We are once again partnering with our friends from the Columbia Gorge Cooperative Weed Management Area to feature invasive knapweeds as the target weed for the month of July.
Clackamas County has many kinds of knapweed, but the three most commonly found in our area are Spotted, Diffuse and Meadow. Although slightly different, they all have similar impacts on the environment.
Knapweeds readily out-competes grasses and other pasture species thanks to a toxin produced in its foliage and roots that slows the growth of surrounding vegetation. Knapweeds prefers full sun and well drained soils. It will invade native prairie, oak savannah and even clear-cuts. It can be found on roadsides, sand and gravel bars, riverbanks, irrigated pastures, moist meadows and forest openings. This invasive weed threatens wildlife habitat, causes significant loss of grazing forage and increases erosion.
Knapweeds are perennials that grow from a deep taproot. Their branched upright stems can grow 1 to 5 feet tall when flowering. Leaves can be up to 6 inches long and 1.25 inches wide. They become smaller as they advance up the flowering stem. The flower heads can be pink, reddish purple or white. They can be oval or almost globe shaped and about one inch wide. Flowering begins in the early summer and can last into the fall. Knapweeds reproduce by seed and can also re-sprout from root crowns. Seeds can remain viable for up to 8 yrs. Seeds are spread by water, animals, humans and vehicles.
Although there are several methods for controlling knapweed, it is best to know which of the three varieties you are trying to eradicate. Check out the Extension publication Identification of Knapweeds and Starthistles in the Pacific Northwest to help with knapweed identification.
The knapweeds are easiest to find and ID once they start to flower. Small infestations can be hand-pulled or dug up. It is important to pull as much of the tap root as possible. If there are too many plants to remove manually, an herbicide labeled for knapweed can be used. Knapweeds can also be mowed before flowering to lessen seed production. This will not eliminate the plant but will slow its spread.
When disposing of pulled plants, be sure to bag flowering plants. Bagged plants should not be composted but placed in the garbage.