Japanese knotweed is a shrubby, herbaceous perennial with reddish, hollow, jointed stems that become woody and rigid with age and resemble bamboo. Sheathlike coverings and swollen nodes are found along the stem where the leaves join. Leaves are alternate, broadly heart-shaped with distinctly pointed tips, and 3–6 inches long. Blooms July through September. Flowers are white to greenish and produced in long spikes that arise where leaves join the stems or at the ends of the stems. Seeds are small, winged, and are formed by August and September.
Similar species: Missouri has 4 Fallopia species. This is the only one with stout, erect stems and is a perennial. The others have slender stems that climb and twine, and they are annuals or short-lived perennials.
Height: 3 to 12 feet.
Scattered throughout Missouri. There are currently 19 infested counties.
Habitat and Conservation
Although Japanese knotweed prefers moist soils, it tolerates a wide range of growing conditions, including full sun, high salinity, and dry soil. It grows along rivers, streams, roadsides, utility rights-of-way, and crop fields. Originally from Asia, it was first introduced to the British Isles as an ornamental plant in 1825. Due to its aggressive nature, many states now prohibit its use.
Invasive. Spreads primarily by vegetative means with its vigorously growing rhizomes, which are very durable and survive for decades. A small piece of rhizome moved to another site will give rise to a new plant, as often happens on eroding stream banks. It also colonizes along streams via seeds and will ultimately dominate a given streamside habitat. Knotweed also spreads along roads during routine mowing and is often transported to new sites in fill dirt.
Brought to North America in the late 1800s for use in landscaping and erosion control, it easily gets out of control and spreads where it’s not wanted. It is not a good erosion-control plant, as it doesn’t hold the soil well. Our country loses billions of dollars a year to invasive species.
It grows quickly, forming dense thickets and outcompeting native plants, including trees and shrubs. Along streams, it survives floods and rapidly colonizes scoured shores and islands. Its rhizomes do not stabilize banks as well as the finer roots of trees or grasses, opening the banks to erosion.