Japanese Knotweed: Choosing Your Battles

In a recent post on this blog, I debunked eight myths about Japanese knotweed, an invasive plant that is certainly formidable, but whose effects on the environment and infrastructure tend to be exaggerated.

Exaggeration or not, it is perfectly legitimate to fight knotweed if it is on your property, whether planted intentionally or introduced accidentally. If left unchecked, it can take over all available space, which is obviously undesirable if you want to maintain a diverse landscape.

In a future post, I will review the scientific knowledge on controlling knotweed, separating the attractive but ineffective “miracle” methods from those that have had some success. But first, I think it is important to put the fight into perspective. Fighting knotweed is certainly worthwhile, but it is important to bear in mind that it will be a tedious and lengthy process. It is therefore important to choose your battles: some are worth fighting, others are not.

Invasive Plants Are… Plants

This statement may seem trivial, since it is obvious. And yet… I often begin my public lectures on invasive plants with this quip. I simply remind the audience that, as detestable as they may be, invasive plants such as Japanese knotweed are plants, not barrels of toxic chemicals. We tend to forget this in the media hype surrounding many invasive species. Control measures must therefore be proportional to the inconvenience caused by invasive plants or the damage they cause to the environment. For example, I am much quicker to take action when it comes to plants that are very harmful to health, such as giant hogweed, than I am with plants whose effects on biodiversity are far from proven.If knotweed is not harmful to health, what about its environmental effects? Is it justified to combat it in order to preserve biodiversity? Yes, but only in a specific context: that of rivers.

Eroded riverbanks

The environmental effects of Japanese knotweed are most noticeable in riparian strips, its preferred habitat. Once established, the plant takes over the entire area and reduces plant diversity to almost nothing. This loss of biodiversity can also be observed elsewhere, but the consequences are more serious in riparian strips, as effective filtration agents and high-quality wildlife habitats are lost.

In Quebec, the impact of knotweed is particularly noticeable in April. As the stems die in the fall, the ground is bare of vegetation when spring returns. The consequences are remarkable, as ice carried during floods scrapes the unprotected soil surface, amplifying erosion in riparian strips. Where knotweed is established, an additional 3 cm of soil is lost each spring compared to banks covered with shrubs or herbaceous plants. This may not seem like much, but after ten years, that’s 30 cm (a foot!) of soil lost, which is enormous. That’s a lot of topsoil that disappears into the environment and settles in riverbeds, along with the fertilizers and pesticides it contains.

But beware: if knotweed causes erosion, it is because we have made its life much easier by removing the riparian shrubbery for agricultural reasons or to obtain an unobstructed view of the watercourse. If the shrubs had been left in place, knotweed, however aggressive it may be, would have found it very difficult to establish itself, as alders, dogwoods, and willows would have competed fiercely for space and light. On the contrary, herbaceous plants such as grasses, which often make up riparian strips in agricultural settings, are not robust or competitive enough to stand in its way. My team observed along the Etchemin River that every spring, the ice deposits hundreds of mounds of soil with knotweed rhizomes from clones located a little further upstream on the herbaceous strips. This makes it easy for knotweed to take root.

Unfortunately, as we will see in the next section, controlling knotweed in riparian strips is the most complex task, particularly for regulatory reasons. Sometimesclones are so huge (1 km long!) that it is unrealistic to try to remove them.

When the invasion is at an advanced stage, we must either give up or at least focus on the least invaded sections of the watercourse. Better still, the riparian strip should be restored by planting shrubs before the knotweed has had time to establish itself. This is how we choose our battles for the preservation of biodiversity.

The Fight in Its Simplest Form

Citizens regularly consult me for advice on how to combat Japanese knotweed on their residential property. In their case, the issue is not environmental, but rather one of space. Sometimes the clones are so widespread that I check whether they have the financial means to undertake the fight and whether they really want to invest so much in it.

Make no mistake: you can eliminate a knotweed clone, no matter how huge, but the means to do so, such as excavation, are so costly that the benefits do not always outweigh the costs.

Here is the story of a cemetery struggling with giant knotweed clones. The plant is a problem because it is about to encroach on neighboring graves. In this case, there are few options. Obviously, the rhizomes (the graves!) cannot be excavated, and synthetic herbicides, the only truly effective means of controlling the invader, are prohibited in the municipality where the cemetery is located. The only option is to leave the clones in place but increase the frequency of mowing the surrounding lawn.

Make no mistake: cutting back the shoots around the edges will have little effect on the knotweed as a whole, and the rhizomes will remain in the soil beneath the gravestones. However, frequent mowing will keep the invader at bay, as a survival strategy in the mowed area. Instead of spreading horizontally (rhizomes), which would exacerbate the problem in the cemetery, knotweed will invest its energy vertically (stems) to counteract the effects of the control measures. This “investment” will in turn be mowed, and so on. It’s not the ideal solution, but it’s the only reasonable (and perpetual) battle the cemetery could undertake in this circumstance.

If such a simple solution doesn’t satisfy you and you want to embark on a more substantial fight, arm yourself with a little more patience, as I’ll discuss this in the next section. In the meantime, your comments, questions, and solutions are welcome.