Spotted knapweed — stop it if you see it |

Spotted knapweed — stop it if you see it

At this time of year, when most people are thinking about their gardens and outdoor activities, Matt Custer, Routt County’s weed supervisor, is making weed management his priority. On his “A” list for weeds this summer is spotted knapweed. Under the Routt County Noxious Weed Management Plan, the control of noxious spotted knapweed is mandatory for all landowners.

Noxious weeds are weeds that are not native to the U.S., and if growth remains unchecked, these same weeds will ruin our grazing lands and force large game animals to find other feeding areas.

Spotted knapweed, a native of Europe, was inadvertently introduced to North America within a shipment of alfalfa and clover seeds in the 1890s. Without any natural predators such as disease or insects, it spread uncontrollably and now covers more than 4.7 million acres in Montana, turning the land into useless open space. It continues its wayward spread in Washington, Oregon, Idaho, California and western Canada. Each plant produces more than 1,000 seeds, and botanists now suspect the plant may release a chemical substance that inhibits the growth of nearby vegetation.

Spotted knapweed blooms in August and is identified by its white to pink or purple flowerhead and black bract or oval bulb under the blossom. Its leaves are slender and lobed or indented along the sides. It grows 1 to 3 feet tall from a rosette of leaves at its base. Spotted knapweed is similar to thistle, but it has far fewer leaves along its branching shoots. It is doesn’t live long, but it is an aggressive, perennial weed.

The first appearance of spotted knapweed usually is in disturbed areas. By sending down stout taproots, it quickly spreads, choking out other plants and native grasses. It is not choosy about soil conditions. Once established, growth of native vegetation is inhibited, and soil erosion begins.

To prevent the spread of spotted knapweed, use Roundup. A new herbicide, Milestone, is now on the market. Always read the label and use as directed. The weed can be mowed, but if done too early in its flowering state, the plant regrows and produces seeds later in the season.

As good stewards of the land, we must be on our guard against all noxious weeds, and spotted knapweed poses a serious threat to agriculture and wildlife by infesting our rich ranchlands. If you find it or need help in identification, call Custer at the Routt County Extension Office. Together we can prevent the spread of spotted knapweed.

Unlike wildfires, noxious weeds almost never make the headlines of newspapers, but their damage can be just as extensive and lasting.

Ann Noyes is a Master Gardener through the Colorado State University Cooperative Extension Office in Routt County. Products mentioned in this article are not endorsed by Master Gardeners or the CSU Cooperative Extension Service. Questions? Call 879-0825 or email

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