How to Get Rid of Japanese Knotweed

Japanese knotweed (image) is a Godzilla weed. You'll know why if you've ever tried to fight it.
David Beaulieu

Japanese knotweed (Polygonum cuspidatum) is one of the world's most invasive plants. Nicknamed  "Godzilla weed," it has indeed wreaked havoc on even more landscapes than that fictional monster. If you have ever attempted to eradicate this noxious weed, you already know of its Godzilla-like qualities. So how do you get rid of Japanese knotweed plants? It usually takes one or more approaches:

  1. Smothering with tarps
  2. Killing with an herbicide
  3. Cutting to the ground
  4. Digging out the rhizomes

How Japanese Knotweed Spreads

Japanese knotweed is a member of the buckwheat family and was introduced from Japan to the West  (first into the U.K., then into North America) in the 19th century as a landscaping ornamental. One must assume that the Westerners who first planted it were drawn to its masses of small, white flowers, its heart-shaped leaves, and its bamboo-like canes. The weed soon spread like wildfire, taking on a life of its own, independent of its human sponsors.

A common habitat for Japanese knotweed is sunny, moist areas, including riverbanks, roadsides, lawns, and gardens. The weed is often spread via landfill or by having loam trucked in. All it takes is one fragment of one root, hidden within a pile of fill or topsoil dumped onto an unsuspecting yard, and next thing you know, this menace is gaining a toehold. Before you know it, all other plants are crowded out by this highly successful competitor for landscaping space.

There is one piece of good news, though: Japanese knotweed tends not to invade forested areas. Rather, the weed typically takes advantage of areas disturbed by humans—areas affording not only ample sunlight but also friable soil for its invasive roots. If you already have a patch of Japanese knotweed at the edge of the woods on your property, whatever you do, do not begin clearing the wooded land until you have completely eradicated this weed.

Using Tarps to Smother Japanese Knotweed

Covering Japanese knotweed with tarps in spring suppresses the weed's growth at the beginning of the season, giving you an early advantage in eradication efforts. Invest in some plastic or poly tarps that are large enough to cover the weed patch. Prepare the area by cutting old weed canes down to the ground and removing away loose material. Old canes have sharp edges that can easily puncture a tarp.

Cover the ground completely with one or more tarps, as needed. Overlap multiple tarps so that no sunlight can penetrate the seams. Weight down the tarps so they don't lift or blow away in the wind. As the new weed shoots emerge, they will act like tent poles, pushing up your tarps. But you can easily trample them down by walking over the tarps. What growth does occur under the tarps will not amount to much, since it will not have enough sunlight.

Leave the tarps in place as long as you can to choke the weeds. In the meantime, you can use the tarped area for above-ground gardening. For instance, you could apply an attractive mulch on top of the tarps and display container gardens in this area. You could even build raised-bed gardens right on top of the tarps. No matter how long it takes the Japanese knotweed down below to be smothered, your raised beds will be safe: The tarps act as a protective barrier against invasion. If there is visible weed growth when you remove the tarps, you can attach the plants with weed killer or

Using Herbicide to Kill Japanese Knotweed

The second tactic used to kill Japanese knotweed is to apply a weed killer. The recommended products are glyphosate-based herbicides, including brands such as Roundup, Gallup, Landmaster, Pondmaster, Ranger, Rodeo, and Touchdown. Glyphosate weed killers typically are mixed with water and applied to the leaves with a garden sprayer. However, you can also inject glyphosate herbicide into the canes.

By general consensus, the best time to spray the leaves with this herbicide is late summer or early fall, when the plant is flowering and the foliage is conducting the most nutrients to the rhizome to build food reserves. But some gardeners have been successful spraying glyphosate repeatedly throughout the growing season, ensuring the plants never get a chance to gain much height.

Glyphosate is a non-selective herbicide popular with farmers, landscapers, and homeowners. "Non-selective" essentially means that its killing powers are not targeted to a specific type of plant. So when you spray with this type of herbicide, you risk killing just about any plant in harm's way. Many gardeners don't like using herbicides, but Japanese knotweed is a good reason to make an exception. While the tarp method serves as a handy stop-gap measure, it's only when you bring in the herbicide that Japanese knotweed realizes you mean business. Note: Restrict the use of herbicide to areas destined to become lawns or ornamental beds. Do not use it to clear the land for a future vegetable garden.

Cutting and Digging Up Japanese Knotweed

Thirdly, Japanese knotweed can be suppressed (but not eradicated) by cutting it back throughout the summer so that its photosynthesis is never allowed to operate at high levels. Since the cuttings easily sprout new roots and take hold in the soil, pick up the cuttings and bag them afterward. Don't rely on the cutting method alone, however. Cutting back should be used hand-in-hand with injections of weed killer into the cane stumps.

As a final method, you can dig into the ground where the weed shoots come up most vigorously. In these areas, you will probably discover the rhizome-clumps from which spring Japanese knotweed's roots and shoots. In stands that have flourished for many years, these rhizome-clumps are very woody and can easily reach widths of a foot or more. Dig up the rhizomes and bag them for disposal.

Do not expect immediate results from using this tactic. No matter how careful you are, some of the rhizome roots will snap off. And from even the tiniest root left in the ground, a new plant will eventually sprout. But remember: This is a long-term war. As with cutting, don't rely on this eradication method by itself. A good reason to dig up the woody clumps is when preparing the ground for tarps so that the tarps don't get damaged.​

A Multi-Pronged Approach

These eradication and control methods are not mutually exclusive. In fact, waging an all-out, multi-pronged offensive will increase your chances of successful eradication of Japanese knotweed from your yard. For instance, you may wish to keep a tarp over the bulk of the problem area during the warm weather months, slashing and/or poisoning along the perimeter as necessary. Then, in late autumn and/or early spring, dig up as many of the rhizomes as you can (only to ensure even ground for your tarps so that they do not become punctured).

Afterward, place the tarps back on, even though winter is on the way. You want the tarps to be already in place for the next growing season. That way, in case you get busy with your gardening in the spring and find yourself pressed for time, you don't have to worry about remembering the tarps.

Getting rid of Japanese knotweed may require several seasons. The key is to stick with your project. This weed can be eradicated from your yard only if you keep after it. Plan on making its eradication your new hobby.