Spotted knapweed [Centaurea stoebe L. ssp. micranthos (Gugler) Hayek][CENMA] [CDFA list: A][Forestry, Ornamental, Turf] Photographs Map of Distribution Biocontrol

Diffuse knapweed [Centaurea diffusa Lam.][CENDI] [CDFA list: A] Photographs Map of Distribution Biocontrol

Squarrose knapweed [Centaurea squarrosa Willd.][CENSQ][CDFA list: A] Photographs Map of Distribution Biocontrol

Purple starthistle [Centaurea calcitrapa L.][CENCA][Cal EPPC list: B][CDFA list: B][Ornamental] Photographs Map of Distribution

Iberian starthistle [Centaurea iberica Spreng.][CENIB][CDFA list: A][Forestry] Photographs Map of Distribution

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GENERAL DESCRIPTION:Noxious bushy weeds with spiny or comb-like phyllaries and white, pink, or purple flowers. Plants exist as basal rosettes until erect, highly branched flowering stems with are produced late spring/summer. Centaurea species produce allelopathic effects and are highly competitive with other plants, often displacing desired vegetation. Centaurea is a large genus comprised of about 500 species, none native to California. Thirteen species occur in California as introduced weeds and escaped ornamentals. Numerous biocontrol agents are known to parasitize Centaurea species in their native habitats. For the knapweeds, only the banded gall fly (Urophora affinis) and UV knapweed seed head fly (Urophora quadrifasciata) are currently established in California at publication time.

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SEEDLINGS:Cotyledons spatulate to oval. Rosette leaves pinnate-divided. Purple and Iberian starthistles develop straw-colored spines at the centers of rosettes.

MATURE PLANT:Upper stem leaves not winged. Foliage variously covered with short to medium interwoven gray hairs. Leaves alternate. Lower stem leaves deeply 1- or 2-pinnate-lobed, ~ 10-20 cm long. spotted knapweed: Leaves resin-dotted. Upper leaves mostly pinnate-divided.

ROOTS and UNDERGROUND STRUCTURES:Taprooted long in diffuse knapweed, stout in spotted knapweed, squarrose knapweeds, and purple starthistle and Iberian starthistles.

FLOWERS:Flower heads consist of few to many fertile disc flowers interspersed with long bristles on the receptacle. Phyllaries overlapping in several rows, with tips variously spiny or comb-like. Phyllary characteristics are important for species identification. Refer to the Flower Head Comparison of White, Pink, or Purple-flowered Centaurea and Related Species Table for a comparison.

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FRUITS and SEEDS:Achenes (1-seeded fruits) oblong, 2.5-3.5 mm long, apex flattened, tapered to a rounded, laterally notched base. Pappus (when present) +/- whitish, composed of unequal, stiff, minutely barbed bristles or tiny, flat scales.

POSTSENESCENCE CHARACTERISTICS:Old flower stems can persist for an extended period after senescence (less common for diffuse knapweed). Phyllaries and achenes remaining on old stems can aid with species identification when plants are overwintering as rosettes.

HABITAT:Fields, roadsides, disturbed open sites, grasslands, overgrazed rangelands, and logged areas. Plants seldom persist in shaded places and colonize most soil types with a disturbed A horizon.


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PROPAGATION/PHENOLOGY:Reproduces by seeds, except where noted. These species have variable dispersal mechanisms described below. However, most seeds or seed heads of all Centaurea species fall near the parent plant, and some can disperse to greater distances with human activities, vehicles, heavy machinery, water, soil movement, and by clinging to shoes, clothing, tires, and feet, fur, or feathers of animals. Germination can occur over a broad range of environmental conditions. Seedling emergence is typically highest after the first fall rains. Mortality of seedlings that emerge in spring can be high when conditions become dry after emergence. Most seedlings emerge from seeds at or near the soil surface. Plants produce fewer viable seeds in dry years. Infestation density correlates with the age of the population and degree of disturbance. Spotted and diffuse knapweed seeds exhibit 3 germination patterns: non-dormant seeds that germinate with or without light exposure, dormant seeds that germinate in response to red light, and dormant seeds that are not light sensitive. All germination types occur on each plant. For spotted and diffuse knapweeds, optimal germination is between 10-28º C (50-82º F).

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Fertilizer applications and poorly timed mowing can encourage survival. Rosettes are usually too low to be affected by mowing. Mowing mature flower stems disperses seed and can stimulate re-growth of stems.Burning removes current growth, but may enhance seed germination. Hand pulling 2-4 times per year or severing plants at least 2 inches below crowns can control small infestations (less effective for spotted knapweed).

