Some of Nederland’s lesser-known but high priority noxious weeds are knapweeds. I have seen these “tumbleweed”-like plants in the “lower” Sunnyside Neighborhood, behind the Community Center on Forest Road, on the recreation path along Barker Reservoir, and over by the veterinary office near the Eldora turnoff. These nasty plants were the target of recent completely organic “weed-steaming” treatments as well as hand-pulling by Teens Inc.

In spring, the knapweeds start to “bolt” or to send up their flowering stem. Diffuse and spotted knapweeds as well as the hybrid between the two are thought to be native to grasslands and shrub steppes of the eastern Mediterranean and western Asia. They were probably introduced from Eurasia as a contaminant of alfalfa seed in the late 1800s or early 1900s.

Knapweeds are a huge problem in Montana and have infested over a hundred thousand acres in Colorado, mainly in the Front Range. Although these noxious weeds are not that common in Nederland right now, they will rapidly take over if not properly managed.

In Nederland, I mainly see diffuse knapweed (Centaurea diffusa). However, spotted knapweed (Centaurea stoebe) and a hybrid between the two occur as well. Diffuse and spotted knapweeds are annuals, biennials, or short-lived perennials growing one to two feet tall. The white to purplish flowers appear from late June to September. At maturity, the many-branched stems are rough to the touch, and after seed set in the fall, the stems break off and the plants tumble with the wind. The seeds and plant parts can also adhere to vehicles or shoes.

Disturbance allows knapweeds to invade a wide range of habitats, where they rapidly establish dense stands. Once knapweeds invade a site, it can take years to get rid of them because the seeds can survive in the seedbank for up to 10 years. Knapweeds are adept at depleting soil moisture and possess chemical substances that inhibit the growth of other species of plants (alleopathy).

One of the best ways to get rid of knapweed is to pull up the plant (easy to do after a rain) or sever the root below the soil surface. If you do this when the plant is in flower, be sure to properly dispose of the flowering cut plants, since seeds can mature and become viable after the plant has been cut down. Management must be persistent to deplete the remaining seeds in the soil.

I recommend putting the plants in a compostable paper yard bag (available at Ace Hardware) and bringing them to the Forestry Sort Yard on Ridge Road. Make sure to leave room to roll down the bag to close it.

Also, please educate yourself about some of our look-alike native plants such as wild tarragon (Oligosporus dracunculus) and tansy aster (Machaeranthera bigelovii). If you remove a large area of knapweed, it’s also good to reseed the site with a native mountain seed mix to get more desirable vegetation growing that will help stabilize soil and provide competition for any re-emerging knapweed seedlings.

For more information see http:// extension.colostate.edu/topic-areas/ natural-resources/diffuse-and-spottedknapweed 3-110/ or http://nederlandco.org/noxious-weeds/