How to Grow: Control Japanese Knotweed

Learn how to identify and control Japanese knotweed invasive perennial in your landscape

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 8 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 2 to 3 weeks, or whenever the plants are 6 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 weaken, 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 6 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.

Excerpted from the Vermont Garden Journal on Vermont Public Radio.