Japanese knotweed, sometimes called Japanese bamboo or Asian knotweed, is an aggressive weed classified as noxious or invasive in parts of the United States. The World Conservation Union, which develops practices to preserve native flora, classifies the plant as one of the top invasive species on the planet.
Part of the buckwheat family, it’s botanically classified as Reynoutria japonica, Fallopia japonica, and Polygonum cuspidatum. Despite its common name, it’s not related to bamboo.
The plant blossoms in the fall, when flowering drops off among other species, making it useful for beekeepers. In the northeastern states, the resulting honey is sold under the name bamboo honey, since bees use it as their primary source of nectar at that time of year.
While it isn’t intentionally cultivated, foragers use the stems as a wild vegetable, and the plant is used in American, Japanese, and European cuisine for its flavor, which is similar to rhubarb.
Japanese knotweed should be removed wherever it is found unless it is being controlled to be used for honey or vegetable production. The plant will smother out natives and degrade the soil where it grows.
Originally indigenous to East Asia in China, Japan, Taiwan, and Korea, Japanese knotweed was brought to North America in the 1800s as an ornamental plant.
It has since naturalized on all continents and in 42 of the 50 states in the US. It isn’t found in the arid Southwestern States, Florida, Louisiana, Colorado, Hawai’i, or North Dakota. While it is banned in many regions, some people still opt to grow it ornamentally, making control difficult.
It grows in a wide range of environments, from deep shade to full sun, in areas with salinity and pollution, and in regions with extreme heat and brief drought. This adaptability has helped it spread far.
Japanese bamboo grows near water sources such as rivers, streams, waste ponds, and canals, though there are exceptions. It favors disturbed areas like roadsides, abandoned lots, and fallow fields.
It’s strong and aggressive enough to break through concrete, so you will find it coming through basement foundations and sidewalks.
The plant can survive flooding and will smother native plant populations. Japanese knotweed also kills nearby plants through allelopathy, which means it releases inhibiting chemicals to retard growth.
Japanese knotweed is sometimes called Japanese bamboo because of the hollow stems with raised nodes, which appear similar to true bamboo. Leaf junctures are swollen and covered in a sheath called an “ocrea.” The mature stems are orangish-green and are covered in a fine white powder.
Young or repeatedly cut stems can be solid or hollow. Stems can reach up to 13 feet tall, though a majority of plants stay shorter, and they cluster in large, dense thickets that can spread extensively.
Young leaves emerge as dark red, gradually changing to green with red veins as they mature. Once fully mature, the fully green leaves are oval with a truncated, flattened heart-shaped base and a smooth margin. The leaves are about six inches long and about four inches wide when mature.
The foliage extends from the stems in an alternate, zig-zag pattern.
The flowers form on upright, six-inch-tall racemes and are light green, cream or white. Once the blossoms fade, small fruits with wings on three sides form. Inside are triangular fruits that are dark brown and glossy. Once the first frost arrives, the leaves die while the stems remain.
The plant is spread by large underground rhizomes and by seed.
Dogwood (Cornus spp.) grows twice as high as Japanese bamboo, but the leaves look similar, so young trees might be mistaken for the invasive plant. Look at the stem for the tell-tale nodules and examine the leaves. They grow opposite one another on dogwoods.
Lilac (Syringa spp.) leaves look similar to young knotweed leaves, but lilacs have woody stems and branches with rough bark, whereas knotweed has herbaceous branches and smooth, woody stems. The leaves grow opposite rather than alternate on lilacs.
Lesser knotweed (Persicaria campanulata) looks similar in most ways, except that it has pink flowers and the undersides of the leaves are light with fine, white hairs. If you look closely, the leaves are attached to a stalk that is more straight than zig-zag.
The leaves on giant knotweed (P. sachalinese) are twice as large as Japanese bamboo, and the plant can grow several feet taller. Otherwise, this invasive weed looks similar to its relative.
Virginia knotweed (Tovara virginiana) looks identical, but for the bristles on the sheat of the stem. It also has more of a vine-like growth habit.
Himalayan balsam (Impatiens glandulifera) is sometimes mistaken for Japanese bamboo, but this plant has pointier leaves, and the blossoms are pinkish-red.
Bamboo is similar to knotweed in that the stems are hollow and have ridges, but the leaves are long and narrow.
The seeds of the plant can be carried on clothing, shoes, pet fur, wind, water, contaminated soil, and tools, further increasing its ability to spread. Cleanliness and sanitation are crucial in order to avoid introducing the plant to an uncontaminated area.
A young, single plant can be removed by hand, though take care to pull all of the roots.
Once established, controlling Japanese knotweed requires killing the rhizomatous roots. A plant can be cut back entirely to the ground, and it will regrow if the roots aren’t killed.
It’s possible, though difficult with larger clumps, to dig up the entire root system. The roots can extend up to ten feet deep into the soil, so extraction requires extensive labor. If a few inches of root are allowed to remain in the soil, a new plant will emerge.
Care should be taken when removing this plant from wilderness areas as ripping out the entire root system and negatively impact native flora and damage soil health.
To destroy the roots, one should repeatedly mow or cut down the stand. This requires several years of consistent effort, but it will eventually deprive the roots of nutrients, and they will starve and die.
In addition to cutting down all of the stems, you can also apply a product that contains glyphosate, triclopyr, imazapyr, or a combination of two. This will speed up the process, but you will still need to continue to mow down or remove any new growth.