Spread, Habitat and Impacts

Spread

Diffuse knapweed flowerheads have effective mechanisms for short- and long-distance dispersal. After seed production, the majority of heads remain closed with seeds intact until fall. Once the heads are fully dry, involucres on the sides of the heads open and seeds drop for short distance dispersal. For long-distance dispersal, plants break off at ground level and tumble in the wind, dispensing seeds individually through small openings at the tip of the heads. Seeds also spread long distances when the tumble weeds attach to vehicles or equipment. Rivers and irrigation systems can also carry entire plants and their seeds long distances. Colonization and spread of diffuse knapweed is enhanced when the competitiveness of the plant community is compromised by disturbance or limiting resource shortfalls. Prescribed grazing to maintain competitive grasses is important to reducing the spread of diffuse knapweed.

Habitat

Diffuse knapweed is native to the grasslands, shrub steppes, and fringe forest of eastern Mediterranean and western Asian locales with a climate characterized by arid summers. This species has demonstrated wide ecological amplitude in colonizing western North America. In North America, it is most invasive in shrub steppe with light, well-drained soils that support bitterbrush/bluebunch wheatgrass communities. It does not grow in poorly-draining soils or intense shade, but can dominate in open ponderosa pine and Douglas fir community types. Dry pastureland and meadows dominated by Kentucky bluegrass are susceptible to invasion. In Montana, the largest infestations are found on native grasslands and fringe forest areas near Big Timber, Ennis, and Helena.

Impacts

Significant environmental and economic impacts have been associated with diffuse knapweed infestations. A Washington State economic analysis estimated losses of nearly $1 million in pasture and rangeland grazing revenue and $2.9 million in lost hay revenue in 1988. A study on the Colorado Front Range found that species richness decreased as diffuse knapweed cover increased, and that total vegetation cover decreased and bare ground increased with increasing diffuse knapweed cover and density. These results support reports that diffuse knapweed infestations reduce biodiversity through their ability to form near monotypic stands. Also, apparent increases in soil erosion and reduced water infiltration and cycling attributed to increased bare ground and reduced bunchgrasses in diffuse knapweed infestations are supported by this study. Diffuse knapweed increases the cost of roadside maintenance, threatens natural area preserves, and reduces land values.

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Last Modified: 06/02/2008