The Enemy - Diffuse knapweed (Centaurea diffusa) this annual (and sometimes biennial) noxious weed is a native of Eurasia and spread into the U.S. around the 1900's. It is one of the only knapweeds that spread by tumbling by the wind like Russian thistle. The plant has a white flower, grows to a height of 24 inches, and has a short taproot. The leaves, like most knapweeds, are innately divided and are lighter green that its relatives. The bud is host to bracts of which project away from the bud and become quite rough to the skin. This plant can be seen around here in great abundance on the dike system that runs from Heike to Roberts along the southern shore of the Snake River.
The Strategy - Diffuse knapweed spread into lightly disturbed areas, and in some cases can invade crops such as alfalfa and grains. The plant is efficient at robbing the soils of nutrients and water and because it spreads quickly it can dominate a site to where there is no forage for livestock and wildlife. It particularly loves riparian and gravely sites.
The Defense - Diffuse knapweed is easily controlled mechanically. Once the plant shows up in abundant numbers then control methods should change to herbicides. As far as I know there are no biological controls, even livestock will not eat the stuff. Herbicides of choice include any of the Pyridine family: Tordon 22K, Milestone, or Curtail. Dicamba based products will work early in the season and it is strongly recommend not to use Roundup or 2,4-D. As this is like the other knapweeds getting an early start is most effective, but remember they spread fast and produce a lot of seeds.
Link to Tordon 22K label. Link to Milestone label.
Link to Curtail label.
PLEASE NOTE - The proper use and application of herbicides can be an effective way to control and eradicate noxious and invasive plants. Before using herbicides, always carefully follow the label and safety instructions on the label. While we recommend the use of herbicides as one of the effective tools for integrated pest management, the Idaho Weed Awareness Campaign assumes no liability for herbicide applications.
For more information, click on the link below to download the Idaho's Noxious Weeds Control Guidelines publication produced by the University of Idaho Extension.
U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)