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How to Get Rid of Knotweed

By S.F. Heron ; Updated September 21, 2017
Use a thick tarp to smother the tenacious knotweed.
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Some weeds simply defy the normal weeding techniques of hand pulling and digging. Knotweed is one such weed that seems to burst forth as a denser plant after weeding. Knotweed can grow over 8 feet tall with oval-shaped medium green leaves. Getting rid of knotweed requires persistent effort, and complete eradication may take more than one growing season.

Manual Knotweed Removal

Schedule the beginning phase of knotweed removal for the fall. Use pruning loppers to clip thicker stalks more than 1/2 inch in diameter down to the soil level to leave no sharp protrusions to puncture landscape fabric or tarp placed over the stumps.

Remove all visible stalks and foliage and discard in yard waste bags for removal from your landscape. The Oregon State University Cooperative Extension Service recommends burning or complete removal of the knotweed stalks and foliage from the property. Knotweed is not suitable for composting.

Stretch a thick tarp over the entire area and extend at least 10 feet beyond the growth site. Knotweed throws out rhizomes (small trailer roots) in every direction underground. Extending the length and width of the tarp ensures maximum coverage of potential new growth areas.

Weigh down the tarp securely with bricks spaced about 1 foot apart along the outer perimeter of the trap. If wind could be a problem, place bricks along the entire edge of the tarp to keep it from shifting during the winter.

Leave the tarp in place until the spring. Keep a close eye on the tarp to make sure shifting doesn't expose shoots. Remove the tarp after the winter and examine the results in mid-spring. Replace the tarp if necessary to smother further growth.

Herbicide Knotweed Removal

Schedule herbicide application for the fall when the plant begins transferring energy to the root system, recommends the Oregon State University Cooperative Extension. Perform the application on a hot sunny day for best results.

Start spraying glyphosate herbicide at the top of the plant to cover all foliage. Allow the plant to remain in place until all foliage appears brown over the next two to three weeks. Reapply if the plant still seems particularly vigorous after 14 days.

Cut back the dead foliage in the spring and dispose of the knotweed stalks in yard waste bags.

Reapply glyphosate herbicide at the first site of any reappearance of the weed. Herbicide removal requires regular and consistent effort to eradicate knotweed. If the knotweed persists during the summer growing season, the Purdue University Cooperative Extension recommends cutting down each stalk and applying glyphosate herbicide within a half-hour of pruning.


Things You Will Need

  • Pruning clippers
  • Pruning loppers
  • Yard waste bags
  • Bricks
  • Thick plastic tarp
  • Glyphosate-based herbicide