Japanese Knotweed

At the request of Town Administration, your Conservation Commission has a public outreach and education campaign to address a serious and widespread infestation of Japanese Knotweed. Knotweed is an aggressive and destructive weed that spreads quickly, shades out native plants and destroys habitat.  We need to act now in a collaborative effort because if we don’t, within a few years it will be virtually impossible to control knotweed.

Here is a concise description of a good way to eliminate knotweed on your property, provided by Storm Connors from the Pleasant Lake Protective Association.  Please note that you should not use RoundUp near water - use Rodeo instead.

Knotweed is blooming. That is the time to spray it with a Glyphosphate containing weed killer. (Use according to directions. More is not better.) When the plant is making flowers, all of the stored energy from the roots is being used to make flowers to produce seeds to make more plants for next year. Spray the foliage every 7 days for 3 applications when you first see the flower buds appear on the plant.

When you use weed killers during the growing season the plant will laugh at you and they will be ineffective.

In the spring, clean up the dead stalks which are non viable. When it begins to grow, pull it out or cut it down as soon as you see it. It will come back in just a few days, so pull it out, or cut it down again and again and again all summer long. If you can, mow the areas weekly during the summer. By destroying the new growth the plant makes and forcing it to use its energy to make new growth again and again, you weaken the plant and in August when you use the weed killers on the plant they will work better.

August first, stop bothering the plant and let it grow all it wants. It will soon begin to make white flowers, and that is the time for you to apply Glyphosphate again. Application is easier as the plants will only be a foot or so high. It won't kill all of the plants but it will kill much of the plant and sterilize the seed produced by the plant in the fall. If you follow these steps you will be able to destroy the plant in just 2 to 3 years.

What your Conservation Commission is doing:

  1. We are maintaining a town-wide inventory of knotweed locations that are visually observable from the 100+ miles of roads in the town.  Each location is a data point with GPS location and incorporated into the Town’s GIS database. We are asking for help from property owners in identifying locations on their own property so we can add it to the database and possibly assist in dealing with the infestation.  We are working with students from Plymouth State University under the guidance of Dr. Amy Villamagna, Assistant Professor of Environmental Science and Policy at Plymouth State University.
  2. We are working with the NH Department of Agriculture and hope to have herbicide applied to a significant percentage of the infestations that are along the state roads in Meredith.
  3. We are working with the town’s Parks & Recreation Department in their efforts to deal with the infestation at Prescott Park.  Your commission is looking to use one of the larger patches by the tennis courts as a “Learning Center” where we hope to demonstrate some of the methods for eradicating Japanese Knotweed that property owners might use.


How To Identify Japanese Knotweed

Knotweed DrawingKnotweed Infestation

Leaves:Knotweed Photo

  • New leaves are dark red and 1/2 to 2 inches long
  • Young leaves are green and rolled back with dark red veins
  • Mature leaves are green and heart-shaped, approximately 6 inches long
  • Grows rapidly to heights over 8 feet tall


  • Bloom in late summer
  • Creamy white in color and 1/4 inch wide
  • Form clustered "spikes" of flowers among the foliage
  • Spike lengths are approximately 5 inches