Dear Burdys:

We applaud your efforts to control the Japanese knotweed. For beginners, we attach a link to a good publication from the New Hampshire Department of Agriculture, which discusses management techniques beginning at page 7. https://www.agriculture.nh.gov/publications-forms/documents/japanese-knotweed-bmps.pdf

If you want to avoid using herbicides to control the knotweed, the choices are mowing and smothering. Some commentators suggest mowing every two to four weeks during the growing season, but this will take years and requires careful disposal of the the mowed refuse to avoid spreading the knotweed. Other commentators contend that mowing is not effective. The NH Dept. of Agriculture publication notes that "Cutting the aboveground portion of the plant usually stimulates dormant lateral buds along the rhizome system, which then send up new shoots further away from the crown, essentially increasing the total number of stems and extending the limits of the stand." 

As you know, another other option is smothering. This option is discussed at length by the NH Dept. of Agriculture booklet at pages 7-8. I believe the method described here is probably different than the method that you have used (3-4 inches of bark mulch or wood chips over the cut stems and non-woven geotextile material or 7 mil black plastic over the mulch), so you may want to try it. Although the smothered area must stay covered for up to 5 years, it doesn't require the constant attention of mowing. According to the NH Dept. of Agriculture, this method has been very successful in NH.

Carefully dispose of the cut knotweed because it can easily resprout. You can put the material in a double black trash bag and let it sit in the sun for a month. After a month, the material should be dead and you can dump it or dispose of it with the trash, assuming there is no viable seed mixed in with the removed material. Your local solid waste district may have composting facility that will take invasive plants. Contact the Bennington County Solid Waste Alliance at https://www.bcswavt.org/contact-us/.

Good luck with this project. It's never too late!

Regards,

Pat

Master Gardener Volunteer

UVM Extension|Master Gardener Helpline|<personal data hidden>|Jeffords Hall University of Vermont|Burlington VT

Thank you for contacting the UVM Extension Master Gardener Helpline. Due to the COVID19 pandemic, on-campus offices are currently closed. We have a limited number of volunteers who have agreed to answer questions from home. We appreciate your patience in allowing 7-14 business days for a response.

We hope that you live to defeat the knotweed and more!