Eradication of Japanese Knotweed Plants

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

The subject of this article is the eradication or control of one of the world's most invasive plants, Japanese knotweed. These noxious weeds go by the botanical namePolygonum cuspidatum, but you will also see the alternate botanical name, Fallopia japonica. You will see it called many other common names, too, including "fleeceflower," "Japanese bamboo," and "Mexican bamboo" (note, however, that this weed is not one of the true bamboo plants).

A local nickname with which this writer is familiar may well capture the essence of this invasive species better than any of those common names: "Godzilla weed." Indeed, it has wreaked havoc on even more landscapes than that fictional monster. If you yourself have ever attempted the eradication of this noxious weed, you already know of its Godzilla-like qualities.

Japanese knotweed, a member of the buckwheat family, 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. In the opinion of most people nowadays, though, those canes are very unattractive when they turn brown in fall (nor do they break down easily). But, alas, there is no accounting for tastes.

The weed soon spread like wildfire, taking on a life of its own, independent of its human sponsors. A common habitat for it is sunny, moist areas, including riverbanks, roadsides, and, yes, your lawn and garden. Japanese knotweed 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 completed the eradication of this weed. You'll only invite it to spread if you clear land next to the currently infested area. Eradicating a mature stand of it, however, is easier said than done.

You slash Japanese knotweed plants to the ground, but they come back. You root them out and burn them on a would-be funeral pyre, but it is only you who feel dead, fatigued from all your labors. Why, it even mocks concrete bonds, bursting up through any available crack in a driveway or sidewalk with its incredible strength. It's the closest thing you'll find to Godzilla in the plant world.

So how do you get rid of Japanese knotweed plants? Let's consider an eradication and control strategy that uses four tactics:

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

Using Tarps to Smother Japanese Knotweed

Is killing Japanese knotweed and reclaiming your yard a realistic goal to have? Well, for those who would like to free up some landscaping space by killing a mature stand of the weed, your hope lies in four tactics, as part of a multi-pronged strategy, carried out faithfully over a long campaign. There's hope for success, but you'll have to stick to your tactics and wage a smart war. And if you'll settle for just suppressing the enemy at first, using tarps, you can at least reclaim the war-torn landscape for the short term, while you continue to battle it over the long term in hopes of finally killing it.

Begin by investing in some plastic or poly tarps, with which you'll cover the weed patch and smother it.

Invest the money in the biggest tarps you can find, because the investment will save you a lot of labor (see below). If the landscape area in which the Japanese knotweed comes up is covered in the early spring with tarps, the weed's growth is held back right away.

The covered Japanese knotweed will still make a fuss, to be sure. It is not for nothing that, in Japan, its native home, Japanese knotweed is referred to as itadori, which means "strong plant." With their Godzilla strength, the new shoots will act like tent poles, pushing your tarps up. But you can then 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. Make sure your tarps overlap each other quite a bit and are weighted down all along the seams and the perimeter, else the sun-seeking shoots will be pushing through the gaps in no time. This is why buying the biggest tarps you can find is a good investment (fewer seams).

One reason why the tarp tactic is a powerful option is the fact that, even while the tarps are still in place, this portion of your landscape can be reclaimed for above-ground gardening uses. 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.

It has often been noted that Japanese knotweed's shoots will push up even through (previously damaged) concrete surfaces. In light of this fact, some landowners might be skeptical about a tactic that is based on holding this plant down with tarps. But what these skeptics would be failing to take into account is that tarps are not only durable but, more importantly, pliable. The pliable nature of tarps means that when the shoots push against them, they give ground, instead of breaking.

The main difficulty with tarps lies in having to prepare the ground carefully before laying them. For, while the soft new Japanese knotweed shoots won't harm tarps, the old canes will most definitely puncture even the toughest of tarps. The old canes are woody, and, when broken, they form sharp, knife-like edges. All of the old canes must be cleared away before tarps are laid.

A variation on the tarp tactic that is free is using old carpeting to smother Japanese knotweed. Stores that sell carpeting have to pay to dispose of the old stuff, so they may well let you haul it away for free. And if you're not above scavenging on the roadsides, check around for old carpeting in your area on trash night. Often, homeowners who have installed new carpeting set out the old carpets for trash pickup.

The problem here is that your town may insist that old carpeting be chopped up into smaller chunks for trash removal. In using these smaller chunks, what you would save in money, you would lose in labor. That is, just as using small, overlapping tarps will be labor-intensive (thus the recommendation to buy big tarps), so will using small, overlapping sections of carpet. Japanese knotweed will poke its head out wherever there is a seam. A large, seamless covering is better for this tactic.

