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.

ON THE MADISON RIVER — Floating along the shoreline in his drift boat, Travis Morris pointed to a 25-yard stretch of purple flowers.

“That’s a huge infestation right there,” Morris said.

The flowers, better known as Canada thistle, look pretty. But the plant is actually a weed that takes over any ground it comes in contact with and chokes out native plants. And without those native plants, the riverbanks erode, water quality deteriorates and fish can’t reproduce.

“This river is in a spot where the weeds are here, but they can still be controlled,” said Matt Wilhelm, education director for the Livingston-based Center for Aquatic Nuisance Species.

Wilhelm and Morris, president of the Bozeman chapter of Trout Unlimited, floated the Madison on Tuesday looking for noxious weeds and recording their location with GPS coordinates.

Read the complete article on the Bozeman Chronicle website.

Dr. Matthew Rinella has completed a project that will allow you in about 30 minutes with a pencil, clipboard, yardstick and a homemade measuring frame to figure out how weeds are impacting you.  Dr. Rinella’s weed calculator will tell you how many pounds of weeds are on your property and if they were gone how much more good forage you would have.

This tool is great for ranchers and small acreage owners.   If you are raising cattle, sheep, a few horses or like to watch elk graze you will find the results valuable.

The tool currently only will provide information on leafy spurge and spotted knapweed.   Dr. Rinella hopes to expand this tool for use with other weeds.   It is available free online.

Summary: Seed head insects, primarily the seed head fly, Urophora affinis, and the seed head weevils, Larinus spp., are reducing spotted knapweed seed production by about 94% in most areas of western Montana. Studies were conducted on the compatibility of seed head biological control agents and mowing for management of spotted knapweed.  Our study demonstrated that mowing of spotted knapweed in the spring and early summer can result in the growth of secondary flower buds which escape attack by seed head biocontrol agents, thereby allowing the knapweed to produce a nearly-normal complement of seed.  Therefore, we conclude that the historical practice of mowing spotted knapweed during the spring and early summer with no follow-up mowings should be avoided if large populations of seed head biocontrol agents are present.

Download a copy of the research article.

Authors: Jim M. Story, Lincoln Smith, Janelle G. Corn & Linda J. White

Summary:  Studies were conducted on the impact of seed head insects on spotted knapweed reproductive potential in western Montana over a 30-year period.  Results indicated that seed head insects are reducing seed production by about 94% in many areas of western Montana.  The reduction of knapweed seed production has resulted in a 98% reduction in the knapweed seed bank which, in turn, has contributed to the decline of knapweed in these areas.  Of the seed head insects, the fly, Urophora affinis, and the weevils, Larinus obtusus and L. minutus, are having the greatest impact on knapweed seed production.  The current decline of spotted knapweed in many areas is likely due to the impact of the seed head insects on the knapweed seed bank and the effects of the root weevil, Cyphocleonus achates, on mature plants.

Download a copy of the research article.

Authors: J.M. Story, N.W. Callan, J.G. Corn, L.J. White

Summary:  Spotted knapweed plant density was monitored over a 11-year period (1993-2004) at two sites in western Montana where the root weevil, Cyphocleonus achates was released.  Spotted knapweed density declined significantly over time at both sites (99% and 77%, respectively), after C. achates numbers increased dramatically at both sites.  The current decline of spotted knapweed in many areas is likely due to the effects of C. achates on mature knapweed plants and the impact of the seed head insects on the knapweed seed bank.

Click here to download and read the complete article.

Prepared by:

Phil Trunkle and Pete Fay
Department of Plant and Soil Science
Montana State University
Bozeman, Montana 59717

Download a PDF version of this document here.

The economic impact of thee invasive, exotic weeds–diffuse, spotted, and Russian knapweed (Centaurea diffusa, C. maculosa, and Acroptilon repens)–on Montana’s economy was estimated using a procedure developed for another invasive weed species. Published data and that from a survey of county weed boards were used to estimate direct negative impacts of over $14 million annually due to infestation of over 2 million acres of rangeland and wildland. This amounts to about $10.63 on each infested grazing land acrea nd $3.95 on each infested wildland acre. Direct plus secondary economic impacts, estimated using an input-output model, are about $42 million annually,, which could support over 500 jobs in the state’s economy. This first approximation suggests the knapweed infestation problem in Montana deserves attention, although more work could be done to refine these estimates and to allow estimation of he impacts at sub-state levels.

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