Notice the dark spots on the bracts under the blossom, this is where the weed gets its name Spotted Knapweed.
Introduction Spotted knapweed is the number one problem on rangelands in western Montana.  Spotted knapweed is dramatically interfering with the Montana’s unique ecosystems.  It was accidentally introduced to the United States through contaminated alfalfa and contaminated soil used as ship ballast.
The first report of this weed in Ravalli County was 1921. Knapweed has infested 2-5 million acres in Montana, with potential to spread to the 34 million susceptible acres in the state.  Spotted knapweed has been recorded in every county in Montana.
Concerns Knapweed has changed the face of our lands in western Montana. Ranchers, wildlife enthusiast, hunters, foresters, hikers, and fisherman are impacted by the dramatic affect that knapweed is having on our ecosystem.  Spotted knapweed is particularly hard to manage due to its ability to occupy a variety of sites and habitats.  This weed will aggressively out-compete native vegetation.
Watersheds throughout the state are reaping the negative effects of the soil erosion that knapweed causes.  The soil erosion caused by spotted knapweed is degrading fisheries and water quality.  Spotted knapweed is impacting Montana’s economy in a serious way. The negative effect knapweed has on Montana’s economy is an estimated $42 million annually.
Identification Spotted knapweed is a pinkish-purple flower and the plant grows 1-3 feet tall.  The bracts underneath the flowers have dark spotted tips that are also fringed. Spotted knapweed blooms from mid to late July and through September.  Each individual flower will bloom for 2-6 days.  The flowers will reopen after 20 days to disperse seeds. One knapweed plant can produce up to 1000 seeds and remain viable in the soil for 8 years. You will begin to see spotted knapweed rosettes in early April.  Knapweed can grow to nearly 4 feet tall on moist sites.
What can you do? For pastures and rangelands that have not been infested, prevention is key.  Avoid transporting seeds from infested sites.  Seeds can attach themselves to vehicles, clothes, and animals.  Use only certified weed-seed free seed and hay in non-infested areas. For infested areas integrated weed management methods need to be employed.  Small infestations can be controlled through timed hand pulling, which should take place when the soil is moist and prior to the plant going to seed. Pulled plants should be bagged and disposed of or burned.
Plowing an infested area is feasible, if plowed down to 7 inches below soil surface and if the area is reseeded with desirable vegetation.  Plowing may cause dormant knapweed seeds in the soil to germinate.  This method is best if used with herbicide treatment or grazing.  Sheep, goats, and cattle will graze spotted knapweed at certain times.  Grazing on knapweed should be timed for when the grasses have gone dormant.  Sheep grazing in combination with herbicide use can be fairly effective.  There are multiple insects that exist in Montana as a mean of controlling knapweed. The success of knapweed eradication requires a long-term commitment.
Visit the MWCA Weed ID pages for additional information and pictures of spotted knapweed.
This series of articles was developed by Ravalli County. If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.
