Squarrose Knapweed
Squarrose Knapweed

The Enemy – Squarrose knapweed (Centarrea virgata). Is a perennial weed that has yet to be found in Idaho. This terrible invader can be found in all the states south of Idaho, especially Utah and it is just a matter of time before it is found in Idaho. It is listed as a EDRR (Early Detection Rapid Response) noxious weed. Squarrose knapweed is a long lived taprooted perennial with pink to white flowers with curved bracts. The leaves are deeply lobed and it grows in a single clump to a height of 2 feet. It looks like Spotted knapweed, but grows like a small bush instead of a plant with few growing shoots.

The Strategy – The difference in this knapweed as compared to the others is that it adapts well to desert habitats and overtakes all the native flora. This is why it is one of the weeds that the BLM in central Utah spends a lot of resources to contain. Each plant can produce thousands of seeds that remain viable for 7-10 years. The plant has no forage value for livestock nor wildlife.

The Defense – The thought that this weed is most-likely in the area and undetected is a great concern to all of us in the weed control profession. Mechanical control is very difficult and there are just a few biological control insects available for control. Numerous herbicides are effective on this weed. Herbicides such as Tordon® 22k, Milestone®, Curtail, and Redeem. All of these herbicides will translocate into the root for better control than 2,4-D. Fall is one of the best times to control this invader. Please call your weed control superintendent if you think you have found this new invader.


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.