Description: Native to Eastern Asia, Japanese Knotweed is an upright, herbaceous perennial that commonly grow to heights over 10 feet. Stems are stout, hollow, and swollen at the joints. Leaves are usually 6-8 inches long and 3-5 inches wide, broad and oval in shape although coming to a point at the tip. When in bloom the tiny white flowers are arranged in attractive branched sprays, often beloved by pollinators. Knotweeds are considered noxious weeds by the Oregon Dept. of Agriculture and are a priority to eradicate.
Ecology: Japanese Knotweed and its similar subspecies (Bohemian and Himalayan) are a serious threat to our native riparian ecosystems. Since it starts growing earlier in the spring than most of our natives (and also grows faster) it easily outcompetes and rapidly forms densely thick monoculture stands that only support very limited food webs in comparison to our diverse native stream side assemblages. It primarily spreads vegetatively, so mowing and tilling are NOT control options. A single rhizome can result in an entire stand if given the chance. Once established it is very persistent and can take multiple years of intensive management to control.
Options: If you observe Knotweed on your property and would like to be part of the solution, there is help available. We perform a local inventory documenting all known stands within the district during late spring and early summer, with follow up treatment occurring in late August and September. If you are open to assistance, please contact Seth Mead, the Watershed Conservationist for the Siuslaw SWCD.
IMPORTANT: Please refrain from mowing, cutting, and/or transporting it, especially in areas adjacent to bodies of water. Disturbance also stresses the plants, resulting in less effective responses to treatment. Please do not discard of it in any river or stream, as it will only float downstream and re-propagate. This is of even greater importance to land owners within the estuary, as the tidal fluctuations can spread it both upstream and down.