SIMILAR SPECIES:Other knapweeds with fringed or comb-like phyllaries that lack spines include black knapweed [Centaurea nigra L.][CENNI], meadow knapweed [Centaurea pratensis Thuill.], and annual cornflower or bachelor’s buttons [Centaurea cyanus L.][CENCY]. Black and meadow knapweeds are less common perennials found in the Klamath Ranges, North Coast, North Coast Ranges, and San Francisco Bay region; to 500 m (1650 ft). Cornflower is an escaped garden annual with showy blue, purple, or white flowers found throughout California, except deserts; to 2000 m (6600 ft). Unlike Centaurea species, Russian knapweed [Acroptilon repens (L.) DC.] and bearded creeper [Crupina vulgaris Cass.] have phyllaries that are ovate and narrowly lanceolate respectively, with papery margins and seeds 3-4 mm long that lack a lateral notch near the base. 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. Refer to the Flower Head Comparison of White, Pink, or Purple-flowered Centaurea and Related Species Table for comparison.

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Prevention: Diffuse and spotted knapweed are excellent pioneer species and rapidly establish in disturbed roadside, rangeland, wildland, or recreation areas. Seed production may range from 1,000 to 18,000 seeds per plant and dispersal occurs by numerous methods. For diffuse knapweed, wind dispersal occurs by a "tumbleweed effect" and seed may be released over long distances. Spotted knapweed primarily spreads by peripheral colonization of infested areas. Both species have spiny seedheads which may be transported on animal hair or fur. Plant fragments also become attached to vehicles and equipment and spread seed for hundreds of miles. Off-road or recreational vehicles may damage existing vegetation and increase soil disturbance. Hikers and hunters can spread seed by picking the flowers or transporting contaminated hay for horse feed. These human dispersal mechanisms can be controlled. Avoid driving vehicles or equipment through infested areas and clean vehicle undercarriages when necessary. Use pelleted feed or certified weed free hay for horses in backcountry areas. Minimize recreational vehicle use in sensitive areas near infestations. For ranchers and rangeland managers, good grazing practices and early detection of new infestations are essential to reduce diffuse knapweed invasion. Avoid overgrazing and allow competitive forage to recover by implementing grazing rotations. Use weed free certified hay to winter feed animals and rotate feeding areas to prevent soil compaction and trampling of newly emerged grasses and forbs. Where intensive management is economical, pasture fertilization and irrigation will greatly reduce the chances of knapweed invasion.

Mechanical: While not a significant problem in agricultural cropping areas, knapweeds proliferate in non-crop areas of soil disturbance. Therefore, tillage in non-crop areas is not recommended. Hand pulling of small infestations of diffuse knapweed has shown considerable success. Since resprouting from the crown can occur, the entire plant must be removed. Hand pulling must be repeated 2-4 times a year and is easiest when the plants have begun to bolt in the late spring and the soil is still moist. Hand pulling of large infestations is very labor intensive and may not always be feasible. Proper disposal of removed plants is important to prevent spread. Piling and burning in a hot fire is a proven method of disposal. Hand pulling spotted knapweed may be less effective, since vegetative reproduction from short lateral roots can occur for several years.
Mowing will reduce, but not eliminate seed production of either species. Timing of mowing is critical. Rosettes are robust to mowing and generally too low to be successfully cut. A single mowing in the bud to early flower stage has been most effective, reducing seed production by greater than 75%. Mowing more mature plants will facilitate seed dispersal and is not recommended.

Fire: The use of fire has demonstrated mixed results for managing diffuse knapweed. Fire followed by vigorous grass regrowth can reduce knapweed stands. However, crown resprouts and increased seedling germination may eliminate any benefits from burning. An integrated approach using fire and herbicides may be more successful than herbicides alone. Applying the correct herbicide to newly emerged plants following a burn is an effective approach.

Biological: The bronze knapweed root borer (Spenoptera jugoslavica) and the banded gall fly (Urophora affinis) are the only two insects currently established in California for control of diffuse and spotted knapweed. They are both compatible for dual release in infested areas. There are several other insects which have been released in the United States for knapweed control.and are listed in Table 1. These biocontrol agents greatly vary in their ability to reduce knapweed seed and vegetative reproduction. Successful establishment may take from one to several years. They will not eradicate knapweeds. However, they may be effectively utilized to reduce knapweed populations over time.

Table 1: Insect species which have been introduced into the United States for the control of C. diffusa, C. stoebe, and C. virgata, and certain other related weedy species.

