Author: Sandy Vanno, Master Gardener Warren County CCE
Japanese knotweed (Polygonum cuspidatum), a member of the buckwheat family is a native of Asia and was first introduced to England in the early 19th century as an ornamental plant. It was later introduced into the United States for erosion control, and on Long Island as an estate-grown ornamental, due to its attractive foliage and cream-colored inflorescence. By the mid-1890s it was reported near Philadelphia, PA, Schenectady, NY, and in New Jersey. Although once sold through seed and plant catalogs, by the late 1930s knotweed, was already being viewed as a problematic pest. It is now widespread throughout New York State and most of the United States.
Knotweed is often confused with bamboo (subfamily Bambu-soideae), another invasive plant. Unlike knotweed, bamboo has slender, papery leaves that persist year-round. Bamboo stems are also jointed, but much woodier while living knotweed stems are herbaceous and will be visibly wet upon cutting. Japanese knotweed stems are hollow and jointed. The leaves are alternate, broadly egg-shaped, and 3 to 6 inches in length. The plant is dioecious, so male and female plants both produce cream-colored flowers that vary slightly in appearance. Flowers appear in late summer and are found in erect clusters 4 to 5 inches long arising from the leaf axils. It can most commonly be found in moist, unmanaged areas, including riverbanks and riparian sites, sodded storm drains and ditches, roadsides, and unkempt gardens. It tends to flourish on moist, well-drained, nutrient-rich soil, especially on shaded banks. Recently it has appeared more frequently along with sunny, dry roadside locations, suggesting the plant is adapting to diverse environments. It creates a dense canopy that prevents the growth of native plants, allowing it to dominate large areas of land. It has the potential to increase soil erosion on riparian banks and flooding potential. Knotweed shoots can also push up through roads, sidewalks, and foundations.
Management of Japanese knotweed typically requires several years and becomes very expensive. One of the best ways to prevent its colonization is to ensure that disturbed habitats are rehabilitated with native vegetation before knotweed can invade. However, if it does invade, digging or pulling can control, or locally eradicate early infestations. Integrated management incorporating chemicals may be more effective for larger infestations.
For more detailed information on Management Strategies, and a detailed list of herbicide treatments and application time for sites near water and away from water, please refer to the document “Japanese Knotweed (Reynoutria japonica): Best Management Practices”, published by New York Invasive Species Research Institute and Cornell College of Agriculture and Life Sciences.
Japanese knotweed strikes fear into the hearts of homeowners in England, as its presence can threaten their property's foundations and make it almost impossible to sell or remortgage. Standard household insurance policies refuse to cover the damage. Due to the amount of damage knotweed causes, if it's discovered at a property as a result of a normal mortgage valuation or property survey, many lenders will either refuse a mortgage altogether or impose specific criteria if they do decide to proceed at all. Under Environmental laws in England, failure to control the spread of the plant can result in civil nuisance claims which can mean legal action and heavy financial penalties. Knotweed can also reduce the value of a property in England between £25,000 and £50,000 if knotweed is formally identified by a surveyor. There is a growing concern in the United States.
CCE Oneida County; Japanese Knotweed
Cornell University Press; “Weeds of the Northeast”
New York Invasive Species Clearinghouse Species Profile “Japanese Knotweed”
Penn State Extension; Invasive Plant Fact Sheet “Japanese Knotweed”
NY Invasive Species Research Institute; Cornell College of Agriculture and Life Sciences; “Japanese Knotweed: Best Management Practices”
Last updated November 9, 2021