War on Weeds: Squarrose knapweed


This perennial weed has a small presence in Idaho, thought to have been brought in in grass seed. This terrible invader can be found in all the states south of Idaho. It is listed as an EDRR (early-detection rapid-response) noxious weed. Squarrose knapweed is a long-lived taprooted perennial with pink to white flowers with downward-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 an individual plant with few growing shoots, and it does not have the dark-colored bracts on the flower head.

Attack: The difference in this knapweed 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 Bureau of Land Management in central Utah spends a lot of resources to contain and control. Each plant can produce thousands of seeds that remain viable for seven to 10 years. The plant has no forage value for livestock nor wildlife.

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 difficult and there are just a few biological control insects available. Numerous herbicides are effective on this weed. On pasture and range sites, herbicides such as Tordon 22K, Milestone, Opensight, Curtail and Prescott are most effective. 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. Call your weed-control superintendent if you think you have found this new invader.