What is the species?
- Polygonum cuspidatum, P. polystachyum, P. sachalinense (Japanese, Himalayan, and giant knotweeds)
- ODA “B” rank
Knotweed, sometimes known as Japanese bamboo, was a popular ornamental in the U.S. until relatively recently. In places like Britain, it has become infamous for its destructive ability. In the Pacific Northwest, it has earned a reputation for creating extensive ecological damage as well as for wreaking havoc on private property.
Why are we concerned?
Knotweed is known to form dense patches that begin in disturbed areas (trails, streams, etc.) and expand into undisturbed areas. In addition, knotweed’s deep, persistent roots make it extraordinarily difficult to kill. Knotweed infestations can lead to:
- Reduced plant and animal diversity as other plant species are choked out
- Increased stream temperatures as the seedlings of shading tree species fail to grow up and replace old trees
How does it spread?
Knotweed is spread primarily by fragments, especially of roots and stem nodes. In natural systems, high water breaks plants and moves them downstream. In urban areas, movement is less well understood. Knotweed persists in places it was planted as a landscape plant; it may also be moving in commercial topsoil. Knotweed seeds, while a possible source of new plants, are not thought to be critical to its spread. Knotweed fragments are moved around by:
- Running water
- Unscreened soil
What does it look like?
Knotweed is known for its hollow, segmented stems and spade-shaped leaves. Under good conditions, knotweed can be 10 feet tall, and forms patches dense enough to be impassable.
Are there any lookalikes?
Not really. Knotweed’s hollow stems strongly resemble bamboo, but the spade-shaped leaves do not.
How do we deal with it?
- Manual: Except for the very smallest infestations, digging is not recommended. Not only does digging require a great deal of effort, it has also been credited with increasing the growth of remaining root fragments. Cutting every two weeks throughout the growing season (May-September) has been presented as a possible manual alternative. Frequent cutting requires careful management of the fragments, though: any piece has the potential to form a new plant.
- Herbicide: Land managers have tried a range of herbicides. Generally, treatments starting in late summer are more effective, and using the same herbicide for more than three years is likely to create resistance.
How can folks help?
If you suspect you’ve found knotweed on your own property, feel free to report it to the city. In general, we can assist with knotweed management only in streamside areas, and only with chemical methods. Management and proper disposal on private property is strongly encouraged.