Weed of the Month: The invasive thistle-like knapweed

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.

Knapweed is a perennial that grows from a deep taproot. The branched upright stems can grow one to five feet tall when flowering. Leaves can be up to six inches long and one and a quarter 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. Knapweed reproduces by seed and can also re-sprout from root crowns.

Seeds can remain viable for up to eight years. 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 with the OSU Master Gardeners for publications about knapweed identification.

Knapweed is easiest to find and identify once it starts flowering. Small infestations can be hand-pulled or dug up. It is important to pull as much of the taproot as possible. If there are too many plants to remove manually, an herbicide labeled for knapweed can be used.

Knapweed 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.

Jordan Kim is Hood River Soil and Water Conservation District district manager.