Invasive in the Spotlight: Japanese Knotweed

Japanese Knotweed (Reynoutria japonica)

Japanese Knotweed (Reynoutria japonica) is an extremely fast growing invasive herbaceous plant in the buckwheat family (Polygonaceae). Native to Asia, it was introduced to the United States sometime during the late 1800’s as an ornamental plant. Unfortunately, it crowds out native species and can grow at a rate of up to 8 inches a day in the summer. It’s found in every state except North Dakota, Nevada, Arizona, New Mexico, Texas, Florida and Hawaii.

Identification

Many people mistake Japanese knotweed for bamboo, as the stems are jointed and hollow. Unlike bamboo the plant has large semi-triangular leaves that alternate on the stem. It is a large, fast growing shrub-like plant that can grow through asphalt and concrete and reach 9 feet in height. It has large woody rhizomes that can grow up to 10 feet deep and 40 feet long, and remain dormant for years. It can regenerate from rhizome pieces as small as ½ inch. 

Control

Knotweed is hard to eradicate and removal is usually a slow process. Small populations can be controlled by continually cutting the canes and digging up the roots. All cuttings should be allowed to dry out in the sun before disposal.  It should never be composted. Knotweed should not be mowed, as mowing can result in spread. Under the right conditions mowed or cut stem fragments can root at the nodes. Smothering is another alternative, using heavy duty (7-mil thick) black plastic or weed fabric. Biological control of Japanese Knotweed is not available yet in the US.

Herbicides containing the active ingredient glyphosate can kill Japanese knotweed, but it may take three to five years of repeated applications to eradicate it. Glyphosate is best applied just after flowering until frost.  

To learn more about Japanese knotweed and its control, check out Preventing the Spread of Japanese Knotweed, a guide created by the NH Department of Agriculture.

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Anna Boudreau Supports Extension

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Anna Boudreau
State Advisory Council Chair, Natural Resources Steward and NH Coverts Cooperator