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Japanese Knotweed

Did You Know?

A few facts:

  • Highly invasive and difficult to control
  • Source of heart-healthy antioxidant
  • Displays white flower clusters in the fall

How to get rid of Knotweed:

  • Frequent mechanical removal
  • Treat with herbicides

Information by Rakesh Chandran, Ph.D., WVU Extension Weed Science Specialist

Japanese Knotweed: A super pest or a super food?

Introduced from Asia in the late 1800s as a fodder or an ornamental, Japanese knotweed has become an invasive weed in West Virginia. Because of its ability to regenerate through extensive underground rhizomes, it spreads rapidly, affecting drainage of waterways and displacing native plants. If it grows in close proximity to housing, it can damage structure and break into sewer lines. Invasive knotweeds are prevalent throughout the state, especially along streams and riverbanks.

Japanese Knotweed Identification

Invasive knotweeds can be recognized by their hollow stems, tall-growing habit, prominent dark green leaves and vibrant display of white flower clusters during autumn. There are three species of knotweeds in West Virginia – giant knotweed (Fallopia sachalinensis), which has large, heart-shaped leaves; Japanese knotweed (Fallopia japonica), which features smaller spade-shaped leaves; and the hybrid Bohemian knotweed (Fallopia X bohemica), which has characteristics that fall in between the two and is considered to be the most prevalent in West Virginia. The term Japanese knotweed is often used to refer to either Fallopia japonica or Fallopia X bohemica.

Japanese knotweed is considered to be one of the best sources of the phenolic compound resveratrol, an antioxidant known to improve organ function, including the heart, and alleviate common ailments, such as diabetes. Resveratrol also is found in red wine, but in much lower quantities.

Controls for Japanese Knotweed

Invasive knotweeds can be difficult to control depending upon their age and the severity of the infestation. Mechanical methods are time consuming and labor intensive. Digging and grubbing can be effective if all underground parts are completely removed and burned. Frequent removal of top-growth by repeated cutting can eventually control a colony.

Herbicides provide effective control of knotweed if applied properly. Established stands may require multiple herbicide applications over several years. An integrated approach where knotweeds are cut back after development of full foliage in spring, followed by regrowth and herbicide application in late summer or early fall is very effective.

The most effective herbicides for knotweeds are glyphosate (Roundup or other formulations for terrestrial use or Rodeo/Shore-Klear when spraying near water) and imazapyr (Arsenal for terrestrial use or Habitat for aquatic use).

If using a 4 pounds per gallon (41%) glyphosate formulation, a 4% spray solution (5.1 ounces of product per gallon of water) may be used. If using a 5.5 pounds per gallon (49%) formulation of glyphosate, a 3% spray solution (3.8 ounces of product per gallon of water) may be used. Add a surfactant, such as methylated seed oil at the rate of 1.25 ounces per gallon of water, especially if a generic formulation is used. Spray to wet most (more than 80%) of the foliage without creating dripping droplets. Do not apply the herbicide under drought-like soil conditions.

Monitor the treated area during the following season to determine the need for repeat applications. After knotweed is controlled, the exposed areas should ideally be revegetated with suitable native species to prevent future infestations. Areas treated with glyphosate may be reseeded two weeks after treatment (to allow weed kill), but areas treated with imazapyr should not be reseeded until the following year due to residual activity of this herbicide.

Author: Rakesh Chandran, WVU Extension Weed Science Specialist
Last Reviewed: May 2019

Recommendations for the use of agricultural chemicals are included as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services does not imply endorsement by West Virginia University Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.