Photo credit: © Ricardo Martins/Flickr
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Species at a Glance
Japanese knotweed is a shrub-like herbaceous perennial that gets the nickname “elephant ear bamboo” from its stems that resemble bamboo when mature. It can reach upright heights of over 4 m (13 ft).
Leaves: Wide, triangular to egg-shaped leaves are pointed at the tips and arranged alternately along the stem. Size varies, but average is about 15 cm (5.9 in) long by 8-10 cm (3.1-3.9 in) wide. Leaf stems are often reddish in color and leaf bases are essentially straight across (truncate).
Flowers: Small, attractive, greenish to white flowers occur in branched clusters about 10-13 cm (3.9-5.1 in) long in the summer.
Fruits/Seeds: Soon after flowering, small winged fruits called nutlets are produced. Triangular seeds are shiny and small, about 0.3 cm (0.1 in) long.
Stems/Roots: Smooth, stout, hollow stems are swollen at the leaf joints and resemble bamboo, especially in older plants. Like all members of this family, the base of the stem above each joint is surrounded by a thin membranous sheath called an ocrea.
Giant knotweed (Fallopia sachalinensis) occurs in some of the same areas as Japanese knotweed but can be distinguished by its larger leaves with heart-shaped bases instead of the characteristic truncate leaf base. It also lacks egg-shaped leaves and the stems that resemble bamboo.
Found in moist, open, partially shaded habitats, Japanese knotweed has been reported along riverbanks, wetlands, roadways, hillsides, and disturbed areas in a variety of soil types and pHs. It also tolerates adverse conditions such as high temperatures, high salinity, drought, and floods.
Long stout rhizomes, which are capable of rapidly producing new plants, result in thick colonies. Plant fragments, seeds, and rhizomes can be spread naturally by water and wind, but also by human interactions.
Native to Japan, China, and parts of Korea and Taiwan, Japanese knotweed was likely introduced to the United States in the late 1800s as an ornamental plant and for erosion control and landscape screening. It is now found throughout the United States and is widespread in the Mid-Atlantic region.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Japanese knotweed emerges early in the spring and grows quickly and aggressively. It forms dense mats that crowd and shade out native plants and grasses along creeks, making riverbanks less stable and more likely to shear off during flooding.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).