Weed of the Month: Spotted Knapweed

Spotted knapweed (Centaurea maculosa) is blooming and suddenly seems to be everywhere.  It’s grey-green wiry foliage help it blend into surrounding vegetation until its pink flowers appear. Then suddenly we see the vast extent of its infestation.

Spotted knapweed is in the aster family, along with many other noxious weeds including Oxeye Daisy, Scentless Chamomile, and all the thistles. The pink flowers of spotted knapweed are reminiscent of Canada Thistle, but knapweed lacks the spiny leaves of thistles. Spotted knapweed can also be distinguished by the black tips on the bracts that make its involucre appear “spotted”(and give the plant its common name).

For those who are not botanists, the involucre is the whorl of bracts (scale-like structures) surrounding the inflorescence of flowers in the aster family.  

Like many noxious weeds in our area, spotted knapweed is native to Eurasia where the climate is similar enough to the rocky mountain west that it can thrive here. In its homeland, it has many limiting factors that keep it from being invasive. These include competition from other plants and herbivores that like to feed on it. However, such limiting factors are not present in the Greater Yellowstone Ecosystem, which allows the spotted knapweed to reproduce and spread, apparently unimpeded.

This creates a challenge for our native wildlife. Spotted knapweed can provide reasonable nutritional value to ungulates when it is young and green. But once it produces flowers, a chemical in its leaves make it unpalatable. Over the course of a year, the nutritional value of spotted knapweed to grazing ungulates is very poor. The knapweed also has the ability to reduce competition from surrounding vegetation by releasing a chemical from its roots that inhibits germination of other plants. So eventually, spotted knapweed can replace the native grasses and forbs that wildlife rely on with a forage of much lower quality.

Elk eat knapweed unwillingly after it has replaced more nutritionally valuable native grasses
Photo credit: Casper Star Tribune

If you have spotted knapweed in your yard, you may notice that it will eventually replace everything else growing in your yard and make your soil less hospitable to other plants, decreasing your property value.

So what can you do if you find spotted knapweed growing on your property?

  1. You can dig it up if you get the entire root out
  2. You can treat it with an herbicide like Milestone, available at Teton County Weed and Pest.
Spraying spotted knapweed along the Gros Ventre River

If you are not sure that what you have is spotted knapweed, click here and submit a Weed ID service request with an uploaded photo of your weed and we will confirm its identity. You can also bring a sample to our office.