Origin:
Japan
Background: Japanese knotweed is one of the Global Invasive Species Database’s top 100 worst invaders. In its native
Japan, however, it is a popular ornamental plant used to stabilize soils and obscure garbage dumps and other unpleasant sights. It is frequently planted by beekeepers for its nectar in the
U.S.
Japanese knotweed was introduced to the
United Kingdom
from
Japan
as an ornamental plant in 1825, and was brought from the
U.K.
to
North America
around 1890. Japanese knotweed is common from
Maine
to
Wisconsin
and south to Louisiana, and occurs sporadically in the western United States. Invasions of Japanese knotweed are found throughout
Long Island
, including in the Peconic Watershed.
Habitat: Japanese knotweed can tolerate a variety of adverse conditions, including deep shade, high temperatures, high salinity and drought. Knotweed is commonly found near water sources, such as along streams and rivers, in low-lying areas, waste areas, and around old home sites.
Ecological Threat: As with many invasive species, Japanese knotweed spreads quickly to form dense stands and can exclude native vegetation. Stands of this plant can also alter natural ecosystems by leaving masses of dead stems behind, making river banks vulnerable to erosion and flooding. Japanese knotweed poses a significant threat to riparian areas, where it can survive severe floods and rapidly colonize disturbed shores and islands. Once established, populations are extremely persistent.
Description and Biology:
Plant: Upright, shrubby, herbaceous perennial that can grow up to 10 feet in height. Stems are smooth, stout, and swollen where the leaf meets the stem. Reddish purple shoots are present in early spring, but as the canes grow, the leaves unfurl and the plant turns green. Mature canes are hollow and have a pattern of purple speckles, leading to its other common name – Long Island Bamboo.
Leaves: Leaves are about 6 inches long by 3 to 4 inches wide. They are broadly oval triangular and pointed at the tip.
Flower, fruit, and seeds: Tiny, greenish-white flowers occur in branched displays during the summer. After the flowers blossom they are followed by small winged fruits. Seeds are triangular, shiny, and about 1/10 of an inch long.
Reproduction: Japanese knotweed spreads by seeds and through long, stout rhizomes. The rhizome may extend as deep as 10 feet and up to 21 feet away from the parent plant. They are knotty and leathery brown. Fresh rhizomes usually possess a dark orange central core with an orange/yellow outer ring. The plant can also be transported to new sites by water, wind, discarded cuttings from gardens, or on the soles of shoes. In addition, Japanese knotweed is continually re-introduced through infested topsoil.
Identification: If examined closely, Japanese knotweed is easy to identify. It may be mistaken at first glance for sweet pepperbush (Clethra alnifolia) or
Virginia
sweetspire (Itea virginica), but the leaves of Japanese knotweed are much larger than those of the two native plants.
Prevention & Control: Because Japanese knotweed can re-grow from vegetative pieces and seeds, it is an extremely difficult plant to control, though mechanical methods are commonly used to eliminate it. Young plants can be removed with a hoe, but all roots and runners must be removed and disposed of carefully to prevent re-sprouting. Cutting, mowing, and pulling can be very effective if done consistently over long periods of time. These methods can be particularly effective when combined with shading.
The Nature Conservancy includes herbicides as a management option for controlling Japanese Knotweed. Please note that a New York State Department of Environmental Conservation freshwater wetlands permit may be required along with a supplemental form for aquatic vegetation control. A certified pesticide applicator must apply the pesticides in wetland areas. Remember to only use herbicides when you are sure that they will do more conservation good than harm.
Native Alternatives:
Sweet pepperbush (Clethra alnifolia)
Virginia
sweetspire (Itea virginica)
Maleberry (Lyonia ligustrina)
Silky dogwood (Cornus amomum)
Fragrant or shining sumac (Rhus aromatica or copallina)
