Invasive Knotweeds (Polygonum spp.)
- Japanese Knotweed (Polygonum cuspidatum)
- Giant Knotweed (Polygonum sachalinense)
- Bohemian Knotweed (Polygonum bohemicum)
- Himalayan Knotweed (Polygonum polystachyum)
Japanese, giant, Bohemian, and Himalayan knotweed are all non-native plants from the buckwheat family. Sometimes called "false bamboo" or "fleeceflower", these tall bamboo-like plants are aggressive colonizers that generally spread by rhizomes, but sometimes by seed. Japanese and giant knotweeds are similar to each other and produce hybrids, one of which is called Bohemian knotweed. Although there are stands of both giant and Japanese knotweeds, botanists currently believe that much of the knotweed found on the Olympic Peninsula is Bohemian knotweed. Giant and Japanese knotweed are distinguished from each other primarily by the size of the leaf and height of the plant. Bohemian, being a hybrid of the two, varies between the two extremes. Himalayan knotweed has a much narrower leaf.
Extremely invasive, knotweed develops a deep, matted root system with rhizomes that can grow to 30 feet or more in length. Originally introduced as an ornamental, plants frequently spread accidentally when root or stem fragments are moved by people, machinery, animals or water. Tiny root fragments as small as 1 inch can produce new plants. Knotweeds form dense thickets that exclude native species and are of little value to wildlife. At the end of the growing season, a mass of dead stems remains that continues to crowd out native seedlings, and leave river banks vulnerable to erosion and flooding. Once established these plants are very difficult to eradicate.
- Thick green to reddish stems that look similar to bamboo
- Large bright green leaves ranging from egg to heart-shaped with a pointed tip
- Sprouts in April, grows to 15 feet tall or more by July
- Spikes of white flowers appear from July to September
- Seen in flood zones along rivers and creeks; also frequents ditches, yards and roadsides.
Several treatment options are described here. Because of knotweed's tremendous ability to resprout following cutting, successful control usually requires herbicides. Please check with your local extension agent, weed board or the Department of Agriculture for information about the proper, safe, and legal use of herbicides.
Spray Herbicide containing glyphosate (e.g. Rodeo, Aquamaster, Roundup, Gly Star) on the leaves and stems in summer or early fall. To avoid spraying very tall plants, it is possible to cut the stems once in May or June and allow the plant to regrow to at least waist height. Most patches require more than one year of treatment.
Non-Spray Herbicide Methods include injecting undiluted herbicide directly into the lower sections of every stem or applying slightly diluted herbicide directly onto stems. Some limitations, as indicated on the label, apply.
Always read and follow directions on the product label and keep herbicides out of waterways. Desirable plants hit with spray will be injured or killed.
Manually Pull or Dig very small, poorly established infestations, removing all the roots of plants in loose soil. Check often for new sprouts and repeat. Or, Cut the stems close to the ground every two weeks throughout the growing season. Both methods will require years of persistent treatment for successful control and should be used only when very few plants are present.