How to Get Rid of Japanese Knotweed
Roger Cook brings some expert advice for getting rid of this invasive weed
My yard is being overtaken by Japanese knotweed. I’ve pulled up its roots, chopped it to the ground, and sprayed it with herbicide and it still comes back. Can it be stopped?
—Bob Wiley, Lynn, Mass.
This stuff is the worst. Japanese knotweed (Fallopia japonica) is highly invasive, smothers native vegetation with its dense foliage and thick rhizomes, and, in this country at least, has no natural predators to keep it under control. It’s already established in the Northeast, Midwest, and Northwest and continues to spread. Hard to believe it was brought here deliberately in the late 1800s as an ornamental for hedges.
It is possible to knock out knotweed if you attack it in a smart and disciplined way. If the patch is small, with fewer than 50 stalks, and only a couple of years old, you might be able to dig it up in one go, as long as you remove and bag every piece of root or rhizome. Leave behind even a half-inch piece, and it can grown back.
But if you find that the roots go deeper than you care to dig—10 feet is the norm for established patches—try this less labor-intensive approach: In the spring, chop back the still-tender shoots with a string trimmer when they’re 1 to 2 feet high. Collect every stalk—they can take root after being cut—bag them, and put them in the trash. In the late summer, when knotweed flowers, chop it back again with loppers a few inches above the ground, and immediately dab each cut stem with Roundup (glyphosate). This short-lived systemic herbicide migrates to the roots, but it’s not likely to kill all of them the first time around. Again, bag all the plant debris and put it in the trash. Repeat the process for the next three years, by which time the plant will have used up the energy stored in its roots and rhizomes and will die.
However, if your thicket of knotweed is near water, you can’t use Roundup; it’s highly poisonous to fish and other aquatic creatures. But if you painstakingly cut down and collect all the stalks three or four times a year, the weed should eventually succumb...hopefully!
Shown: When Japanese knotweed flowers, as shown above, it’s time to chop it down and dab systemic herbicide on the stubble.
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