Identification and Control of Knapweed Species in Central and Eastern Oregon Amy Jo Waldo Four Species of Centaurea Are (Centaurea Solstitialis)
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EC 1559 • February 2003 $2.00 Identification and Control of Knapweed Species in Central and Eastern Oregon Amy Jo Waldo Four species of Centaurea are (Centaurea solstitialis). These Potential hazards commonly found in central and four species are a serious prob- eastern Oregon, including spot- lem throughout Oregon and the Knapweed species are con- ted knapweed (Centaurea Pacific Northwest. Stands of sidered noxious weeds. Their maculosa), diffuse knapweed knapweed can choke out natu- abundance threatens to exclude (Centaurea diffusa), Russian ral vegetation by competing for native and desirable plants and knapweed (Centaurea repens), water and nutrients. to cause economic and ecologi- and yellow starthistle Knapweeds’ ability to spread cal losses. Knapweed species rapidly by seed and roots on can be toxic to the skin for some disturbed soil makes control individuals, so be sure to wear difficult. Seeds can be carried gloves when pulling them. Pro- and spread by wind, water, foot longed consumption of Russian traffic, wildlife, and livestock, knapweed and yellow star- and in hay feed. Once a seed thistle by horses can lead to the “bank” is established in the soil, development of “chewing it can take several years to disease,” a nervous system dis- eradicate a knapweed species order that can result in death. from a site. Knapweeds bloom white, pink, purple, or yellow from Management June through October. See Timing is important for effec- page 3 for descriptions of these Figure 1.—Russian knapweed, rosette tive control. Unfortunately, stage. weeds. Figures 1–9 show the most people do not recognize a four species in their rosette and knapweed species until it is mature stages. Control methods flowering. By that time, hand are discussed below. Figure 2.—Russian knapweed, flowering. Figure 3.—Diffuse knapweed, rosette Figure 4.—Diffuse knapweed, flowering. stage. Amy Jo Waldo, area Extension horticulture agent, Oregon State University. removal and chemical control All of these weeds can be left • Grasses that could be used in are less effective than they are on the ground once pulled or areas with greater annual against younger plants. mowed, as long as they are not precipitation (15 to 20 inches) Practicing a combination of flowering and have not gone to include bromes (Bromus mechanical, cultural, chemical, seed. If the plant is flowering or spp.), orchardgrass (Dactylus and biological control is by far setting seed, remove the flower glomerata), Idaho fescue the most effective way to man- or seed head. Place it in a plas- (Festuca idahoensis), tall fescue age these species. The key to tic bag and dispose of it in your (Festuca arundinacea), and control is a sustained, continu- garbage. meadow foxtail (Alopecurus ous effort. Moderate grazing by live- pratensis). stock (except horses) is effective Mechanical control for suppression of yellow Chemical control starthistle, spotted knapweed, If you have large areas of and diffuse knapweed in Several herbicides are effec- spotted knapweed, diffuse pastures. tive against diffuse knapweed, knapweed, or yellow starthistle, Russian knapweed, spotted you can initiate control by Cultural control knapweed, and yellow burning, mowing, or pulling by starthistle. Current herbicide hand. Once you have cleared an options are listed in the annual Prescribed or controlled area of knapweed, consider edition of the Pacific Northwest burning can be a very success- how you want to use the land Weed Management Handbook (see ful way to control noxious and how to prevent reinfes- “For more information”). Be weeds. Fire quickly destroys all tation. Planting and managing sure to read the label and fol- aboveground parts of knap- competitive grasses and wild- low all instructions when weed plants. In some cases, flowers is one of the best ways using a pesticide! heat from the fire may stimulate to prevent new weeds from Successful weed treatment rapid seed germination, result- establishing. Native or peren- with herbicides depends upon ing in quick emergence of a nial grasses are a good choice in timing and appropriate chemi- great number of weeds. By most areas. cal use. Herbicide effectiveness speeding up the germination Be sure to consider your varies depending on the growth process, the seed bank is planting site, soil, and amount stage of the weed. For example, reduced more quickly and you of available irrigation when some knapweed species are can manage these weeds sooner selecting plants. The following more susceptible to certain her- rather than later. grasses are well-adapted to the bicides as a young plant in the For spotted and diffuse knap- high desert environment. rosette stage (Figures 1, 3, 5, weed, burning is effective any • For an area with 9 to and 7) than they are in the flow- time during the growing sea- 15 inches of annual precipita- ering stage. The herbicide label son. For yellow starthistle, tion, you could use several of will indicate directions for use burning is most successful at the wheatgrasses, including