By Emilie Inoue, VT Agency of Agriculture, Food & Markets
July 1, 2023 | Montpelier, VT - The invasive weed known as Japanese knotweed (Fallopia japonica) is actually part of a complex of species that also includes giant knotweed, (Fallopia sachalinensis) and Bohemian knotweed (Fallopia X Bohemica). These plants are herbaceous shrubs that can reach up to 19 feet in height. They have hollow, jointed stems reminiscent of bamboo, but can be distinguished from it by the presence of a sheath around the joint where the leaf emerges. In addition, they have large (3-5 x 2-4”), alternate, oval shaped leaves with a tips that taper to a point. The species that make up the complex are similar in appearance, though giant knotweed has considerably larger leaves.
Japanese knotweed, like many invasive weeds, outcompetes native, locally adapted plants. It spreads though rhizomes, sending up fast growing shoots from an underground stem which allow it to form dense thickets to the exclusion of other plants. Members of the knotweed complex spread primarily as stem fragments or shoots from rhizomes. Seeds spread via water and wind, and by animal and industrial activities (such as the movement of soil, including as commercial fill). Because this plant can be easily spread and have adverse impact on local plant and wildlife populations, Japanese knotweed is listed on Vermont’s noxious weed quarantine rule list. Plants that are listed on the noxious weed list are prohibited from possession, cultivation, sale, movement, and distribution. The Agency does provide permits for the management of Japanese knotweed.
Knotweeds can be controlled physically though burning, smothering, and pulling. Growth can also be controlled though cutting or mowing; if the plant is cut back multiple times per season for several seasons in a row, the root-borne starch reserves can eventually be depleted. However, the plant’s extensive root system can make both cutting and pulling less effective. Pulled or cut plant remains must be destroyed to prevent further propagation. This can be accomplished by burying the stems at least 6 feet deep or allowing them to dry until biologically inert. It is possible to control knotweed with chemical and biological control are methods as well. Chemical herbicides can pose a danger to aquatic habitats, so they often prove difficult to use against riparian populations. Aphalara idatori is a psyllid insect that was approved for release by the USDA in 2020, though it should be noted that previous releases of this insect in other countries have not led to the establishment of biocontrol populations. USDA is continuing to work on additional solutions.
Sources:
https://nyis.info/invasive_species/japanese-knotweed/
https://www.nyisri.org/research/biological-control/biocontrol-updates-resources/
ITIS - https://www.itis.gov/servlet/SingleRpt/SingleRpt
https://www.agriculture.nh.gov/publications-forms/documents/japanese-knotweed-bmps.pdf
https://vtinvasives.org/invasive/knotweed-japanese