Japanese Knotweed: What Can You Do About It?

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Front Page / Sep. 7, 2017 9:06am EDT

By Marybeth Hanley


Japanese knotweed is one of the most invasive plants in the world. (Provided) Japanese knotweed is one of the most invasive plants in the world. (Provided) Japanese knotweed (Fallopia japonica), commonly called “bamboo,” imported from Asia as an ornamental in the 1800s, is one of the most invasive plants in the world.

Huge monocultures of this plant stretch for miles along every river in the state of Vermont including the White River. Though not limited to rivers, Japanese knotweed incursions are found along forest edges, meadows, fields, shores of streams and lakes, roadsides and waste areas.

These plants are large, and grow in huge stands by themselves to the exclusion of native plants. Japanese knotweed changes our rivers— causing erosion by blocking establishment of trees and grasses better able to stabilize riverbanks, clogging waterways, and creating an ecologically adverse chain reaction that compromises riparian habitat—native plants are eliminated, insects that feed on them disappear (insects that are food for birds, mammals, and fish).

Japanese knotweed is easy to identify. It’s an herbaceous perennial shrub that can grow to over 10 feet high. It has large heart-shaped leaves, small white panicle flowers, and winged fruits. The stems of Japanese knotweed resemble bamboo— they are large, round, jointed, and hollow. Plants grow from underground stems (rhizomes) that spread laterally, up to 65 feet. Plants can regenerate themselves from small pieces of rhizomes that remain viable several feet under ground. Tiny bits of roots and buds from rhizomes can also readily reproduce plants.

What can you do? Japanese knotweed is a difficult but not impossible plant to manage—if you try, and keep trying. Because of its superability to reproduce itself from small rhizome fragments, persistence and commitment over time is essential to deplete the “rhizome bank” in the soil. In the absence of herbicide application, the best method of mechanical removal is a combination of repeated pulling of young plants early, followed by stem cutting and smothering of mature plants later in the growing season.

First, choose a small isolated population of Japanese knotweed. Plants emerge in spring, can reach six feet by May, flower in July, and produce seeds in August/September. Small entire plants (including rhizomes) may be pulled up early in the season by hand.

Disposal methods are very important to prevent spread—do not compost “green” plants on your own. Either bag them and bring to a Chittenden Solid Waste District (CSWD) designated facility where invasive plants are safely composted (for drop off locations see https://cswd.net/a-to-z/diseased-ornoxious plants/), burn them, or pile and smother them (see instructions below). Make sure to clean all clothing, boots, and equipment before leaving sites to avoid spreading even small bits of plant parts. Japanese knotweed can reproduce from pieces of roots as tiny as ½ inch.

Smothering Method—follow these steps once a month during the growing season over five years:

•In summer, cut mature stems to their base and pile on a tarp, plastic, or pavement to dry.

•Cover piles with heavy-duty dark colored tarp or layers of thick (= 7 mm) black plastic. Before covering, spread material (mulch/grass clippings) over stems to prevent puncturing. Layer multiple pieces of tarp/plastic using a two-foot overlap between then and around pile perimeter. If working near a wetland make sure that your stem pile is above the flood line.

•Place weight on top of the tarp/ plastic and seal edges with rocks, sticks, mulch, or sand. If holes or tearing occur, patch these to prevent sun penetration.

•After five years, remove covers. Evaluate site for revegetation using robust native plants.

Start small, work together, be patient, and stay positive—managing Japanese knotweed takes dedication over multiple years. Continue to monitor sites; remember that plants will reemerge if there are viable rhizomes in the soil. Take photographs to document progress, and don’t give up!

For more information, contact MaryBeth Deller at mdeller@fs.fed.us.

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