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Killing Japanese Knotweed (Polygonum Cuspidatum)

Using Tarps to Control Japanese Knotweed and Reclaim Garden Space


Fallopia japonica in bloom.

Fallopia japonica in bloom.

David Beaulieu

Also nicknamed "Japanese bamboo" or "Mexican bamboo," the scientific name for Japanese knotweed is usually given as either Polygonum cuspidatum (as stated on Page 1) or Fallopia japonica. To get you used to these names (both of which you will no doubt encounter if you engage in further reading on the subject), I use the former on the present page and the latter on Page 3.

Note, however, that so-called "Japanese bamboo" or "Mexican bamboo" is not really a bamboo at all. You can read about true bamboo plants, as well as about other bamboo impostors, in my FAQ on bamboo plants.

But is killing Japanese knotweed a realistic goal to entertain? Is it possible to kill the killer Japanese bamboo and reclaim your landscape? Well, for those who would like to free up some landscaping space for a garden by killing an entrenched stand of Polygonum cuspidatum, your hope resides in four tactics, as part of a multipronged 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 Polygonum cuspidatum for the long term.

Begin by investing in some plastic or poly tarps, with which you'll cover your patch of Polygonum cuspidatum 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, Japanese knotweed'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 bamboo 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 Japanese bamboo shoots will be pushing through the gaps in no time. This is why buying the biggest tarps you can find is a good investment.

One reason why I would call this tarp tactic perhaps the most powerful of your options 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 Japanese knotweed 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 the sturdiest of tarps. Old Japanese knotweed 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 multipronged attack on a large stand of Polygonum cuspidatum. That's why you need to know about three more tactics, discussed on Page 3. For, unless your tarps are quite extensive, you'll still find Japanese bamboo pushing up shoots beyond the perimeter of your tarps....

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