Species include:
- Japanese knotweed (Polygonum cuspidatum or Fallopia japonica) also known as Japanese bamboo, false bamboo, Himalayan fleece vine.
- Giant knotweed (Polygonum sachalinense or Fallopia sachalinense) also known as Japanese bamboo, sacacline, Sakhalin knotweed.
- Bohemian knotweed (Polygonum bohemicum) also known as Japanese-Giant hybrid knotweed.
- Himalayan knotweed (Polygonum polystachyum or Fallopia polystachyum) also known as Kashmir plume, bell-shaped knotweed, cultivated knotweed.
Have you ever seen a plant destroy a building? Well, that’s exactly what would happen if woody knotweeds were left unmanaged! While there are many species of knotweed, they all have similar impacts. These species have rhizomatous roots and form dense stands. Worse yet, it can sprout from small root fragments, making it extremely difficult to control.
Description:
| Life Cycle: | Perennial (life cycle lasts more than one year) |
| Early Detection and Rapid Response species: | Yes |
| Height: | Up to 15 feet (4.5 meters) |
| Root Description: | Its rhizomatous roots are fast growing. They can grow 10 feet (3 meters) deep and 30 feet (9 meters) long. |
| Leaf Description: | The best way to distinguish knotweed species from one another is to examine their leaves. Each species has smooth leaves that grow alternate along the stem. • Japanese knotweed’s leaves are the smallest, measuring 3 to 6 inches (7.5 – 15 centimeters) long. Its leaves are tear-drop shaped with sharp tips at the ends. • Giant knotweed has the largest leaves, measuring up to 12 inches (30 centimeters) wide and 18 inches (45 centimeters) long. Each leaf is leathery and heart shaped. • Bohemian knotweed’s leaves look like a combination of Japanese and giant knotweed leaves and usually 7 inches (20 centimeters) long. • Himalayan knotweed’s leaves grow up to 12 inches (30 centimeters) long and are lance-shaped with a tapered base. |
| Fruit & Flower Description: | Small white flowers form in dense clusters near the end of its stems and branches. Winged seeds spread by flying in the wind or floating on water. Seeds can remain viable in soil for up to 15 years. |
| Bloom Time: | July to September |
Habitat:
- Knotweed thrives in moist soils. They can be found in riparian areasRiparian Areas Areas of land that occur along the edges of rivers, streams, lakes, and other bodies of water., wetlands, roadsides, and neglected gardens.
- These plants tolerate partial shade but prefer areas that receive full sun.
Impact:
- Knotweed forms dense thickets that shade out native plants and exclude wildlife.
- New plants can regrow from small root fragments, often as small as half an inch. This enables it to outgrow and suppress other plants.
- Its roots lack of fine root fibers that hold soil. Areas with knotweed are more at risk for erosion.
- Roots also damage asphalt, concrete, and building foundations.
What we’re doing about it:
- All knotweed species are a priority for the Tualatin SWCD. As such, our Invasive Species Program has been actively monitoring and treating it throughout the watershed.
- If identified within Washington County, a specially trained crew can come out survey for it. If found, the crew will treat the infestation for free.
Look-alikes:
Native red-osier dogwood (Cornus sericea) is frequently confused with woody knotweed. Although dogwood flowers grow in clusters and are white like knotweed, the clusters are umbrella-shaped and produce white berries. Dogwood’s stems are also woody and turn bright red in the winter.
What you can do about it:
- Please report any infestations to the Oregon Invasive Species Hotline.
- Do not mow, cut, dig, or compost this plant! These actions cause it to rapidly spread.
- If you have cut or dug knotweed material, please dispose of any plant fragments in a tied plastic bag and throw it away in the trash – not the yard debris or home compost.
Noxious Weed Listing:
| State of Oregon: | Class B |
| State of Washington: | Class B |
More Resources:
Download the Woody Knotweed Best Management Practices Factsheet
- Pacific Northwest Weed Management Handbook: Woody knotweed
- University of California, Weed Research & Information Center: Woody knotweed
Japanese Knotweed
- Oregon Department of Agriculture: Japanese knotweed
- Washington State Noxious Weed Control Board: Japanese knotweed
Giant Knotweed
- Oregon Department of Agriculture: Giant knotweed
- Washington State Noxious Weed Control Board: Giant knotweed
Bohemian Knotweed
- Oregon Department of Agriculture: Bohemian knotweed
Himalayan Knotweed
- Oregon Department of Agriculture: Himalayan knotweed
- Washington State Noxious Weed Control Board: Himalayan knotweed