ID Links Flower Flowers & Buds Plant Leaves Seed Seedling Young Plants Winter1 Winter2 Knapweed Pulling Infestations Knapweed With Spittle Bug Leaves
Left: spotted knapweed leaves grown in the Whitehall High School greenhouse. Photo by WHS student Brian Edsall.
Right: spotted knapweed flower and buds. Photo by WHS teacher Todd Breitenfeldt.
Spotted knapweed is a short lived perennial that grows from 0.3 to 1 meter tall (3).
Spotted knapweed produces a stout taproot. (1) Seedlings usually stay in the rosette stage through the winter and continue growing in early April. (3)
The weed itself has alternately arranged, pale green leaves measuring about 2.5 to 7.5 centimeters (1-3 in.) long. Rosette leaves are deeply divided into lobes. Stem leaves have fewer lobes and become smaller toward branch tips. (2)
Spotted knapweed blooms from July through August, and bolts (grows long stems from the basal rosette of leaves) in early May. The individual flower heads bloom for two to six days before the bracts close. After about twenty days, the bracts reopen, and seeds are dispersed. (5) The flower buds occur at the end of the main stem and upper branches. The flowers are pink to light purple and occasionally white. (2, 5) Spotted knapweed seed heads are different of those of diffuse knapweed in that spotted knapweed's have a shorter center spine of the bracts and generally a black spot near the top. (2)
PROBABLE ENTRY INTO NORTH AMERICA/ NATIVE RANGE:
Spotted knapweed was probably introduced to North America as a contaminant of hay or alfalfa seed from Eastern Europe or Asia in the early 1900's (1, 3). Herbarium records at Montana State University indicate that spotted knapweed was first collected in Gallatin County in the mid- 1920s. It spread to additional counties by 1950, and now can be found in all 56 Montana counties. (3)
While an average spotted knapweed plant produces about 1,000 seeds annually, up to 18,000 seeds can be produced. This means that as few as 100 plants per acre can produce more than one million seeds.
The seeds may stay close to home or be transported by wind, water, vehicles, humans, or animals to other places. The smooth seeds are brown to black with pale, longitudinal lines. (4)
ENVIRONMENT FAVORABLE TO INFESTATION:
The threat of spotted knapweed is greatest in range dominated by blue bunch wheat grass, needle and thread or Idaho fescue, and in woodland dominated by Ponderosa pine or Douglas fir. Large infestations also develop on well-drained, light soils receiving summer rainfall. Spotted knapweed prefers a more moist environment than diffuse or squarrose knapweed. (3)
MONTANA INFESTATION/ IMPACT:
Spotted knapweed is the number one problem weed on Western Montana range lands. Infestations cause soil erosion, decrease biodiversity, and reduce forage for wildlife and livestock. (5)
The main way of controlling spotted knapweed is by determining the size and location of the infested areas. Small patches of spotted knapweed can be permanently destroyed with a persistent herbicide or cultural control program. However, an approach which uses cultural and biological methods, in conjunction with herbicides, is necessary to control large infestations and to slow the rapid spread of this weed. To be successful, management programs must continue annually over a period of 10 or more years; until the soil seed bank is used up. (3)
Long-term control of spotted knapweed is not possible with a single treatment because of the fact that the seeds remain viable in the soil for at least seven to ten years (the soil seed bank). Therefore, treatment programs must be continued until seed reserves in the soil are exhausted. (3)
The key to stopping the spread of spotted knapweed is early detection and treatment of invading plants. Picloram (Tordon), clopyralid (Stinger), 2,4-D and dicamba (Banvel) will control spotted and diffuse knapweed on range land. These herbicides are selective for broadleaf weeds and generally do not harm grasses when applied at recommended rates. (3)
Tordon is the most effective herbicide for controlling spotted and diffuse knapweed. Picloram at one-fourth pound of active ingredient per acre applied in early June will give 100 percent control for two to five years, depending on the sight condition. (3)
Herbicides should be applied to spotted and diffuse knapweed at the proper growth stage for best control. MSU research has shown that 2,4-D, Banvel, Stinger, and Tordon were most effective when applied in late May to early June. Herbicide effectiveness declines rapidly after flowering. (3)
Careful, tedious hand-pulling will control very small infestations of knapweed. Since seeds can remain viable in the soil seven to ten years, weeds must be continuously pulled until there are no more viable seeds. The entire plant must be pulled since regrowth can occur from the crown or root. For the best results, plants should be pulled in the spring when the soil is wet and before seeds are formed. Plants with seeds should be disposed of by deep burial or burning. (3) Other suggested methods are tilling (plowing) and grazing by goats (1).
Biological weed control is environmentally safe, selective, and very economical. Eight natural enemies, all native to Eurasia, have been introduced into Montana for control of spotted and diffuse knapweed. (4)
Two flies (Urophora affinis and U. quadrifasciata) attack seed heads and were introduced into Western Montana in 1973 and 1980. A seed head- attacking moth (Metzneria paucipunctella), was released in Montana in 1980. The larvae feed on the florets and seeds of the spotted knapweed. A root moth (Agapeta zoegana) and a root weevil (Cyphocleonus achates), were released in 1984 and 1988. The larvae of both insects cause big damage, often killing small knapweed plants. A root beetle (Sphenoptera jugoslavica), was released on spotted knapweed in 1983. Two root moths (Pelochrista medullana and Pterolonche inspersa) have been unsuccessful to date. (3)
Other controls of spotted knapweed are things such as goats. They will often eat the weed. Washing the bottoms of your vehicle or shoes after traveling in an infested area is always a good idea, to prevent the spread of this noxious weed. (1)
1. Breitenfeldt, Todd, Personal Interview, Biology teacher, Whitehall Schools, Box 1109,Whitehall, Mt. 59759. (406)-287-3862. 9-1-99.
2. Cranston, Roy. "Field Guide to Noxious Weeds and other Selected Weeds of British Columbia." [Website] http://www.agf.gov.bc.ca/croplive/cropprot/weedguide/spotknap.htm, 9-3-99.
3. Lacey, C.A, and Lacey, J.R. "Controlling Knapweed on Montana Rangeland." Montana State
University, Bozeman, Montana, 59717. "Identifying Knapweed," and "Methods of Control." 3-8-99.
4. Malone, Marty. "Spotted Knapweed (Centaurea maculosa.)" [Website]
5.) Rees, Norman., et. al., Ed. Biological Control of Weeds in the West, Western Society of Weed
Science, in cooperation with USDA ARS, Mt Dept. of Ag, and Mt state Univ., Color World
Printer. Feb. 1996.
By April Dyson
Published By Joe Wilson
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