The invasive plant known as Japanese knotweed (Reynoutria japonica) is notoriously difficult to eradicate, but the key is patience, persistence and a plan that can take years to succeed.
We heard from Sue, a Homegrown NH listener:
“The Japanese knotweed in the wooded areas around my home has proliferated to the point where I feel like a tiny bastion of light in the dark."
She continues, "I am too old to go around ripping it out, as I used to, and pesticides are out of the question: suggestions?”
Japanese knotweed can be a frustrating presence in the landscape, especially when it spreads through wooded areas around a home. For gardeners hoping to control the invasive plant without using herbicides, the options are limited — but not nonexistent.
“Japanese knotweed is a really tough one,” says Emma Erler, lead horticulturist at Squam Lakes Natural Science Center. “Whichever management technique you decide to use, it’s going to be a marathon, not a sprint.”
Smothering Japanese knotweed can take years
Digging or pulling Japanese knotweed can work when a population is small and newly established. If you find just one or two shoots growing in your landscape, removing them may be effective.
But for a patch that has been growing for years, Erler recommends smothering as one of the most effective non-chemical control methods.
The process involves cutting the knotweed down to the ground and covering the area with heavy-duty black plastic or nonwoven landscape fabric.
Without sunlight, the plant must draw on the energy stored in its extensive underground root system, known as rhizomes, to produce new growth. Over time, the goal is to exhaust those reserves.
In New Hampshire, the recommended approach is to let the knotweed grow through the spring and into early summer, then cut the stems close to the ground and cover the patch.
The covering should extend several feet beyond the visible edges of the infestation. Japanese knotweed has extensive rhizomes that can send up new shoots beyond the covered area, so a tarp that is too small may allow the plant to keep spreading.
Secure the covering with stones, logs, or cinderblocks. For a more attractive appearance, put mulch or arborist wood chips over the material.
The biggest challenge is the length of time required.
“Realistically, that tarp is going to need to be in place for at least five years,” Erler says. It may take as long as 10 years to kill a patch of Japanese knotweed.
Even after the covering comes off, gardeners should continue monitoring the area. The plant can persist underground for a long time, and new shoots may appear after an extended period of dormancy.
What about herbicides?
For gardeners willing to consider chemical control, herbicides can be an effective tool when used appropriately. Erler says glyphosate has generally shown good efficacy against Japanese knotweed.
Timing matters. Late summer or fall is typically the best time to apply a systemic herbicide, which moves through the plant to target the roots.
Japanese knotweed flowers are an important late-season source of nectar for bees and other pollinators. Beekeepers also value the plant for its abundant blooms.
For that reason, Erler recommends allowing the plant to finish flowering before considering herbicide treatment.
However, applications must be made before frost or freezing temperatures.
Erler cautions that anyone using an herbicide should read and follow the product label instructions carefully.
Persistence is essential
There is no quick fix for an established Japanese knotweed infestation. Even the most promising approach requires repeated attention and a willingness to monitor the site over many growing seasons.
“It requires a lot of patience,” she says. “It can be done. Absolutely.”
Homegrown NH is a collaboration between NHPR and Squam Lakes Natural Science Center.
