Q. How do I control Japanese knotweed?
Scientific name: Reynoutria japonica (Fallopia japonica)
Common names: Japanese knotweed, fleeceflower, Mexican bamboo
This stubborn Asian invader usually requires 3-4 years to eliminate. A combination of a few different removal methods often works best.
Removing manually: One of the simplest (but not easiest) methods is cutting down the plants and then digging out the rhizomes--knotweed's primary method of spreading. This can be a challenge because a new plant can grow from even a very small piece of remaining rhizome.
Cutting/Mowing: Small patches of knotweed can be eradicated with repeated and persistent mowing or weed trimming. The patches must be mowed or cut twice a month between April and August and then at monthly intervals until frost. When using a hand pruner, lopper, or weed eater, cut the stalks as close to the ground as possible. Do not let the regrowth exceed six inches before again cutting to the ground. Stack the cut stalks away from moist ground, so they will dry out and not root. When using a weed eater, ensure that scattered plant parts do not land in moist areas where they can take root.
Covering: There are reports of successful control of small patches of plants using a combination of cutting, hand pulling, and/or tilling, followed by covering. After cutting the plants down to ground level, cover the area with several layers of black plastic, thick landscape fabric, or several layers of cardboard. Hold in place with a few strategically placed rocks and cover with a layer of sand. Extend the coverage area at least 20 feet or more beyond the knotweed, and check periodically that shoots are not coming up outside of the cover or through the cover. Knotweed has been known to grow through asphalt! The cover needs to be left in place for at least one full year and probably longer.
Chemicals: This most hazardous option calls for several considerations:
- Be aware of drift, so as to avoid killing off your or your neighbors' other plants if chemicals are applied on a windy day. "Painting" herbicide directly onto plants with a brush or non-porous glove will help avoid drift.
- Chemicals can be injected directly into the plant. In late summer or early fall, cut the stems down to a few inches and inject into the base.
- Alternatively, chemicals can be poured into cut stems. Insert a stick or rod down the hollow center of the stem to break the remaining node barriers and then pour concentrated Roundup® or other herbicide down the stem.
- Avoid chemical leaching into nearby streams and ponds.
An alternative to stacking the cut material above ground is to burn all the cut material and bury the ashes.
For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service