- Japanese knotweed (Polygonum cuspidatum or Fallopia japonica) also known as Japanese bamboo, Mexican bamboo, false bamboo, Himalayan fleece vine, crimson beauty sally rhubarb, fleece flower.
- 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, they all have similar impacts on the landscape. These species have vigorous rhizomatous rootsA continuously growing, horizontal underground stem. and form nearly impenetrable stands. Worse yet, it can sprout from small fragments, making it extremely difficult to control.
|Life Cycle:||Perennial (life cycle lasts more than one year)|
|Early Detection and Rapid Response species:||Yes|
|Height:||Up to 15 feet (5 meters)|
|Leaf Description:||The best way to distinguish species from one another is to closely examine their leaves. Each species has smooth leaves that alternate along the stem, but that’s where their similarities end:|
• Japanese knotweed’s leaves are smaller, measuring 4 to 6 inches (10 – 15 centimeters) long. Its leaves are oval shaped with sharp tips at the ends.
• Giant knotweed has the largest leaves, measuring 12 inches (30 centimeters) wide and 18 inches (45 centimeters) long. Each leaf is leathery and heart shaped.
• Bohemian knotweed’s leaves have similar characteristics to both Japanese and giant knotweed but are shorter, measuring 7 inches (20 centimeters) long.
• Himalayan knotweed’s leaves are long and slender with pointed tips.
|Fruit & Flower Description:||Small white or greenish flowers form in dense clusters near the end of the stems and branches. Winged seeds help spread this species by allowing them to fly in the wind and float on water. Seeds can remain viable in soil for up to 15 years.|
|Bloom Time:||July to August|
- Knotweeds thrive in disturbed and moist soils. They can be found in riparian areasAreas that are directly adjacent to flowing streams, creeks, or rivers., wetlands, roadsides, and neglected gardens.
- They tolerate partial shade but prefer full sun conditions.
- Knotweeds form dense thickets that shade out native plants and exclude wildlife.
- These species have a fast growing, extensive rhizomatous root system that enable it to outgrow and suppress other plant species. Their root systems can also damage asphalt, concrete or building foundations, decreasing property value.
- They can clog waterways, increase bank erosion, and lower the quality of riparian habitat for fish and wildlife.
What we’re doing about it:
- All species are categorized as a priority species 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 the species. If found, the crew will treat the infestation at no cost to you.
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! It rapidly spreads by plant fragments.
Noxious Weed Listing:
|State of Oregon:||Class B|
|State of Washington:||Class B|
Download the Woody Knotweed Best Management Practices Factsheet
- Pacific Northwest Weed Management Handbook: Woody knotweed
- University of California, Weed Research & Information Center: Woody knotweed
- Oregon Department of Agriculture: Japanese knotweed
- Washington State Noxious Weed Control Board: Japanese knotweed
- Oregon Department of Agriculture: Giant knotweed
- Washington State Noxious Weed Control Board: Giant knotweed
- Oregon Department of Agriculture: Bohemian knotweed