DISTRIBUTION, BIOLOGY, AND MANAGEMENT OF DIFFUSE KNAPWEED (CENTAUREA-DIFFUSA) AND SPOTTED KNAPWEED (CENTAUREA-MACULOSA)

Diffuse knapweed, a biennial or short-lived perennial, and spotted kna pweed, a perennial, are taprooted Eurasian weeds invading rangeland in the western United States and Canada. Knapweed (Centaurea spp.) invas ion is associated with reductions in biodiversity, wildlife, and lives tock 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 diff use 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 conta ining infestations and control efforts. Hand pulling in areas with sma ll infestations can be effective for controlling spotted and diffuse k napweeds. Picloram applied at 0.28 kg ha(-1) provides control for abou t 3 yr. Effective long-term control of knapweeds requires periodic app lications of picloram, which are only cost-effective on highly product ive range sites with a residual grass understory. About 12 insect spec ies have been released for knapweed biocontrol. Seed production has be en reduced by 46% by insects feeding in the flower heads. Although ins ects have not reduced spotted knapweed densities, they may stress the weed and shift the competitive balance to associated species. Sheep gr azing reduces the density of very young seedlings and may limit seedli ng recruitment into the population, In areas without a residual unders tory of desired plant species, revegetation of knapweed-infested range land is required, Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improv ed strategies.