Invasive Species: Knotweeds

Polygonum sp.

There are four species that occur in Oregon. Japanese knotweed (Polygonum japonica) is the most common in Yamhill County.

Why is it important to control?

Knotweeds grow vigorously and create dense colonies that exclude native vegetation. It is a significant threat in riparian areas.

A green Knotweed branch.

How can you identify it?

Image of knotweed floweringBrown Knotweed seeds
Habitat: Riparian, wetlands, roadsides, neglected gardens
Life Cycle: Perennial
Height: Up to 15 ft
Leaf Description: Heart or teardrop shaped
Fruit & Flower Description: Cream colored flowers in plumes at the end of stems and branches
Bloom Time: July to September

How do you control it?

  • Do not mow, cut, dig or compost this plant. These actions cause it to rapidly spread.

  • For chemical control, use a broadleaf selective herbicide and make sure to follow all label directions.

  • The Oregon Department of Agriculture has no biocontrol agents.

How to Report it?

There are various ways you can report this noxious weed.

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