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Distribution, Biology, and Management of Diffuse Knapweed (Centaurea diffusa) and Spotted Knapweed (Centaurea maculosa)

Published online by Cambridge University Press:  12 June 2017

Roger L. Sheley ,
James S. Jacobs  and
Michael F. Carpinelli
Roger L. Sheley
Affiliation:
Plant, Soil and Environmental Science, Montana State University, Bozeman, MT 59717-3120
James S. Jacobs
Affiliation:
Plant, Soil and Environmental Science, Montana State University, Bozeman, MT 59717-3120
Michael F. Carpinelli
Affiliation:
Plant, Soil and Environmental Science, Montana State University, Bozeman, MT 59717-3120
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Abstract

Diffuse knapweed, a biennial or short-lived perennial, and spotted knapweed, a perennial, are taprooted Eurasian weeds invading rangeland in the western United States and Canada. Knapweed (Centaurea spp.) invasion is associated with reductions in biodiversity, wildlife, and livestock forage, and increased erosion. Spotted knapweed grows to about 1 m and usually has purple flowers, whereas diffuse knapweed is slightly shorter, usually with white flowers. Persistent flower bracts on diffuse knapweed bear a rigid terminal spine about 8 mm long with four or five pairs of shorter lateral spines. Bracts on spotted knapweed have dark spotted tips. Knapweed management involves a combination of containing infestations and control efforts. Hand pulling in areas with small infestations can be effective for controlling spotted and diffuse knapweeds. Picloram applied at 0.28 kg ha−1 provides control for about 3 yr. Effective long-term control of knapweeds requires periodic applications of picloram, which are only cost-effective on highly productive range sites with a residual grass understory. About 12 insect species have been released for knapweed biocontrol. Seed production has been reduced by 46% by insects feeding in the flower heads. Although insects have not reduced spotted knapweed densities, they may stress the weed and shift the competitive balance to associated species. Sheep grazing reduces the density of very young seedlings and may limit seedling recruitment into the population. In areas without a residual understory of desired plant species, revegetation of knapweed-infested rangeland is required. Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies.

Keywords

Picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic aciddiffuse knapweed, Centaurea diffusa Lam. #3 CENDIspotted knapweed, Centaurea maculosa Lam. # CENMANoxious rangeland weedsintegrated weed managementsustainable weed managementCENDICENMA
Type
Symposium
Information
Weed Technology , Volume 12 , Issue 2 , June 1998 , pp. 353 - 362
Copyright
Copyright © 1998 by the Weed Science Society of America 

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