and the best time to apply the the very early flowering stage. crested (Agropyron deser- product to the targeted weed. Burning is not as effective torum), bluebunch (Agropyron Consider whether to use a against Russian knapweed, spicatum), Sherman big (Poa selective or a nonselective her- which spreads by roots. ampla ‘Sherman’), western bicide based on the location of Spotted knapweed, diffuse (Pascopyrum smithii), the knapweed species. (Selec- knapweed, and yellow thickspike (Elymus lanceo- tive herbicides target only spe- starthistle can be mowed latus), Siberian (Agropyron cific kinds of weeds, whereas repeatedly depending on the sibiricum), intermediate nonselective herbicides target a time of year and plant growth (Agropyron intermedium), and broad range of weeds with few stage. Mowing is effective until streambank (Elymus exclusions.) Special consider- the plant reaches the flowering lanceolatus ‘Sodar’). Indian ation should be given to herbi- stage; after the plant flowers, ricegrass (Oryzopsis cide use in a pasture; some mowing will spread new seeds. hymenoides), sheep fescue selective herbicides will target a Mowing is not as effective (Festuca ovina), Idaho fescue knapweed species without kill- against Russian knapweed. (Festuca idahoensis), and ing desirable grasses. All four species can be pulled Canada wildrye (Elymus Do not add chemically by hand anytime throughout canadensis) also can be used treated plants to your compost the year. Be sure to wear gloves. in these locations. pile. Biological control production of new diffuse and These biological control spotted knapweed seeds by methods still are being Several biological control attacking the flowers. This evaluated. They seem to be agents such as weevils and flies insect is providing excellent showing success; however, the are being introduced for knap- control in these areas. Other result is slow control, not weed management. In drier insects, such as the hairy weevil immediate eradication. A list of parts of Oregon and Washing- (Eustenopus villosus), provide biological control agents for ton, Larinus minutus (lesser effective control of yellow knapweed is available in the knapweed flower weevil) is starthistle. current edition of the Pacific effective in reducing the Northwest Weed Management Handbook. Identification of Knapweeds Spotted knapweed Diffuse knapweed Russian knapweed Yellow starthistle Centaurea maculosa Centaurea diffusa Centaurea repens Centaurea solstitialis Growth habit 1–3 ft tall. Biennial or short- 1–2 ft tall. Upright branching 1.5–3 ft tall. Creeping, clump- 1–3 ft tall. Winter annual that lived perennial. A rosette annual or short-lived perennial forming perennial that spreads forms a rosette in the beginning forms the first year, followed (biennial). Smaller in size than by both seed and rhizome. of the season, followed by by flowering stalks the second spotted knapweed. upright, rigid, branching stems. year. Leaves Medium green color with a Silvery-green. Lower leaves are Silvery-green. Lower leaves are Grayish-green. Basal leaves are silvery-gray cast. On young divided; upper leaves are long and usually lobed; upper deeply lobed. Upper leaves are plants, the leaves are deeply narrow and elliptical in shape. leaves are smaller and toothed smaller and pointed. lobed, becoming more narrow or entire. and elliptical on mature plants. Root system Stout taproot. Elongated taproot. Dark brown to black, Taproot. horizontal. Flower Solitary purple, pink, or White, rose, or purple. Bracts Pink to purple. Bracts are Single, terminal, bright yellow sometimes white ray flowers are covered with distinct somewhat pearly and papery, bracts are covered with spines about 1–1.5 inches across. A “comblike” spines (Figure 4). similar to parchment paper that can be up to 0.75 inch long key feature is the flower (Figure 2). (Figure 8). bract, which has dark, fringed tips (Figure 6). Culture Tolerates both dry and moist Establishes in dry, sunny Tolerates both dry and moist Adapts to both dry and moist areas, sunny or shady condi- locations on roadsides, waste environments, sunny and environments, most soil types, tions. This plant will occupy areas, and pastures. shady locations. Usually and a sunny or shady environ- roadsides, pastures, or hills. establishes in pastures. ment. Establishes in pastures, More common than diffuse or along roadsides, and in waste Russian knapweed in central areas. Oregon. For more information French, K., Burrill, L.C., Butler, T.V. 1999. Problem Thistles of Oregon, EC 1288. Larson, L., Sheley, R., McInnis, M., and Kiemnec, G. 1995. Yellow Starthistle Ecology and Management on PNW Range- lands, EM 8580. Whitson, T.D., Burrill, L.C., Dewey, S.A., Cudney, D.W., Nelson, B.E., Lee, R.D., Parker, R. 1992. Weeds of the West. Pacific Northwest Weed Manage- Figure 5.—Spotted knapweed, rosette Figure 6.—Spotted knapweed, flowering. stage. ment Handbook (revised annually). Many OSU Extension Service publications may be viewed or downloaded from the Web. Visit the online Publications and Videos catalog at http://eesc.oregonstate.edu. Copies of our publications and videos also are available from OSU Extension and Experiment Station Communi- cations.