8 Myths About Japanese Knotweed

Radio-Canada recently reported on the despair of a citizen whose land was invaded by Japanese knotweed after he accepted fill dirt that clearly contained rhizomes of the plant. It is never a good idea to accept soil from a stranger, but we can sympathize with this citizen, who will have to fight an uphill battle to get rid of the invader. However, he will not be able to count on readers’ comments in his quest for Japanese knotweed-free land. People say all sorts of things about this plant.

As an invasive plant specialist, I have been working on Japanese knotweed for over 15 years. It is a formidable species, invasive everywhere it has been introduced around the world. It is one of the most studied plants in the world, and yet many myths persist about it. Here are a few:

1. Japanese Knotweed Roots Can Grow Up to 5 Meters Deep Into the Ground

The reality is somewhere between 1 and 2 m, perhaps a little more in porous soil such as sand, but never up to 5 m. This famous “5 m” figure, which is not based on any reliable studies, is propagated by British companies that capitalize on fears about knotweed to promote questionable methods such as excavation or deep burial with encapsulation. These solutions are certainly effective, but excessive and, above all, very costly… and profitable for those carrying out the work.

2. Japanese Knotweed Rhizomes Can Penetrate Foundations

This is a persistent myth that is also cleverly perpetuated by the same British companies selling miracle solutions, such as anti-knotweed membranes that are resistant to perforation (but not concrete… spot the mistake). However, there are plenty of images on the internet of knotweed infiltrating cement and brick. Some are hoaxes (knotweed in houses!), others are real, but beware: it is not the knotweed that “pierces” the foundations, it only infiltrates existing cracks. From then on, it can widen them, but it is not the source of the problem. British researchers have clearly demonstrated this in an abandoned neighborhood overrun with knotweed: houses with solid foundations are highly resistant to this terrible invader.

3. Japanese Knotweed Rhizomes Remain Dormant for 20 Years

Dormancy exists in seeds: it is a physiological mechanism that prevents them from germinating when conditions are not favorable for the emergence or survival of the resulting plant. However, there is no “dormancy” in rhizomes. A piece of knotweed rhizome contains a lot of reserves (sugars, nutrients) that allow it to survive in the soil for a long time. After a few years, this piece may produce a stem that will eventually emerge. However, to date, no one has been able to prove that a piece can survive for as long as 20 years.

4. Japanese Knotweed Releases Toxins Into the Soil

Laboratory tests have shown that when knotweed decomposes, it releases substances (known as allelochemicals) which, in high doses, can reduce the growth of other plants. These are not “toxins,” but simply molecules that have a harmful effect on plants, much like a herbicide. However, to date, no one has been able to find sufficient quantities of these substances in the soil beneath knotweed to have any effect on plants. If there are no other plants under knotweed, it is simply because competition between the stems for space and light is so fierce that no other species is equipped to cope with it. Step inside a knotweed clone and you will quickly understand why.

5. Japanese Knotweed Spreads by Seed

In theory, it could, but in practice, it never happens. Virtually all Japanese knotweed plants in Europe and all those I have tested in Quebec (more than 200) are genetically identical, indicating that the plant spreads only through fragments of stems or rhizomes, either intentionally or accidentally. This gigantic clone originated from a single individual introduced to the Netherlands in 1830, multiplied in a nursery, and then exported all over the world.

6. Japanese Knotweed Can Be Eliminated by Mowing It

The idea is to remove the stems at regular intervals, which would deplete the rhizomes’ reserves. As I always tell those who want to try it, you will exhaust yourself long before the knotweed. All attempts at repeated mowing have failed to date. The reserves are abundant in the massive rhizomes and can be replenished by the few stems that invariably grow back. The knotweed clone may not spread—which is something—but as soon as mowing stops, it won’t take long for it to regain its vigor.

7. Japanese Knotweed Can Be Eliminated With a Herbicide

In theory, yes, especially for very small patches measuring a few square meters, but in practice this happens very rarely. Make no mistake: if the treatment is carried out correctly by skilled technicians who are familiar with knotweed, systemic herbicides such as glyphosate, which penetrate the tissue and reach the rhizomes, can eliminate up to 99% of the plant. However, to date, no one has managed to completely eradicate a large clone using only herbicides. Simply stop spraying and the knotweed will reclaim the lost ground. In any case, synthetic herbicides are now banned in several municipalities. Biological herbicides do not reach the rhizomes and are therefore ineffective. As for miracle solutions such as electric shocks, diesel, boiling water, bleach, salt, etc., knotweed is completely unaffected.

8. Japanese Knotweed Can Be Eradicated With a Tarp

What would you say if I offered you a solution to your knotweed problem in the form of a herbicide that would kill all vegetation for six years, kill soil invertebrates, prevent water from penetrating the soil, and leave your land looking decidedly ugly? It’s unlikely that you would accept it straight away. Yet this is the “ecological” solution that is frequently proposed for knotweed: occultation.

With two exceptions, occultation is in many ways worse than a herbicide, mainly because the tarps (geomembranes), which are made of plastic (petroleum), end up in landfills. Tarps are useful against common reed and spiked water milfoil, two other formidable invaders, because the duration of the tarping is short (a few months to at most two years) and the tarps are reusable in these circumstances. But for knotweed, it doesn’t work, simply because the rhizomes have long-lasting reserves.

Even after six years of covering, the minimum recommended duration in a recent study, the plant still survives in most cases, mainly because it does not give up easily and takes advantage of every crack in the membrane to produce stems. The same study is a little more optimistic than I am about the effectiveness of the technique, but only if the covering is done very rigorously, which is rarely the case.

So, should we give up on Japanese knotweed? Not necessarily, but we need to choose our battles wisely. We will look at this in a future article.