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!
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.
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.
Why It Is Important to Get Rid of Japanese Knotweed
At any of the homes where you've lived, have you ever had a stand of Japanese knotweed (Polygonum cuspidatum) in the yard? If so, you know that the area consumed by such a stand is useless. You can't grow a lawn there, you can't plant a garden there. In a word, the space is wasted. And if you've ever tried to get rid of this weed and reclaim the wasted space, you know that this is no easy task.
But is that as bad as it gets? Or can matters get still worse? Can Japanese knotweed even reduce home values? The rather surprising answer is "Yes"; that answer will be explained and qualified later in the article. But first, some background information.
My adversarial relationship with Japanese knotweed plants goes back decades. When I was a child, though, it began as something of a love-hate relationship.
"Love" may be a bit too strong a term, but I did have a use for these tall, bamboo-like weeds when I was a little tyke. I would "tunnel" into the thick stand of them on our property, carving out the interior portion to form a green cave of sorts. It was a fun place to hide.
The "hate" part of the relationship evolved as I grew older. My father detested these fast-growing weeds and assigned me the chore of cutting them down with a sickle. As you may well imagine (kids being kids), I grew tired of having to wield the sickle every week or so to cut them down to size.
As an adult, I picked up where my dad had left off in battling this nuisance (a change in strategy resulted in eradication, I am happy to report). My dad and I were hardly alone: this is a battle waged on many fronts in the U.S. and Canada. Untold numbers of homeowners have tried this, that and the other thing to wipe it out. Many, unable or unwilling to ascertain through research what the best way is to get rid of the menace, simply give up after a while.
But I recently learned that, in the U.K., Japanese knotweed is more than simply a nuisance: it can affect mortgages and, thereby, home values. Karen Platt, perhaps best-known as the author of books on black plants, sent me the following information in an email exchange:
"I see we have something else in common -- Japanese knotweed has just penetrated my land from next door. Next door is owned by a landlord who rents out to a tenant. She is not a very nice person and has caused me problems with noisy tenants in the past. Now she won't deal with the Japanese knotweed. Nightmare. I'd be interested in comparing notes -- found your idea of smothering Japanese knotweed very interesting.
"Here [the U.K.] it's affecting mortgages -- no mortgage lender will lend on a property that has untreated Japanese knotweed. If you have a management plan with a 5 year insurance backed guarantee, then they will consider it. I was just about to sell, that's how I found out about the plant.
The valuer noticed it on next door's land and sure enough it's come through on the boundary of my land.
"I think it's because of the Olympics held in London this year . The cost of eradicating Japanese knotweed from the site was 70 million sterling, so the Royal Institute of Chartered Surveyors instructed their members to report any Japanese knotweed in domestic gardens on their reports. The mortgage lenders then got scared and hey ho -- no mortgages on properties with untreated Japanese knotweed found within 7m of a boundary, and if it's nearer to your property than that chances are no mortgage at all anyway, whether it is being treated or not. You also have to have an insurance backed guarantee which makes eradication rather more expensive.
"This policy has almost wiped out the entire value of some properties. It's kind of ridiculous because people are just going to start cutting it down and pretend they haven't got it, as one of my neighbours seems to be doing.
"I'm glad this approach is not being taken in the USA."
But don't be lulled into a false sense of security, America: at the time of writing (November, 2012), we are, after all, becoming more and more like Europe. It would not surprise me at all to see Japanese knotweed eventually reducing home values here in the U.S.
Nor should you think for a moment that the Japanese knotweed on your property will somehow die out on its own at some point. If it comes down to a waiting game, you can be sure of this much: these invasive plants will win every time.
If you try to play the waiting game with Japanese knotweed, it will outlive you. Trust me, what you must do is to learn how to kill Japanese knotweed.