Japanese Knotweed

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.

Management strategies:

  • digging and pulling – requires all roots and runners be removed; special care must be taken to avoid missing any roots or rhizomes; should only be done for mature stems, ensuring that it is pulled from the base of the stem; deep digging for well-established plants lead to a significant increase in stem density but if it is integrated with herbicide treatments it can be effective.
  • cutting and mowing – well suited for houses, resorts and lawn borders, parks and gardens – cutting stems in early summer followed by herbicide application can improve effectiveness of treatment. Some suggest against this treatment as there is less leaf area for herbicide treatment than knotweed that is left alone; also, it is difficult to time mowing properly.
  • biocontrol – Aphalara itadori is a species of psyllid from Japan which feeds on Japanese knotweed and may reduce the growth rate; this biocontrol was released in the UK in 2010 and Canada in 2014 and has been approved for release within the US.
  • chemical control – systemic herbicides are generally the most efficient and effective means of managing knotweed due to the chemicals being trans-located to rhizomes following foliar applications; the ideal time to apply an herbicide is when flower buds are developing in August or September after-stems were cut in late spring or early summer; after treatment, all dead stems should be knocked down to ensure easy access for future management; significant decreases in Japanese and hybrid knotweed abundance can be achieved by different combinations of herbicides; however, there is no robust evidence available regarding their long-term effectiveness.
  • disposal – plant material must be removed and disposed of properly because the stems and rhizomes can resprout; solarized biomass in a bag in the sun for at least two weeks or plant material can be spread on a contained, impervious surface in a thin, even layer to ensure even heating. Contaminated soil and plant material should be buried at least 5 feet under the surface in a disposal pit with annual monitoring to ensure there is no regrowth; if necessary, re-sprouting plants can be treated with herbicides; this material should not be composted. Stem and other plant material could be burned to ensure there is no regrowth and then put into a landfill. For contaminated soil, the environmental agency waste regulation department should be contacted or disposal can be in situ, to reduce landfill charges and decrease the risk of spread and ensure that effectiveness of the disposal can be monitored. It is extremely important to tamp down soil after removing plants. The key is to avoid plant material from making contact with any type of watercourse. Always inspect your shoes and clothes before leaving your garden if it is contaminated with knotweed. Ensure you don't carry any plant or oil material from your garden. You can easily spread the knotweed to other parts of your garden.

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.

References:

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”

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Last updated November 3, 2020