Vermont Public is independent, community-supported media, serving Vermont with trusted, relevant and essential information. We share stories that bring people together, from every corner of our region. New to Vermont Public? Start here.

© 2024 Vermont Public | 365 Troy Ave. Colchester, VT 05446

Public Files:

For assistance accessing our public files, please contact or call 802-655-9451.
Play Live Radio
Next Up:
0:00 0:00
Available On Air Stations

For information about listening to Vermont Public Radio, please go here.

Vermont Garden Journal: Getting Rid Of Pesky Japanese Knotweed

Japanese knotweed is considered an invasive species in many states and Canadian provinces.
Japanese knotweed is considered an invasive species in many states and Canadian provinces.

There are invasive weeds and then, there is Japanese knotweed. This weed takes over wetlands, stream banks, roadsides, and moist landscapes, crowding out other plants and destroying habitat.

In Japan, knotweed is kept under control by a variety of native insects and diseases. It arrived in North America via Europe in the late 1800s and it's considered invasive in 42 states and eight Canadian provinces. It grows quickly in spring from underground rhizomes into a 10-to-15-foot tall mass of greenery. Right now its greenish-white flowers are blooming.

While Japanese knotweed spreads by seed and by stems rooting in moist soil, the real culprit is the roots. If you dig out the roots, any pieces left in the soil will sprout and form more plants.

So, what to do? First, be realistic. A single season of mowing or digging won't stop Japanese knotweed. This is a long term proposition. You'll need to use a number of techniques to be successful. Mow or cut down the stand every two-to-three-weeks, or whenever the plants are six inches tall, from May until October. Gather the cuttings and place them in black plastic garbage bags in the sun for weeks to kill them. Then, cover the stand with a heavy duty tarp used for erosion control. Both of these methods are designed to weaken the roots and may need to be used for a number of seasons. After it's been weakened, try to dig out a small stand by hand. You'll have to repeat this annually, for years, since it will resprout.

I'm not one to usually recommend herbicides, but for large infestations you may have to resort to chemicals. Check with the Department of Agriculture for the best ones to use and where you can apply them. Consider using the injection or foam method. In fall, cut stems six inches above the soil line and apply the herbicide directly to the cut to translocate into the roots. This will reduce the herbicide spread to other plants.

Charlie Nardozzi is a nationally recognized garden writer, radio and TV show host, consultant, and speaker. Charlie is the host of All Things Gardening on Sunday mornings at 9:35 during Weekend Edition on Vermont Public. Charlie is a guest on Vermont Public's Vermont Edition during the growing season. He also offers garden tips on local television and is a frequent guest on national programs.
Latest Stories