Non-Native Invasive Species

Japanese knotweed

Fallopia japonica

Japanese knotweed

Japanese knotweed
Illustration by Anne Hunter

Have you ever wondered how it happens that stream banks are usually covered with vegetation? After all, they're torn at by flood waters and ice jams, or alternately inundated and then left exposed to the air. It would seem to be a difficult environment in which to take root and thrive.

But some native riparian trees and shrubs are perfectly adapted to this difficult situation. Willows and dogwoods, for example, have pliable stems that "give" rather than fight against high water or ice floes. And if by chance the stems should break, they're likely to take root where they next land on land.

Japanese knotweed, an increasingly common plant of streambanks and wet areas, has a similar feature. Should its rhizomes break, they too often take root and sprout in their new location downstream - so often that this invasive native of east Asia can out-root and out-sprout our native riparian species. River edges that were once clothed in a diversity of species, each filling a very particular niche in that plant community, can quickly become linear bands of nearly nothing but knotweed. Think, for example, of the Roaring Branch in Bennington.

Japanese knotweed

Japanese knotweed
Photo: Jan Samanek, State Phytosanitary Administration, Bugwood.org

Japanese knotweed (also known as Mexican bamboo and fleece flower, and by its Latin name Fallopia japonica var. japonica) was brought to North America in the late 1800s. It is now considered one of our worst environmental weeds, from the Gulf and East Coasts north to much of Canada and west to Alaska and central California.

Its rhizomes can reach sixty feet in length. Its sprouts can penetrate concrete. It leafs out earlier than anything else on the streambank and quickly casts a heavy, growth-depressing shade. Its canes grow profusely and, when dead, form thick mats which decompose slowly. It is, however, beautiful, with deep green leaves on reddish petioles, colorful jointed canes, and, at this time of year, clouds of white flower clusters.

Japanese knotweed

Japanese knotweed
Photo: Jan Samanek, State Phytosanitary Administration, Bugwood.org

(The seed capsules that follow are also good-looking. Interestingly, the seeds are sterile because they have not been pollinated. It turns out all the Japanese knotweed plants in North America are in fact divisions or root sprouts of the same clone - and it is female. There are no male Japanese knotweeds growing wild on our continent, that we know of.)

Cutting won't control it. Herbicides are better than cutting, but require much labor to be effective (most managers inject each stem with the pesticide, using a special nail-gun like tool) and usually need follow up.

We should not, however, lose hope. Beginning in 2000, a consortium of several agencies and companies from the United Kingdom, the United States, and Japan, including the U.S. Forest Service, began researching biological controls for this pest. A beetle and a fungus (something like a rust disease of beans) seem most promising. The consortium is now looking for funds to trial these organisms to make sure they are in fact effective and pose no threat to native organisms.