20 May 2005 Page 1 of 4 

Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

http://www.nps.gov/plants/alien/

FACT SHEET: SPOTTED KNAPWEED

Spotted Knapweed 

Centaurea stoebe L. ssp. micranthos (Gugler) Hayek 

Sunflower family (Asteraceae) 

NATIVE RANGE 

Central Europe, east to central Russia, Caucasia, and western Siberia 

DESCRIPTION 

Spotted knapweed, previously known as Centaurea biebersteinii, is a biennial or 

short-lived perennial. Its name is derived from the spots formed by black margins on 

the flower bract tips. Spotted knapweed typically forms a basal rosette of leaves in its 

first year and flowers in subsequent years. Rosette leaves are approximately 8 

inches long by 2 inches wide, borne on short stalks, and deeply lobed once or twice 

on both sides of the center vein, with lobes oblong and wider toward the tip. The 

taproot is stout and deep. Flowering stems are erect, 8 to 50 inches tall, branched 

above the middle, and sparsely to densely hairy. Stem leaves alternate along the 

stem, are unstalked, and may be slightly lobed, or linear and unlobed. Leaf size 

decreases towards the tip of the stem. 

Flowers are purple to pink, rarely white, with 25 to 35 flowers per head. Plants bloom 

from June to October, and flower heads usually remain on the plant. Flower heads are oblong or oval shaped, 1⁄4 inch 

wide and 1⁄2 inch across, and are single or borne in clusters of two or three at the branch ends. Leaf like bracts 

surrounding the base of the flower head are oval and yellow green, becoming brown near the base. The margins of these 

bracts have a soft spine like fringe, with the center spine being shorter than the lateral spines. The brown, oval seeds are 

1/16 to 1/8 inch long, with pale longitudinal lines and a short fringe on one end. 

ECOLOGICAL THREAT 

Spotted knapweed infests a variety of natural and semi-natural habitats including barrens, fields, forests, prairies, 

meadows, pastures, and rangelands. It outcompetes native plant species, reduces native plant and animal biodiversity, 

and decreases forage production for livestock and wildlife. Spotted knapweed may degrade soil and water resources by 

increasing erosion, surface runoff, and stream sedimentation. It has increased at an estimated rate of 27% per year since 

1920 and has the potential to invade about half of all the rangeland (35 million acres) in Montana alone. 

DISTRIBUTION IN THE UNITED STATES 

Spotted knapweed is a widely distributed species reported to occur throughout 

Canada and in every state in the U.S. except Alaska, Georgia, Mississippi, 

Oklahoma and Texas. It has been designated as a noxious weed in Arizona, 

California, Colorado, Idaho, Minnesota, Montana, Nebraska, Nevada, New 

Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and 

Wyoming. 

It has been identified as invasive in natural areas by eighteen organizations in 

twenty-six states (Arizona, California, Colorado, Connecticut, Delaware, 

Kentucky, Idaho, Illinois, Massachusetts, Maryland, Michigan, Minnesota, Montana, North Carolina, New Jersey, New 

York, Oregon, Pennsylvania, South Dakota, Tennessee, Utah, Virginia, Washington, Wisconsin, West Virginia, and 

Wyoming). Fifteen national parks also identify spotted knapweed as an invasive plant and a threat to natural habitats. 

HABITAT IN THE UNITED STATES 

Spotted knapweed is found at elevations up to and over 10,000 feet and in precipitation zones receiving 8 to 80 inches of 

rain annually. Spotted knapweed prefers well-drained, light-textured soils that receive summer rainfall, including open 

forests dominated by ponderosa pine and Douglas fir, and prairie habitats dominated by Idaho fescue, bluebunch 
20 May 2005 Page 2 of 4 

Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

http://www.nps.gov/plants/alien/

wheatgrass, and needle-and-thread grass. Disturbance allows for rapid establishment and spread; however, spotted 

knapweed is capable of invading well managed rangelands. Spotted knapweed does not compete well with vigorously 

growing grass in moist areas. In seasonally dry areas, spotted knapweed's taproot allows it to access water from deep in 

the soil, beyond the reach of more shallowly rooted species. 

BACKGROUND 

Spotted knapweed was introduced to North America from Eurasia as a contaminant in 

alfalfa and possibly clover seed, and through discarded soil used as ship ballast. It was 

first recorded in Victoria, British Columbia in 1883 and spread further in domestic 

alfalfa seeds and hay before it was recognized as a serious problem. 

BIOLOGY & SPREAD 

Spotted knapweed plants in North America generally live 3 to 7 years but can live up to 

nine years or longer. Plants regrow from buds on the root crown. Reproduction is by 

seed, and plants are capable of producing 500- 4,000 seeds per square foot per year. 

