The Strategy – Spotted Knapweed (Centaurea stoebe (maculosa) ssp. micranthos) is a short lived perennial native to Europe. It tends to grow to a height of 3 foot and has a single tap root that can go as deep as 18 inches. This plant has a pink flower with brown bracts (spots) on the flower head and a minimal amount of leaves on the mature plant.

The Attack – This is probably the most widely spread weed in North America. Due to it’s ability to be spread by the wind it can quickly establish itself in dry and moist areas. The plant releases a chemical which prevents other plants from growing near it (allelopathic) and the seed can remain viable for up to 8 years. The plant is rarely feed on by livestock and will replace the grasses and forbs in pasture and range situations. In fact in Montana there have been parcels of land that have been condemned by the authorities due to the abundance of the plant.

The Defense – This plant is not difficult to control, just grows fast and everywhere, thus people have a tendency to give up on it. Mechanical control is very effective and is why we do not see it in cultivated crops (except hay). Numerous insects have been released to control the flowers as well as the root growth of the plant. Sheep and goats can be made to eat the invader.

Herbicides such as Milestone®, Opensight® (formerly known as Chaparral®), Tordon 22k®, and Curtail® offer great control in the spring or even fall.

Multi-year programs including re-planting of desirable species is a must to control this plant. Many species of knapweed exist so make sure you know your pest prior to controlling them. Call your county weed superintendent for positive identification and advise.


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.