But as already stated, smothering via tarp is only one tactic to be used in a multi-pronged attack on a large stand of Japanese knotweed. That's why you need to know about three more tactics, discussed below. For, unless your tarps are quite extensive, you'll still find the weed pushing up shoots beyond the perimeter of your tarps.

Using Weed Killers to Kill Japanese Knotweed

A second tactic used to kill Japanese knotweed in your landscape focuses on weed killers for Japanese knotweed. The recommended weed killers for use against this perennial are those that are glyphosate-based. Trade names for weed killers containing glyphosate include Roundup, Gallup, Landmaster, Pondmaster, Ranger, Rodeo, and Touchdown. 

Glyphosate is usually applied with a garden sprayer, onto the leaves, after being mixed in a tank. 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 have been successful spraying glyphosate repeatedly throughout the growing season, basically never giving the plants a chance to put on much height.

One hates using herbicides, but this may be a case where you'll consider making 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. And this weed is too tough a customer to be fighting with one hand tied behind your back. Restrict the use of the herbicide method to areas destined to become lawns or ornamental beds. Do not use it to clear the land for a future vegetable garden.

Less Effective Methods: Cutting and Digging

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 in isolation, though. Cutting back Japanese knotweed regularly is a tactic meant mainly to be used hand-in-hand with injections of weed killer into the cane stumps. But this is a lot of work and surely not the preferred method.

Finally, dig into the ground where the shoots come up most vigorously in your yard. 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.

The rhizomes can be dug up and bagged. Do not, however, expect immediate results from using this tactic. For, 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. In this case, the nourishment your enemy requires to fight you most vigorously is stored in its rhizomes. Think of the rhizomes as fortresses. Although enemy soldiers will fan out and hide after their fortress has been destroyed, the loss of the fortress makes their long-term success less likely.

As with cutting, don't rely on this eradication tactic in isolation. A good reason for wanting to dig up the woody clumps of Japanese knotweed, however, is preparing the ground for the laying of tarps, so that the tarps don't get damaged. For, no matter how close to the clumps you trim the old canes, sharp edges are still likely to stick up through the soil from these clumps. Those sharp edges will puncture your tarp. So to get a nice, smooth surface for the laying of smothering tarps, it may be best to dig out some of the bigger clumps.

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.

Using Glyphosate Herbicide to Control Japanese Knotweed

What is glyphosate? Why is there so much controversy surrounding this herbicide? How can it be used to control Japanese knotweed? These are some of the issues treated below. Let's begin with a brief look at what it is and how it works, before moving on to the more controversial aspects of this herbicide.

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.

Technically, the active ingredient in this type of herbicide is called  "isopropylamine salt of glyphosate." One sustainable-agriculture site explains how it works in this way: "Glyphosate is absorbed through the leaves and is then transported throughout the plant. It inhibits a metabolic pathway required to form essential amino acids...."

Controversy: Glyphosate, Surfactants

Chemical herbicides as a whole are frowned upon by the most vocal segment of the gardening community, which exhibits a strong organic bent and therefore prefers weed control without chemicals. Because glyphosate is so popular in other circles, it has become the poster child for unwelcome chemical herbicides among organic gardeners. The chemical companies' claims that these ingredients are safe (that is, have low toxicity) and biodegradable stand in stark contrast to the objections of their detractors.

Further stoking tensions is the connection between glyphosate and the hot-button issue of GMOs (genetically modified organisms). Some plants have been genetically modified to be more tolerant not only of drought and insects, but also of glyphosate, itself. Seeds are harvested from these plants and marketed with the selling point that they are meant to work well with glyphosate use. The idea is to be able to use a non-selective herbicide as if it were more of a selective herbicide.

This solution is, not surprisingly, controversial. MIT sums up the controversy thusly: "Concerns about the effects of genetically engineered foods on health have to do with both the crops themselves along with the toxicity of the herbicide...."

Organic gardeners suspected that this herbicide was toxic as soon as it hit the market in the 1970s, but subsequent studies have thrown fuel on the fire by adding a wrinkle to the debate: Maybe testing for the toxicity of glyphosate isn't enough to determine the product's safety. That is, in recent years a spotlight has been thrown on the inactive ingredients in these herbicides, which include soapy substances called surfactants ("surface-acting agents").

The use of surfactants in herbicides is common. A plant's leaf has a waxy coating to it that would inhibit penetration by herbicides without the help of surfactants, which are wetting agents that reduce a liquid's surface tension and make it easier to spread. The result is better penetration, without which the herbicide wouldn't be able to do its work of inhibiting the plant's metabolism. But many have questioned whether it might not be the surfactant that makes these herbicides potentially harmful to humans in a significant way.

Why Glyphosate is Used to Control Japanese Knotweed

In light of these concerns, why is it, then, that not a few people who think of themselves as being more or less "green" give their blessing (however grudgingly) to the use of glyphosate in certain situations? Well, sometimes it comes down to a matter of the lesser of two evils.