About 90% of the seeds are viable at the time of dispersal, and they can remain viable 

in the soil for 5-8 years. Most seeds are dispersed near the parent plant but can be 

transported by people, wildlife, livestock, vehicles, and in soil, crop seed, and 

contaminated hay. Gravel pits, soil stockpiles, powerlines, grain elevators, railroad and 

equipment yards are important seed distribution points. 

MANAGEMENT OPTIONS 

The most cost effective management strategy for spotted knapweed is to prevent its 

spread to non-infested areas. Spread by seed can be minimized by avoiding travel 

through infested areas; by cleaning footwear, clothing, backpacks, and other items after hiking through infested areas; by 

not grazing livestock when ripe seeds are present in the flower heads; and by using weed free hay. 

Manual and Mechanical 

Small infestations of spotted knapweed can be controlled by persistent hand-pulling done prior to seed set. Gloves should 

be worn because of the possibility of skin irritation. Because spotted knapweed can regrow from the base, care must be 

taken to remove the entire crown and taproot. 

Biological 

A variety of natural enemies are used as biological control agents for large infestations of spotted knapweed. Most 

biocontrol techniques use insect larvae to damage the root, stem, leaf, or flower. Two species of seed head flies, 

Urophora affinis and U. quadrifasciata, are well-established on spotted knapweed. The larvae of these species reduce 

seed production by as much as 50% by feeding on spotted knapweed seed heads and causing the plant to form galls. 

Three moth species (Agapeta zoegana, Pelochrista medullana, and Pterolonche inspersa) and a weevil (Cyphocleonus 

achates) that feed on spotted knapweed roots have also been released. 

The collective stress on the plant caused by these insects reduces seed production and may lead to reduced 

competitiveness. Biological control agents may be more effective when combined with other control methods such as 

herbicides, grazing, and revegetation with desirable, competitive plants. 

Chemical

Control of spotted knapweed infestations using three chemical herbicides (2,4-D, clopyralid, and picloram) has been 

reported but is problematic. Existing plants can be killed with 2,4-D but it needs to be reapplied yearly to control new 

plants germinating from seed stored in the soil. Picloram is a more persistent herbicide and has controlled knapweed for 

three to five years when applied at 0.25 lb/acre at any stage of plant growth; or with clopyralid (0.24 lb/acre) or clopyralid 

(0.2 lb/acre) plus 2,4-D (1 lb./acre) applied during bolt or bud growth stage. In the absence of desirable native grasses, 

longevity of control may be increased by revegetating with competitive grasses and forbs. Picloram may pose a risk of 

groundwater contamination where soils are permeable, particularly where the water table is shallow. 

Other 

Long-term grazing by sheep and goats has been found to control spotted knapweed. Burning, cultivation, and fertilization 

typically are not effective on spotted knapweed unless combined with other methods of control. 
20 May 2005 Page 3 of 4 

Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

http://www.nps.gov/plants/alien/

USE PESTICIDES WISELY: Always read the entire pesticide label carefully, follow all mixing and application instructions and wear all 

recommended personal protective gear and clothing. Contact your state department of agriculture for any additional pesticide use 

requirements, restrictions or recommendations. 

NOTICE: mention of pesticide products on this page does not constitute endorsement of any material.

CONTACTS 

For more information on the management of spotted knapweed, please contact: 

• Michael Carpinelli, USDA-ARS, Burns, OR (541-573-8911, michael.carpinelli at oregonstate.edu) 

• Steve Dewey's Weed Web (http://www.ext.usu.edu/ag/weeds/index.htm) 

• Peter Rice, Montana Noxious Weed Trust (http://invader.dbs.umt.edu/) 

OTHER LINKS 

• http://www.invasive.org/species/subject.cfm?sub=3013 

• http://nbii-nin.ciesin.columbia.edu/ipane/icat/browse.do?specieId=45

AUTHOR 

Michael Carpinelli, USDA-ARS, Burns, OR 

EDITOR

Jil M. Swearingen, National Park Service, National Capital Region, Center for Urban Ecology, Washington, DC 

REVIEWERS 

Ruth Douglas, Charlottesville, VA 

Karen Enyedy, Austin, TX 

Jim Jacobs, Montana State University, Bozeman, MT 

Lori Makarick, National Park Service, Grand Canyon National Park, Grand Canyon, AZ 

Phillip Moore, AHTD, AR 

Monica Pokorny, Montana State University, Bozeman, MT 

Leila M. Shultz, Harvard University Herbaria, Cambridge, MA 

Ed Shadrick, V3 Consultants, Woodridge, IL 

PHOTOGRAPHS 

Washington State Noxious Weed Control Board 

REFERENCES 

Boggs, K.W. and J.M. Story. 1987. The population age structure of spotted knapweed (Centaurea maculosa) in Montana. 

Weed Sci. 35:194 198. 

