One of the most severe weed problems we have in the state of Montana is the spotted knapweed. It infests more than 4.5 million acres in the state, costing tens of millions of dollars in control costs, displacing native vegetation, wildlife and livestock.
In urban and suburban settings, the weed can negatively impact property values, increase costs for control, and it create a real eyesore once blooming is finished. As subdivisions have proliferated in Montana, so has spotted knapweed. The disturbed soil in these areas is perfect for the establishment of the weed. Many homeowners do not recognize the weed, and fail to implement control measures, adding to the already widespread problem; in fact, it is sometimes cultivated and cared for as a "beautiful wildflower" by folks who do not realize the plant is an invasive species accidentally introduced from Eurasia.
Spotted knapweed is 1-3 feet tall, with leaves that are about 2-3 inches long and deeply lobed, or indented. Flowers are similar to those of Canada thistle, both of which are shaped like a small, lavender dandelion blossom. Spotted knapweed is a short-lived perennial, meaning it lives for only a few years, sometimes only 2. The first year it grows to the rosette stage, which is about 6 inches in diameter and 1-2 inches tall. The second year it grows to its full height, and has a rather spindly appearance.
Management of spotted knapweed in small areas, such as the yard, can be achieved by hand pulling, making sure to remove as much as the root as possible to prevent regrowth. Mowing is not recommended because flower heads can be produced very close to the ground, below mower blades, when upper parts of the plant are removed. If the number of plants is not prohibitive, spot application with glyphosate will also be effective. Glyphosate is relatively safe for both humans and the environment. Remember that glyphosate will kill any plant it touches, including desirable plants. Apply it in such a way that it does not touch any plants other than the ones you want to kill.
If the infestation is too large to treat individual plants, try Weedmaster herbicide, which is a premixed combination of 2,4-D and dicamba on an annual basis if needed. Remember that Weedmaster is not as benign in the environment as glyphosate, so if possible, use hand pulling or the glyphosate to manage the weed. The dicamba in Weedmaster is highly soluble in water and can move through the soil to cause damage to trees and shrubs; so follow the label instructions very carefully when using this product. A closely related species of knapweed, Russian knapweed, is not controlled using these methods. If Russian knapweed is present, contact your county weed supervisor or your county Extension agent for further information. When using any pesticide, be sure to read all label instructions and follow them carefully, wearing necessary protective clothing and equipment.
To help keep infestations minimized, encourage healthy, deep-rooted lawn grasses, ground covers, or pasture grasses for larger acreages. For assistance with identification and control of spotted knapweed, contact your county weed office or Extension agent.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997