Rea Orthner Wrobel, Nederland.  Some of Nederland’s lesser known Noxious Weeds are knapweeds. I have seen these “tumbleweed” like plants in the Sunnyside Neighborhood, around the Community Center, by Barker Reservoir, and over by the veterinary office near the Eldora turnoff. I believe they also occur in the Big Springs area.

Right now, in early June, the knapweeds are just starting to “bolt” or to send up their flowering stem. Diffuse and spotted knapweeds (the kinds we have in Nederland) 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 1800’s or early 1900’s.

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. Diffuse and spotted knapweeds are annuals, biennials or short-lived perennials growing 1 to 2 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. Knapweeds are adept at depleting soil moisture and possess chemical substances that inhibit the growth of other species of plants (alleopathy).

In Nederland, I mainly see diffuse knapweed (Centaurea diffusa), however, spotted knapweed (Centaurea stoebe) and a hybrid between the two occurs as well. These knapweeds are all similar in appearance and the key to their control is “early detection – rapid response”. 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.

One of the best ways to get rid of knapweed is to 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.

Also, it’s best to educate yourself about some of our native look-alike native plants such as wild tarragon (Oligosporus dracunculus) and tansy aster (Macaeranthera bigloveii).

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-spotted-knapweed-3-110/.

http://nederlandco.org/noxious-weeds/