Scientific name: Polygonum cuspidatum

What Is It?

Japanese knotweed, an escaped ornamental, is a shrubby perennial that was first introduced in the United States from Asia. It grows very aggressively along roadways, neglected gardens, streambeds, and in moist, wet places.

Is It Here Yet?

Yes. It can be found throughout Washington.

Why Should I Care?

Japanese knotweed grows vigorously, creating dense colonies that make it hard or impossible for other native plants to survive. Its ability to out-compete other species results in an altered natural ecosystem. Once established, it is very difficult to get rid of.

How Can We Stop It?

To stop the spread of Japanese knotweed, care must be taken to prevent new plants from becoming established. New growth can sprout easily from very small fragments left behind, so when working to control or remove the plant make sure that all plant material is removed and then burned or dried. Various control methods have been documented. Visit the Washington Noxious Weed Control Board or Washington Department of Ecology Web sites for more information.

This species is listed as a Class B noxious weed, meaning that it is designated for control in certain state regions. Japanese knotweed also is on Washington’s Noxious Weed Seed and Plant Quarantine list, meaning it is prohibited to transport, buy, sell, offer for sale, or distribute Japanese knotweed plants, plant parts, or seeds.

What Are Its Characteristics?

  • Japanese knotweed can grow up to 8 feet tall.
  • Its flowers are whitish with clusters that are longer than the leaves.
  • The plants stems are reddish brown and hollow.
  • The leaves are 4-6 inches long with a triangular tip and a blunt base.

How Do I Distinguish It From Native Species?

Japanese knotweed may be confused with other knotweed species, but Japanese knotweed is much shorter than bohemian or giant knotweed.

Additional Photographs