Japanese knotweed is native to eastern Asia and was first introduced into North America in the late 1800s. It was used as an ornamental plant on properties and also for erosion control due to its deep and interwoven root system.
Japanese knotweed is a dense growing shrub reaching heights of 10 feet and looks like a bamboo. The semi-woody stem is hollow with enlarged nodes. Leaves are alternate, 6 inches long, 3-4 inches wide and broadly-ovate. Japanese knotweed commonly invades disturbed areas with high light, such as road sides, stream banks and shore lines, but can also grow in full shade conditions with a high drought tolerance, a high temperature tolerance and high salinity conditions.
Reproduction occurs both by rhizomes (lateral growing roots) and seeds, making this plant extremely hard to eradicate. The plant has also been known to reproduce simply from cuttings which allows for many means of dispersion.
The stands are so dense that they shade out other plant species, reducing wildlife habitat for native species. This plant is extremely hard to eradicate once established, so the key it preventing establishment by manually removing immature clusters. Along river banks, the shallow root growth can cause unstable banks, which is exacerbated by knotweed dying back in the fall.
As previously stated, knotweed has the ability to regrow full plants from its cuttings as well as from its rhizomes (root structure) and seeds. Due to this, knotweed cannot simply be cut down, but must be dug up with the entire root structure and disposed of fully. Plants should be removed from the site and either disposed of in black plastic bags, or at the town composting facility. A "cut-and-dab" approach can be used if woody root can be exposed. Foliar spray is not recommended as it can be harmful to the surrounding floura and fauna. See the invasive removal page for how to carry out these methods. Any removal within 100 feet of wetland resource areas, including certified vernal pools, or within 200 feet of a perennial stream may require approval from the Concord Natural Resources Commission. Please contact the Division of Natural Resources before you begin.
The following native plants can serve as a good replacement to knotweed in a garden: