Weed Of The Month: Spotted Knapweed
Author: Allison Kosto, MSU Broadwater County Extension Agent
Weed of The Month: Spotted Knapweed
Allison Kosto
MSU Broadwater County Extension Agent
This month is in recognition of my very first “weed of the month” article back in 2016 in the Broadwater Reporter. Even though I’ve already talked about Spotted Knapweed, many years ago, it’s time for a refresh because spotted knapweed is one of the most costly and widespread weeds in the state of Montana, estimated to cost the state $42 million per year in direct and indirect costs.
Description & Habitat
Native to Europe and Russia, spotted knapweed was introduced to the United States as a contaminant in alfalfa and through discarded soil used as ship ballast. It was first recorded in Montana in 1920 in Ravalli County. A state-listed noxious weed; it is now found in every county in the state making it the most widespread weed in Montana. It is highly adaptable and likes sunny areas with well-drained or gravelly soils and disturbed areas. However, it can also be found in forests, rangeland, roadsides, pastures and even lawns.
There are several different types of knapweeds, and they are sometimes easily confused. Spotted knapweed is a tap-rooted perennial (life cycle of more than two years) plant that spreads by seed. Plants are 8 to 50 inches tall. The stems are grayish green with many branches, slender leaves and have a single flower at the tip of each branch. The flower is usually pink-purple but can also be light purple or white. The black bracts underneath the flower head give the “spotted” appearance. One plant can produce up to 300 flowerheads and 140,000 seeds.
Spotted knapweed can displace desirable plants, reduce native plants, decrease forage available for wildlife and livestock, alter ecosystems and decrease land value. Spotted knapweed is known for having allelopathic chemicals that suppress the growth of other plants around it giving it a competitive advantage. It can also cause “chewing disease” in horses which is a neurological disorder that causes brain lesions and ulcers.
Management
Prevention is key to stopping spotted knapweed from getting established. Vehicles are a large spreader of seeds even in dry conditions. If you are on land with knapweed, wash vehicles, ATVs and any equipment before taking it home. Do not drive or walk in areas that are heavily infested if possible. Purchase certified noxious weed seed-free hay.
Hand-pulling spotted knapweed is difficult due to the long taproot. The entire crown must be destroyed to be effective. Mowing can reduce seed production if done consistently right before flowering. However, spotted knapweed can adapt and still flower when mowed in certain situations. Burning is not effective.
Biological control has been highly effective on spotted knapweed. Thirteen insects have been introduced to Montana. Most effective are a root feeding weevil, seed head feeding weevil and a seed head feeding fly. Control can be as high as 77 to 99 percent. Sheep, goats and cattle have also been known to graze weeds to low to moderate levels. They may need to be trained to graze the weeds.
Selective herbicides can provide good control and are often most cost-effective in small or new infestations. The best time to spray is early spring or fall. Commonly used chemicals include Curtail, Grazon (restricted use), Milestone, Redeem, Stinger, Transline and Tordon (restricted use). Always read and follow the entire label when applying herbicide. Common chemical names are used for clarity but does not imply endorsement of a product or brand.
For assistance with weed identification and control, contact the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu. Broadwater County has a cost-share program for controlling noxious weeds including spotted knapweed. For more information contact the Broadwater County Weed District at 406-266-9243.
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PhotoCredit: Photo Provided by MSU Broadwater County Extension Agent
Image 1 Caption: Spotted Knapweed Lavin
Photo Provided by MSU Broadwater County Extension Agent
