Troublemaker 1: Bohemian knotweed

Posted: Wednesday, April 09, 2008

Invasive species are increasingly in the headlines these days. Many of us have them in our yards or know someone who does. Not just yards, but wild places are being invaded and considerable harm could come from it. Prevention, early detection, and rapid response are the least costly avenues for dealing with invasives. This column will address plant and non-plant species of concern in Southeast.

Courtesy Al Schneider
Courtesy Al Schneider

An invasive species is one that is not native to an ecosystem, and is likely to cause harm to human health, the economy or the environment.

One of the most visible invasive plants in the Juneau area is Bohemian knotweed, aka, Japanese knotweed, Japanese or Mexican bamboo. It has an invasiveness ranking of 84, on a scale of 1-100, on the Alaska Natural Heritage Program list. Our version is the hybrid cross between Japanese knotweed and Giant knotweed. This plant is in the buckwheat family and is native to Japan, but has been spread worldwide. It most likely came to us from England as an ornamental.

The patches in Juneau yards and roadsides are clones which expand by rhizomes (modified underground stems). Rooting may also occur at stem nodes if they are left on the ground. This hybrid is capable of producing seeds, but so far that doesn't appear to have occurred in Juneau. Dispersal is most often by transport of stems or soil containing rhizome pieces through human or water activity.

The dense thicket of rhizomes and stems excludes native plants and is unsuitable habitat for birds and mammals. Even though the rhizomes are dense, they do not stabilize slopes. This knotweed is an aggressive invader of riparian habitat, completely taking over riverbanks and streambeds. Spread in riparian areas may have negative impacts on reproduction of salmon and other aquatic species.

So, what can be done about this beautiful but bad actor? There are several methods of treating knotweed; combinations often work best. Any treatment that involves the removal of plant parts requires proper disposal to prevent spread. You may leave parts to dry where they will not infect new areas, burn them, or dispose of them at the dump. Do not compost.

• This plant loves light, so shading is one option. Cover with several layers of materials such as black plastic, old carpet, cardboard, plywood, etc., including areas several feet beyond the visible growth. Leave covered for at least one year, preferably two. Watch for new growth sneaking up around the edges. Local reports claim good results on small patches with consistent oversight.

• If you enjoy working in your yard, cut the patch to ground level every two to three weeks, for several years. You will at least give it a good scare.

• Pulling and digging involve removing the rhizomes; easiest in loose soil. Remember that the small pieces you miss will sprout, so expect to continue the process for several seasons.

• The herbicide, glyphosate, is effective for killing knotweed. There are several approved and effective methods for herbicide application. Follow label recommendations and guidelines. Remember, the label is the law. Contact the Cooperative Extension Service for additional information.

• Not in My Backyard is a monthly feature about invasive species created by U.S. Forest Service botanist Ellen Anderson. She can be reached at

The Alaska Natural Heritage Program Weed Ranking Project is available online at Contributors to this column will include Juneau Invasive Plant Action, Juneau Watershed Partnership, Forest Service State & Private Forestry and the National Marine Fisheries Service. Others may be added in future columns.

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