Jim Glaister perused my article on the removal of Japanese knotweed, or "fleeceflower" and offered some interesting feedback, for which I am grateful. Glaister is Marketing Manager for Wreford Limited, one of the UK’s leading Japanese knotweed specialists:

I notice your favourite method of eradication treatment is to smother Japanese knotweed with tarps and that your preferred application rate with glyphosate is 'as often as possible,' so to speak. Are you aware of the issues of dormancy in Japanese knotweed?

This is currently an area of insufficient research on Japanese knotweed, but enough observations have been made to recognise that Japanese knotweed dormancy offers a serious threat to some eradication treatment methods. If Japanese knotweed is threatened too strongly, or circumstances prevent it from growing, the plant will go into dormancy – effectively extending what it goes through during the winter. This means that smothering it with membrane will eventually stop it growing but will not eradicate it. Once the tarps are removed, and the Japanese knotweed feels conditions are suitable for recovery, it will start growing again. It could take a season or two (or even longer), but the risk of re-growth is there. Current estimates on Japanese knotweed dormancy suggest the plant can remain in this state for up to 20 years – and some estimates are suggesting even longer than that!

Removal of Japanese Knotweed: The Dormancy Factor

Let me interject a few points at this juncture. The above observations are certainly worth discussing, as awareness of Japanese knotweed's "dormancy defense" could influence the strategy you're planning for the removal of Japanese knotweed. For instance, let's say you have an infestation of the weed at the edge of your lawn. Laying tarps down over such an area (to smother the Japanese knotweed) for a few years -- then removing the tarps to re-seed -- wouldn't be an effective strategy for the removal of Japanese knotweed. The weed would very likely "pop up" again, meaning you'd have to repeat the process all over again.

I must say, however, that one of the arguments for using the "tarp method" (where applicable) for the removal of Japanese knotweed is that it's organic, to the extent that it is not supplemented with spraying. Furthermore, in many cases the 20-year waiting period (to get through Japanese knotweed's dormant period) shouldn't cause undue hardship: I do plan on leaving my tarps on 20 years or longer.

One reason for my patience -- and another virtue of the tarp method -- is that one can simply garden right over the tarps, using raised beds. And mulch can be applied over the tarp areas outside the raised beds, both to disguise the tarps and to protect them from UV rays.

Japanese knotweed's defenses do, however, go beyond the ability to "play possum." A related challenge you may experience if you employ the tarp method for the removal of Japanese knotweed is what I like to term the "safety valve defense." That is, if prevented from pushing up through the ground in one area (a 30' x 30' square covered by tarps, let's say), Japanese knotweed has been known to find a way to send up shoots along the perimeter, effectively escaping its bounds.

If you wish to stay organic and avoid spraying to control these escapees, you might consider experimenting with bamboo barriers in conjunction with tarps for the removal of Japanese knotweed. I say "experimenting," because I have not tried this method, myself.

Removal of Japanese Knotweed: How Often to Spray With Herbicides

But let's continue with the interesting thoughts of Jim Glaister, Japanese knotweed specialist:

Similarly, if it is hit hard with herbicide, Japanese knotweed can induce dormancy as a defence mechanism before the chemicals have fully eradicated it. After a couple of seasons or so, the remaining viable Japanese knotweed material can begin to re-grow. I’ve seen this personally with residual chemicals – the top growth is effectively sterilised, but the deeper rhizomes remain in the ground, viable but dormant, until they feel happy to start producing new shoots again. We have found that gradual poisoning of Japanese knotweed gives better long-term results than frequent or stronger chemical applications.

It may be worth advising your readers, too, about the extent of rhizome spread before they start trying to dig it up. I appreciate that you have told them that they will almost certainly leave some material in the soil after they attack it with a shovel, but they might not be aware of just how much. On a mature stand of Japanese knotweed it is not uncommon to find rhizome spreading 4-5m in any direction (and sometimes further), as well as 2-3m deep. All it takes is a piece of rhizome the size of a fingernail to constitute a new plant.

Very helpful advice from a Japanese knotweed specialist for the removal of Japanese knotweed. Thanks, Jim!

Jim Glaister, Japanese knotweed specialist, is currently (2008) working on a book about the removal of Japanese knotweed.