Diffuse knapweed is a non-native biennial or short lived perennial noxious weed that originated in Eurasia. It rapidly invades pasture, rangeland, roadsides and meadows. It can cause a serious decline in forage and crop production, and is not preferred by livestock as forage.
Research shows that people are the major cause of knapweed spread. Knapweeds are spread in hay, and on vehicle undercarriages. Producers should use caution when using hay from road ditches, and when purchasing hay from other states.
Diffuse knapweed is 1-3 feet tall, and has fine gray-green foliage. The flowers on diffuse knapweed are usually white and bloom July – September in this area. Diffuse knapweed can reproduce by re-growth from the root, but it mainly reproduces by the spread of seed, with each plant producing 400-900 seeds.
The EC-130 Guide for Weed Management in Nebraska recommends several products to control diffuse knapweed. Herbicide applied at the rosette stage is successful, because that is when there is the most leaf surface to apply herbicide to. If the diffuse knapweed has been allowed to go to seed, monitor the areas for several years, because of the seed bank that may be present. If using mechanical control, pulling or cutting the plant, it is best to remove the rosette and part of the roots if possible, as the plant can regrow from the root.
There are two biological control insects approved for use on spotted and diffuse knapweed, however with the small number of acres infested at this time, herbicide control is encouraged.