Japanese Knotweed (Polygonum cuspidatum) and its close cousin, Himalayan Knotweed (P. polystachyum) are all abloom this time year, leading the curious to ask, “what is that flossy flower along the streams of the North Coast?” Some call it lovely, as did the gardeners and nursery owners who imported it at the turn of the 20th century, but many know it today as the potential “kudzu” of the Pacific Northwest. The plant grows rapidly into huge, monocultural thickets along streams–as you can see by this colony along the North Fork of the Wilson River that I photographed just a couple of weeks ago.
Knotweed is a pernicious plant–propagating itself easily from even the tiniest fraction of a stem or root. Brousing animals and annual floods can easily move the plant throughout watersheds. Like the mythical Medusa, the plant responds to cutting by growing yet more stems. While the plant is edible (and eaten in it’s native Asia), it is of dubious wildlife value. According to experiments completed by University of California at Davis, the Lumi Tribe, and The Nature Conservancy in Oregon, small, isolated patches of knotweed can be controlled by FREQUENT cuttings, at least in April and again in August, but kept up for at least two years. No one, however, is sure if this really works and how long the plant may survive below ground. We do know that the plants can spread roots up to 25 feet in all directions of a patch. Note that composting is NOT recommended as the plants will simply sprout in the pile.
Knotweed has posed a singularly difficult problem for watershed councils and land managers interested in preserving water quality and maintaining high quality salmonid habitat. This comes from the fact that applications of herbicides must be sufficiently strong to control the plant, but at the same time, follow EPA guidelines and keep water free of the potentially toxic chemicals.
Thus far, two herbicides are recommended for treatment of riparian knotweed: glyphosphate and imazapyr. Different formulations of glyphosphate can be found under the trade names “Aquamaster,” “Rodeo,” “Gly Star,” and “Round-up” among others. Imazapyr appears under the trade name “Arsenal.” Whichever is used, be sure to follow the directions on the packaging and do not use these chemicals near water unless indicated that they are safe for aquatic life. Of these, “Aquamaster” and “Habitat” contain this indication. Now is the time to spray Knotweed plants as they are flowering and sending energy into their root systems for the winter. For details on some still experimental techniques consult The Nature Conservancy’s on-line publication “Controlling Knotweed in the Pacific Northwest.” Note that this does not constitute an official OSU endorsement of the techniques outlined therein. At the same time, I would love to hear from you if you try something and think it works (or not).
University researchers, The Nature Conservancy, the USDA, and numerous land managers are working out treatment methods as fast a possible. Stay tuned for more information here as a team of North Coast researchers and land managers work together to find some locally appropriate solutions.
In the meantime, don’t smile when you see those lovely flossy blooms along a stream near you…