Chicoine, T.K. 1984. Spotted knapweed (Centaurea maculosa L.): Control, seed longevity and migration in Montana. MS 

Thesis Montana State University. Bozeman, Mont. 96 pp MIN 25 8430038. 

Chicoine, T.K., P.K. Fay, and G.A. Nielsen. 1985. Predicting weed migration from soil and climate maps. Weed Sci. 34:57 

61. 

Groh, H. 1944. Canadian weed survey. 2nd Ann. Rep. Can. Dep. Agric. 

Kartesz, J.T. 1994. A Synonymized Checklist of the Vascular Flora of the United States, Canada, and Greenland. Vol.1 - 

Checklist. 2nd ed. Timber Press, Portland, OR. 622 p. 

Lacey, J.R., C.B. Marlow, and J.R. Lane. 1989. Influence of spotted knapweed (Centaurea maculosa) on surface water 

runoff and sediment yield. Weed Technol. 3:627 631. 
20 May 2005 Page 4 of 4 

Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

http://www.nps.gov/plants/alien/

Lacey, C.A., J.R. Lacey, P.K. Fay, J.M. Story, and D.L. Zamora. 1995. Controlling knapweed in Montana rangeland. 

Circular 311, Cooperative Extension Serv., Montana State Univ. Bozeman. 

Mauer, T., M. J. Russo, and M.Evans. 1987. Element Stewardship Abstract: Spotted Knapweed (Centaurea maculosa). 

The Nature Conservancy 

Muller, H., D. Schroeder, and A. Gassmann. 1988. Agapeta zoegana (L) (Lepidoptera: Cochylidae), a suitable prospect 

for biological control of spotted and diffuse knapweed, Centaurea maculosa Monnet De La Marck and Centaurea 

diffusa Monnet De La Marck (Compositae) in North America. Can. Ent. 120:109 24. 

Rees, N.E., P.C. Quimby, Jr., G.L. Piper, E.M. Coombs, C.E. Turner, N.R. Spencer, and L.V. Knutson. 1996. Biological 

control of weeds in the west. Western Society of Weed Science, USDA Agric. Res. Serv., Mont. Dept. Agric., 

Mont. State Univ. 

Roché, B.F. Jr., and C.J. Talbott. 1986. The collection history Centaurea found in Washington State. Agri. Res. Center. 

Res. Bull. XB0978. Washington State University Cooperative 

Roche, B.F., G.L. Piper, and C.J. Talbott. 1986. Knapweeds of Washington. Washington State Univ. Coop. Exten. 

EB1393. 41p. Extension, Pullman, WA. 36 pp. 

Sheley, R.L., M. Manoukian, and G.Marks. 1996. Preventing Noxious Weed Invasion. Rangelands. 18(3):100 101. 

Sheley, R.L., J.S. Jacobs, and M.F. Carpinelli. 1998. Distribution, biology, and management of diffuse (Centaurea diffusa) 

and spotted knapweed (Centaurea maculosa). Weed Technol. 12:353 362. 

Sheley, R.L., J.S. Jacobs, and M.F. Carpinelli. 1999. Spotted knapweed. In: R.L. Sheley and J.K. Petroff (eds.), Biology 

and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis, OR. pp. 350 61. 

Shirman, R. 1981. Seed production and spring seedling establishment of diffuse and spotted knapweed. J. Range 

Manage. 34:45 47. 

Spoon, C.W., H.R. Boweles, and A.Kulla 1983. Noxious weeds on The Lolo National Forest. A situation analysis staff 

paper. USDA Forest Ser. North. Reg. 33p. 

Story, J.M., K.W. Boggs, W.R. Good, P. Harris, and R.M. Nowierski. 1991. Metzneria paucipunctella Zeller (Lepidoptera: 

Gelechiidae), a moth introduced against spotted knapweed: its feeding strategy and impact on two introduced 

Urophora spp. (Diptera: Tephritidae). Can. Entomol. 123:1001-1007. 

Swearingen, J. 2009. WeedUS Database of Plants Invading Natural Areas in the United States: Spotted Knapweed 

(Centaurea stoebe ssp. micranthos). http://www.invasive.org/weedus/subject.html?sub=3013. 

Thompson, M.J. 1996. Winter foraging response of elk to spotted knapweed removal. Northwest Sci. 70(1):10 19. 

Tyser, R.W. and C.H. Key. 1988. Spotted knapweed in natural area fescue grasslands: an ecological assessment. 

Northwest Sci. 62:151 160. 

USDA, NRCS. 2009. The PLANTS Database (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 

70874-4490 USA. 

Virginia Department of Conservation and Recreation. 1999. Invasive Alien Plant Species of Virginia: Spotted knapweed 

(Centaurea maculosa Lam.). 2 p. 

Whitson, T.D. et al. 2001. Weeds of the West. Western Society of Weed Science. 628 pp.