Spotted knapweed, Centaurea stoebe, is widespread along forest roadsides and open areas in the Swan Valley where the soil has been compacted and disturbed. This member of the sunflower family has pinkish-purple flowers. The plant has a tap root. The leaves of the basal rosettes are deeply lobed, grayish green, and up to 6 inches long. Knapweed can have many branches and grow 4-feet tall.
Spotted knapweed can take over pastures and log deck sites. The good news is knapweed doesn't grow well under trees. It prefers open, sunny sites. If the forest is allowed to reestablish itself, knapweed will gradually decline.
Spotted Knapweed Prevention and Control
  Knapweed is a biennial, or short-lived perennial, spread by hundreds of seeds that drop from the seed heads. Small patches of knapweed can be eradicated by pulling. But it must be repeated for several years because the seeds can remain viable in the soil for a decade or more. Gloves should be used to pull knapweed. The blossoms can be picked and the seed heads destroyed before they drop their seeds to help slow the spread.
Grazing with sheep is effective in some parts of Montana. But raising sheep in the Swan Valley is not recommended, due to our high numbers of predators, including grizzly bears, black bears, gray wolves, mountain lions and coyotes.
The weevil Chyphocleonus achates has recently been tried in the Swan Valley. It and other biological controls may prove to be helpful at reducing knapweed infestations.
Spraying may be necessary to control large knapweed patches. Keeping the plants contained to the affected site may be the only practical option if herbicide costs are prohibitive. Herbicides should be applied by a trained applicator at the rates recommended by the product manufacturer. The best time to apply herbicides is when the plants are vigorously growing, usually in May, June or early July.
Any effective treatment should include re-vegetation with native plants or other desirable vegetation.
For more information visit Missoula County Weed District. |
