical control is good place to start. Pulling Russian knapweed plants two to three times annually contained, but did not eliminate, an infestation in Washington. Often, the plants that do reemerge are smaller in size and lower in vigor. This is a good indication that the plants are under stress and that their nutrient reserves are declining. If an infestation is too large to be treated mechanically, herbicides can be applied for effective control.
Biological control agents can place additional stress on Russian knapweed plants. Two biological agents for Russian knapweed have been released in the United States; Subanguina picridis , a gall forming nematode, and Aceria acroptiloni , a seed gall mite.
Once the initial infestation has been controlled, native species should be replanted to act as a vegetative suppressant.
Acroptilon repens (L.) DC.
Rhaponticum repens , synonym Acroptilon repens ,  with the common name Russian knapweed , is a bushy rhizomatous perennial, up to 8 dm tall. Stems and leaves are finely arachnoid-tomentose becoming glabrous and green with age. The lower cauline leaves are smaller, pinnately lobed; the upper leaves become much reduced, sessile, serrate to entire. The heads are numerous terminating the branches. Flowers are pink to purplish, the marginal ones not enlarged.
EPPO Global Database