Invasive Knotweeds

Agriculture Topics

Invasive knotweeds were introduced to the United States as ornamentals from Asia in the late 1800s. Although these knotweeds are collectively referred to as Japanese knotweed, there are two parent species and a third hybrid widespread in West Virginia.
The two parent species are giant knotweed and Japanese knotweed, and the hybrid species is Bohemian knotweed. Do not plant them for ornamental purposes, because they can quickly invade the area.
Invasive knotweeds can be easily recognized by their hollow stems, tall-growing habit, prominent broad leaves and a vibrant display of white flower clusters during fall months. A variety with bright red flowers has been observed lately.
Giant knotweed has large soft leaves with fine hairs on its edges. A fully-expanded giant knotweed leaf is heart-shaped, has distinct lobes at the leaf base, and is large enough to easily mask a letter-size paper.
Japanese knotweed leaves are usually less than half the size of a giant knotweed leaf, with a leathery texture, and spade-shaped with no distinct lobes at the base.
Bohemian knotweed has characteristics that fall in between the parent species described above. This hybrid is considered to be even more invasive than giant or
Japanese knotweed, and possibly more common in West Virginia.
These invasive knotweeds are an ecological threat because they can spread and regenerate through extensive underground rhizomes, potentially displacing native plants. If established on property, knotweeds may depreciate its land value. Removal and control can tie up considerable amounts of time and resources.
Knotweed thrives along streams of any size and is also spread by streams. It can easily be found in many places in Lewis County, including along the stream behind the Horner post office, the bank above the Willow Street park in Weston, and multiple locations along the West Fork River.
Successful control methods mean destroying the underground plant parts. One option is to remove top growth by repeated cutting to eventually kill knotweed. This technique’s success will depend on how often it is mowed, when and the number of times it is mowed over. Allow the knotweed to grow fully in spring, then cut it back and repeat the process several times during the growing season.
Chemical control can be cost effective. Isolated infestations may be easily managed by a landowner, however, professional assistance may be required to manage large scale infestations of well-established knotweed stands.
Older stands also respond better to herbicides if they are cut back after complete emergence in spring, regrown and herbicide is applied in late summer or early fall (July to September).
Herbicides effective against these knotweeds are glyphosate and imazapyr. Glyphosate is available as Roundup® for land use or Rodeo® for aquatic use (when spray can come into contact with water). Various generic preparations are also available for both purposes.
Imazapyr is available as Arsenal® for land use or Habitat® for aquatic use.
If using glyphosate, a 3 percent to 4 percent solution made from a concentrate is needed to get good control. Add methylated seed oil (MSO) at the rate of 1.25 oz./gal. water to better absorb the herbicide, especially if a generic preparation is used. Do not use ready-to-use preparations, since most are not concentrated enough to control invasive knotweeds.
Imazapyr (Arsenal® or Habitat®) is best applied by a professional due to its persistent nature and potential to cause injury to nearby desirable vegetation by root uptake.

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