Eradication of Japanese Knotweed Plants

Getting Rid of "Godzilla Weed" Difficult, But Possible

Japanese knotweed (image) is a Godzilla weed. You'll know why if you've ever tried to fight it.
I call Japanese knotweed "Godzilla weed." If you've ever tried to fight it, you'll know how I came up with that nickname. 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 name, Polygonum 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 really a bamboo at all; you can read about true bamboo plants, as well as about other bamboo impostors, in this FAQ on bamboo plants).

 But regardless of the common name you prefer, these indomitable plants deserve a more provocative nickname ("killer bamboo" works well, or, better yet, "Godzilla weed"). This invasive species is not a killer of people, but of landscaping hopes and gardening dreams. Having wreaked havoc in lawns and gardens across both North America and the UK, Japanese knotweed has caught the attention of stewards of the land. And if you yourself have ever attempted the eradication of this noxious weed, you already know of its Godzilla-like qualities.

Japanese knotweed was introduced from Japan to the West -- first into the U.K., then into North America -- in the 19th century as a landscaping ornamental (alas, there is no accounting for tastes). It soon spread like wildfire, a mad killer 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, furtively submerged within a pile of fill or topsoil dumped onto an unsuspecting yard and, next thing you know, a burgeoning menace is gaining a toehold. Before you know it, all other plants are crowded out by this highly successful competitor for landscaping space, resulting in a monoculture.

Fortunately, Japanese knotweed is not inclined 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 killer bamboo. You'll only invite it to spread if you clear land adjacent to the currently infested area. Eradicating an entrenched 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. Japanese knotweed may not be the fabled Godzilla let loose to wreak havoc across a cringing populace. But it's one of the closest things that 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 employs four tactics:

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

Using Tarps to Smother Japanese Knotweed

Is killing Japanese knotweed a realistic goal to entertain? Is it possible to kill the killer bamboo and reclaim your landscape? Well, for those who would like to free up some landscaping space by killing an entrenched stand of the weed, your hope resides in four tactics, as part of a multi-pronged strategy, carried out diligently over a long campaign. There's hope for your garden -- but you'll have to be persistent with 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 maintain the siege that will (hopefully) kill your nemesis for the long term.

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 -- the investment will save you a lot of labor (see below). If the landscape area from which the Japanese knotweed emerges is covered in the early spring with tarps, the weed's growth is immediately impeded.

The covered Japanese knotweed will still make a fuss, to be sure. It is not for nothing that, in Japan, home of this killer bamboo, 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 be ineffectual, deprived of sufficient sunlight. Make sure your tarps overlap each other significantly 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, during implementation, 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 Godzilla-shoots will push up even through (previously damaged) concrete surfaces. Dismayed by this observation, some landowners might be skeptical about a tactic that is based on suppressing this plant 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 pliability of tarps means that when the bamboo 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 sturdiest of tarps. The old canes are woody, and when broken they form knife-like sharp 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, meaning that what you save in money, you lose in labor. That is, just as using small overlapping tarps will be labor-intensive (the reason for my recommending big tarps above), 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 weed killer recommended for use against this Godzilla is glyphosate. Trade names for weed killers containing glyphosate include Roundup, Gallup, Landmaster, Pondmaster, Ranger, Rodeo, and Touchdown. I am most familiar with Roundup.

Roundup is usually applied as a foliar spray; i.e., the weed killer is sprayed on Japanese knotweed's foliage with a sprayer after being mixed in a tank. I use Green Gorilla's pump-free garden sprayer. However, you can also inject glyphosate herbicide into the canes, using an injection product that I have reviewed. By general consensus, the best time to spray the leaves with Roundup 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 -- essentially never giving the plants a chance to put on much height.

Ones 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. I would restrict the use of the herbicide method to areas destined to become lawns or ornamental beds; I would not use it to clear the land for a future vegetable garden.

Additional information regarding Roundup herbicide is supplied at the end of this article.

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 re-entrench themselves 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 in conjunction with injections of weed killer into the cane stumps (see above). But this is a lot of work -- and certainly not my 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 implementing 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 emerge. 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 more problematic.

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 (see above for a description of the tactic that employs tarps). For, no matter how close to the clumps you trim the old canes, sharp edges are still likely to protrude 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, lest they 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.pring 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. Persistence is the key. This weed can be eradicated from your yard only if you are consistently vigilant. Plan on making its eradication your new hobby.

Using Roundup Herbicide to Control Japanese Knotweed

What is Roundup? 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.

Roundup is a non-selective herbicide put out by Monsanto Company and 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.

The active ingredient of Roundup is isopropylamine salt of glyphosate. The ecochem site explains how it works: "Glyphosate is absorbed through the leaves and is then transported throughout the plant. It inhibits a metabolic pathway required to form essential amino acids...."

Roundup 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 Roundup is so popular in other circles, it has become the poster child for evil chemical herbicides among organic gardeners. Monsanto's claims that Roundup's ingredients are safe (i.e., have low toxicity) and biodegradable stand in stark contrast to the objections of their detractors, who publish articles with provocative titles such as "The Horrific Truth About Monsanto's Roundup Herbicide."

Further exacerbating tensions is the connection between Roundup and the hot-button issue of GMOs (genetically modified organisms), in which Monsanto, once again, is involved. Monsanto sells seeds for what are termed Roundup Ready® crops. These plants have been genetically modified to be more tolerant not only of drought and insects, but also of Roundup, itself. Thus Monsanto has figured out a way for a non-selective herbicide to be used as if it were 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 Roundup."

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

The use of surfactants in herbicides is hardly restricted to Roundup. 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 called questioned whether it might not be the surfactant that makes Roundup potentially harmful to humans in a significant way.

Why Roundup is Used to Control Japanese Knotweed

In light of these concerns, why is it, then, that not a few people sympathetic to the environmental movement give their blessing (however grudgingly) to the use of Roundup 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 indigenous 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 such as Roundup is judged to be the best method to control a weed such as Japanese knotweed in a given case, its use might be considered 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 optimal time to spray. By thus restricting yourself to one spraying per year, you're 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, believe me, 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 Roundup to control Japanese knotweed. Here's the story she emailed me:

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 3-6 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 3-4 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.

Want to learn about another reader's experiment in controlling this plant? Here's how Ryan tried choking out Japanese knotweed.

As a companion piece to the article above, please consult "Tips From Japanese Knotweed Specialists."