Environmentalists are concerned not only about the release of toxins into the environment, but also about the extinction of native plants (among other issues). Invasive plants such as Japanese knotweed sometimes form monocultures that put pressure on the populations of their native rivals. Therefore, if an herbicide is judged to be the best method to control a weed such as Japanese knotweed in a given case, its use might be thought justifiable. It's a case of "pick your poison."

If this is the course of action you choose to control Japanese knotweed, consider limiting your spraying to late summer or early fall. This is when the weed blooms and is thought by some experts to be the best time to spray. By thus restricting yourself to one spraying per year, you're also reducing the amount of the herbicide released into the environment.

Organic gardeners sometimes use natural weed killers such as vinegar as alternatives to chemical herbicides, but vinegar won't touch a tenacious pest like Japanese knotweed.

One Reader's Method for Using Herbicide to Control Japanese Knotweed

Reader, Stacey W., who lives in upstate NY, sent in her own story on this topic, relating the specific manner in which she used a glyphosate-based herbicide to control Japanese knotweed. Here's her story:


"I've read (and reread) your article on controlling Japanese knotweed because I had the luck of moving into a house where there were not 1, but 2 very well established stands on the property and property line. Without realizing what it was, I had some excavation done and now have 2 very prolific stands. One is about 30 feet x 20 feet. Which led me to your informative article."

"What I wanted to share is the luck I've had in battling it. I don't know if it's dying or just going dormant, but I'm hoping for the former based on some of the symptoms I'll describe.

"I tried the smothering method you mention using tarps or old carpeting first, but due to the size of the stand and the ground it covered (hilly), it didn't work. So in April, when I first started seeing shoots come up through the cracks in the tarps (and the weed was actually pushing aside the tarps I couldn't get secured tight enough!), I started spraying the leaves with Roundup (I waited until it was 3 to 6 feet tall). After 2 applications I noticed it hadn't done much for the existing plants (now 8+ feet tall) except some brown leaves. But the new growth, where I had sprayed the Roundup at the base of the existing plants, was weak looking, with stems that curved toward the ground, unable to bear their own weight. And it seemed to be growing more slowly.

"The directions on the bottle actually suggest cutting plants 3 to 4 feet above the ground and pouring Roundup into the stems so I figured I would try that. I presume this is similar to what you discuss where you're talking about using herbicide injection to kill Japanese knotweed.


"Over the past several weeks I've been cutting stems and pouring Roundup into the hollow stems, and that has had a startling effect: the stems themselves die at the tips after "rotting." By rotting I mean that the stems take on a purplish cast and the water in the middle becomes foul. Then slowly the top either closes itself into a point, without the stem below it dying, or the top section of the stem dies, but the stem below it doesn't. Spraying the stem itself with Roundup doesn't kill the plant, but it does induce more of this "rotting" appearance throughout the stem. In either case (although the effect is much slower when the stem is sprayed) the stem continues to remain alive but the plant loses the ability to produce leaves. It's like the plant is doing all it can to keep the old stem alive while it tries to send up new stems. And that old stem is a drain on its resources.

"I've been doing this slash and inject method once every 2 weeks, for about 2 months now. The stand does seem to be dying. New growth is still coming through, but it's slower, and often has twisted leaves (triangular instead of the standard shape), with bent stems. Plants where I've sprayed the stems also adopt the bent stem look, but the stems are purplish, and new growth has the triangular leaves, suggesting that I'm weakening the plant itself.

"Rather than trying to kill it all at once, I'm only treating stems once they are at least 3 feet tall. Plants shorter than that I spray with the Roundup to target the root system. The really interesting part of this experiment is that the large stems I've poured Roundup into die only in the top segment. Every segment of the stem below the top one remains green and healthy unless I soak the stem itself with Roundup. And even when soaked with Roundup it remains alive, but it stops its growth. The period between treatments seems to be prompting new growth, which is what I'm hoping will keep the plant from slipping into a dormant state before I can kill it.

"Obviously this is a lot of work and I hate having to use chemicals like this, but it does seem to be helping and I feel a little better that this is a spot treatment rather than a broadcast spraying. I plan on taking the next several weeks off from any treatment in order to prompt new growth so I can spray the leaves again and get the Roundup into the root system. My hope is that my perseverance will pay off next year and I'll be able to plant the area, with only the occasional treatment.

"I'd be thrilled to know if anyone else has tried this approach of spraying on and into the stems while spraying the leaves of the small plants. I'm hoping it's the right mix of getting poison to the roots, while not letting it go dormant, to do some serious damage to this plant.

"Thank you for the article and your advice! It was very helpful in identifying what I was dealing with and putting me on the right track to end it."