Species name Common name Stage,Damage Species attacked Habitat Establishment Current availability
Sphenoptera jugoslavica Bronze knapweed root borer Larvae, root feeding; some adult leaf damage C. diffusa, C. stoebe Warm, dry areas CA, ID, MT, OR, WA OR, WA
Urophora affinis Banded gall fly Larvae, developing seed heads C. diffusa, C. stoebe, C. virgata, Northwestern US in infested areas CA, ID, MT, OR, UT, WA Northwestern US
Agapeta zoegana Sulfur knapweed moth Larvae, root feeding, C. stoebe, C. diffusa Moderately humid and temperate CA, MT, OR, WA Limited due to collection difficulty
Bangasternus fausti Broad-nosed seed head weevil Larvae, seed head feeding C. diffusa, C. stoebe, C. virgata, Areas with hot, dry summers CA, MT, OR, UT Limited
Chaetorellia acrolophi Knapweed peacock fly Larvae, seed head feeding C. stoebe, C. calcitrapa, C diffusa, C. leucophaea, C. vallesiaca, C. virgata Moist habitats MT, OR Currently not available for general redistribution
Cyphocleonus achates Knapweed root weevil Larvae, root vascular tissue C. stoebe, C. diffusa Well drained soils with dense knapweed CA, CO, MT, OR, WA OR, Europe
Larinus minutus Lesser knapweed flower weevil Larvae, seed head; adult, rosettes and flowers C. diffusa, C. stoebe Hot, dry areas CA, MT, OR, WA Limited in OR and WA
Larinus obtusus Blunt knapweed flower weevil Larvae, seed head C. stoebe, C. diffusa More moist areas than L. obtusus WA Currently unavailable
Metzneria paucipunctella Spotted knapweed seed head moth Larvae, seed C. stoebe, C. diffusa Sites with good winter snow cover ID, MT, OR, WA ID, OR, WA
Pelochrista medullana Brown winged root moth Larvae, root cortex C. diffusa, C. stoebe Dry sites with high plant density None currently found Currently unavailable
Pterolonche inspersa Grey winged root moth Larvae, root vascular tissue C. diffusa, C. stoebe, C. virgata Mediterranean climate, no harsh winters MT Limited
Terellia virens Green clearwing fly Larvae, seeds C. stoebe, C. diffusa Wide range CA, MT, OR Limited in OR
Urophora quadrifasciata UV knapweed seed head fly Larvae, seed head C. diffusa, C. stoebe, C. virgata Western US CA, ID, MT OR, UT, WA Northwestern US

Chemical: Herbicides can be used to control existing stands of C. diffusa and C. stoebe and substantially reduce seed production. However, since the seed of both species is viable in the soil for up to seven years, retreatment will be necessary. Long term reductions in the seed bank must be the goal for effective knapweed management with herbicides. Proper timing of herbicide applications is critical to effective control. In California, there are three herbicides important for knapweed management: 2,4-D, dicamba, and clopyralid. All three are most effective when applied in the spring, when plants are beginning to bolt. 2,4-D should be applied at 2 lb ae/A; dicamba at 1 lb ae/A; and clopyralid at 0.25 lb ae/A. Clopyralid and dicamba are the most effective treatments. Both will provide some residual control, particularly clopyralid, and retreatments may be necessary in the second, third, or fourth years. Dicamba will injure or kill most other broadleaves it contacts, including desirable forage and native broadleaf species. Clopyralid is more selective, but will injure legumes such as clovers. 2,4-D is the least expensive treatment, but is less effective than dicamba and clopyralid, and retreatment will be required every year. 2,4-D will also injure other broadleaves, similar to dicamba. The most effective knapweed treatment is picloram applied at 0.25 lb ae/A. However, it is not labeled for use in California. Picloram will provide 100% residual control of C. diffusa and C. stoebe for 2-5 years after treatment. This herbicide will kill or severely injure most other broadleaf species it contacts in the soil or on the foliage including trees, forbs, and shrubs. No resistance to these herbicides has been documented in C. diffusa or C. stoebe. However, any herbicide management program should integrate rotation between herbicides to prevent the development of resistance.

Integrated management strategies: Seed production by C. diffusa or C. stoebe is typically 1000 times greater than required to maintain infestations. Thus, relatively few plants per acre are needed for rapid reinfestation. Management must be continuous or reinfestation is inevitable. Reseeding and establishment of competitive grasses or other native species is critical. Native bunchgrass communities are generally very resistant to knapweed invasion. However, almost any form of disturbance (including inclement weather such as hailstorms) may open a niche for invasion. Research has also shown that areas receiving 10-14 inches of precipitation annually are most susceptible to knapweed invasion, even in established perennial bunchgrass communities. The severity of knapweed infestations in other states such as Montana and Wyoming should serve as an indicator of the potential economic and environmental problems that knapweeds pose to California.

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