Exotic Invasive Species - Japanese Knotweed

What is an exotic invasive species?
An alien species whose introduction does or is likely to cause economic or environmental harm, or harm to human health due to its aggressive nature of growth, colonization and dispersal..

What's the problem?
Fat, red-speckled shoots of Japanese knotweed (Polygonum cuspidatum and P. sachalinense), like a forest of blood-spattered asparagus spears, emerge each April from a mound of dead canes to form a jungle of 8-10-foot-tall plants. Their rhizomes spread underground 25 feet in all directions, sending up shoots tough enough to pop through pavement. If rhizomes reach a brook, bits break off and take root miles downstream. When rhizome-infested soil is hauled away by ditch-digging highway crews and later used as fill, a new jungle will sprout there.

The speed with which it has spread to all parts of Pennsylvania has been spectacular when you consider that it does not leave seeds behind but grows from pieces of the plant or root system that is cut and transported by people or by water. These plants, introduced to the United States a century ago as ornamentals, have been found in 36 states, with heaviest infestations in the East. It crowds the banks of the Monongahela, Allegheny, Ohio and other major rivers and their tributaries, and is considered a serious problem.

Because Japanese knotweeds do not originate in the United States (brought in as ornamentals from Asia in the 19th Century), it does not compete "fairly" with our native species and is able to spread unchecked. Once established, they shade out native plants by producing a dense canopy of leaves early in the growing season. Although Japanese knotweeds are not toxic to humans, animals or other plants, they offer a poor habitat for native insects, birds and mammals.

If invasive species are left unchecked, native plant life is lost. Multicolored meadows of orange hawkweed, bluebells, and black-eyed susans could give way to monotonous stretches of exotics. Wetlands become choked by purple loosestrife and giant reed grass. This has happened in other prized natural areas, as well as in much of the nation's urban and rural landscapes.

Text modified from: Environmental Network News; Seattle Times

Photo courtesy of L. A. Seiger - Ecology Research Laboratory, San Diego State University

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