Invasive Plant: Russian Knapweed

Photo shows a small purple, brush-like flower on a green plant with a brown background.
Although not currently known to be in Portland, Russian knapweed is a Required Eradication species. Infestations cause big problems for Oregon's farmers and ranchers. Growing in dense clumps, Russian knapweed forces out crops or native plants and is inedible to livestock and wildlife.
  • Photo shows a small purple, brush-like flower on a green plant with a brown background.

    Russian knapweed is on the Required Eradication List. Report all sightings. Photo from the Oregon Department of Agriculture.
    Scientific name: Acroptilon repens
  • Annual, perennial, or biennial: Perennial
  • Conditions it likes best: Clay soils and semi-arid environment
  • How it spreads: Spreads by root fragments and seed
  • How it causes damage: Overruns native grasslands or irrigated crops
  • Best time and way to manage: Not commonly found in Portland
  • Oregon Department of Agriculture noxious weed rank: B
  • Report patches of this plant? Yes. City code lists this as a Required Eradication species. Report all sightings to Environmental Services. Contact information is on this page. Management of this species is required, but free help is available.

How to Identify Russian Knapweed

Russian knapweed blooms summer to fall with cone-shaped flowering heads one-quarter to one-half inch wide with one flower, pink to lavender, growing at each branchlet tip. Plants grow into dense patches with straight, branched stems.

Winter-Spring. Russian knapweed forms a rosette in the first autumn of growth, with pale or gray-green leaves through the winter. Plants bolt (grow a stem) in the following spring.

Summer. The plant bolts (grows a stem) in June, becoming 1-4 feet tall with a central stem, several spreading branches, and a long main root. Most plants flower in the summer from June to August. 

Fall. When some plants dry out and die in the summer and fall, the stems break off and roll away in the wind spreading seed.

How and When to Remove Russian Knapweed

Russian knapweed is usually in central, north central, and southeastern Oregon. It is not yet known in Portland, but it is on the Required Eradication List so report any sightings of patches within Oregon.

Prevention is Best Practice

Clean your boots, pets, and maybe even your tires when you finish a hike or trail ride in Pacific Northwest natural areas, or if you have invasive plants on your own property. Cleaning boots and pets keeps invasive plants from spreading to new places.  Be careful when trading plants with neighbors and fellow gardeners to make sure you’re not trading this or any other plant of concern.  

Properly Dispose of Invasive Plants

To prevent spreading invasive plants:

  • Never place plants or seeds in your compost or yard debris.
  • Never throw pulled plants on the ground or into the street or sidewalk.
  • Put all pulled plants, bulbs, or seeds into a plastic bag and put the bag in the trash.
  • Wash all garden tools and gloves.