By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties

Like just about any small woodland, the Matteson Demonstration Forest has its share of invasive weeds. Besides familiar and ubiquitous foes such as Scotch broom and thistles, one of particular concern is knapweed.

With purple flowers emerging from roundish bases at the top of a tall stalk, knapweeds superficially resemble a sort of spineless thistle, and in fact they are relatives of thistles, botanically speaking. They are biennial (2-year life cycle) to perennial plants and reproduce by seed. Fairly inconspicuous in the winter and spring; at this time of year, their purple flowers betray their location on and along roadbeds and other disturbed areas.

There are several species of knapweeds (Centaurea spp.) in Oregon, and all are classified as noxious weeds. Meadow knapweed, actually a hybrid of two other species, seems to be the most prevalent in the Willamette Valley.   Spotted knapweed is more of a problem on the eastside though has been documented on the westside too.

These two species can be tricky to tell apart, but we think we have meadow knapweed at Matteson, based on its wider distribution on the westside, the shape of the foliage and the color of the bracts (the tiny scale-like leaves at the base of the flowers).

So why are knapweeds a problem in forestlands? The biggest concern is their impact to native plant communities. They are tough competitors that can crowd out other desirable herbaceous plants, posing particular challenges to pasture or grassland managers, or those trying to restore meadows or oak savannahs. Even for those that do not have those particular objectives for their property, knapweeds are “road runners” in that they are easily spread by foot and vehicle traffic along roads and trails. So if you find that you have a knapweed infestation on your property, you can do surrounding property owners a favor by keeping it in check.

On the Matteson tract, knapweed is largely confined to gravel roads and open areas along the roads. Our main concern is limiting the spread of the weed, not only on this property but also to other College Forests and to other properties. For that reason, we are trying to keep OSU College Forest vehicles off the property as much as possible, so that tires and vehicle undersides do not pick up the seeds.  Multi-year herbicide treatments will also likely be necessary to reduce knapweed on the property; although because the weed is also prevalent along the county road ringing Hagg Lake, we may be fighting an uphill battle. We will be regularly surveying where our access roads join up with county roads and neighbors to contain and prevent its spread.

For more on knapweed management, consult these fact sheets: Invasive Weeds in Forestlands: Knapweeds from OSU Extension and Meadow Knapweed Best Management Practices from the King County Noxious Weed Program.

Thanks to Michelle Delepine from West Multnomah SWCD for her helpful expertise on this subject.