Weed of the Month: This is the best time to eradicate Knotweed

Often mistaken for bamboo, knotweed grows to 15 feet tall with 2.5 foot long leaves. Stems are hollow with swollen joints and plumes of white flowers emerging in late summer and fall.

There are four species of knotweed invading the gorge: Japanese, Himalayan, Giant and Bohemian. While it prefers streamside areas, it can be found along roadsides and other upland sites.

Imagine a weed that you can’t cut, mow or weed because doing so will encourage it to spread. Knotweed reproduces readily from small pieces of its rhizomes (lateral roots) and stems as well as seeds.

Knotweed can grow up through asphalt and concrete. It outcompetes native vegetation. Along coastal Oregon, it has taken over large tracts of riverbank. It is doing the same in parts of the Columbia River Gorge.

If you have this plant, please help keep it from spreading!Chemical control and diligent multi-year monitoring are often the only effective ways to stop this invader.

August to October is the best time to spray an herbicide for knotweed control. That said, if you do nothing else, please cut, bag and dispose of flowers in the garbage before they go to seed.

Information on knotweed and how to control it is contained in the Oregon State University publication EM 9031 or online at goo.gl/frZQNt.

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Jordan Kim is director of Hood River Soil and Water Conservation District.