I garden at a summer house in Ulster County, New York, and would like to clear the property of Japanese knotweed (Polygonum cuspidatum). Can you tell me how to get rid of it?
Melanie Small, New York, NY
Japanese knotweed is extremely invasive. This tenacious weed spreads by thick, underground rhizomes as well as by self-seeding to form large, dense colonies.
Photo/Illustration: Jennifer Blume
Chris Mattrick, Rare Plant Curator at the New England Wild Flower Society's Garden in the Woods, replies: You’re not alone in your desire to rid your property of this invader. This fall-blooming, herbaceous perennial can overrun natural areas, gardens, and backyards. It has even been rumored to push its way through several inches of asphalt. It was first introduced from Japan in the late 1800s as an ornamental species, but soon escaped from gardens into lawns, roadsides, and the wild. It was further spread by its use as erosion control along streams and rivers.
Once it is established, Japanese knotweed forms large, dense colonies that spread primarily by rhizomes that can extend up to 65 feet. Seeds are also a source of new plants, and colonies of Japanese knotweed are often transported to new locations in fill dirt. It takes just an inch-long piece of knotweed rhizome in fill or compost to regenerate into a new colony. So, be careful when disposing of removed plants and seeds. The best technique is to double-bag the plant remains in plastic trash bags and take them to your local landfill.
Japanese knotweed is a tenacious weed, but there are techniques that are successful in reducing populations of this species. These methods need to be repeated for two to three years. The easiest, but most time-consuming, technique is cutting knotweed colonies down to the ground at least four times over a growing season. This will reduce the plants’ underground reserves and cause considerable dieback.
Another tactic is to apply glyphosate, an ingredient in Roundup. Glyphosate is most effective when used in conjunction with cutting back plants. Apply the herbicide to the regrowth of cut stems, but remember that glyphosate is a nonselective herbicide which will kill any plant it comes in contact with. Always use caution and read the label when using garden chemicals.
If your infestation is small, digging out the knotweed plants is an option. But remember, you must completely remove all parts of the plants. Then check regularly for any signs of new growth, and remove it promptly.