29 April 2017

Japanese Knotweed Wilting

Virtually all the populations of Japanese Knotweed (Fallopia japonica) in Neath Port Talbot are exhibiting catastrophic wilt symptoms.


I brought this to the attention of a few people a few days ago and several people have told me that they have noticed this elsewhere in South Wales. In a walk along along the Neath Canal between Neath and Briton Ferry, almost every plant I looked at was showing severe wilt symptoms (as shown in the photo below). It almost looks as if there has been a mass herbicide spraying event!


I haven't noticed Japanese Knotweed wilting on such a wide scale before and it is interesting to speculate on what is going on here. The rapid onset and scale of this phenomenon suggests that it is not the result of disease or herbivore attack. Rather, it appears to be  the result of a  physiological shock. But it doesn't appear to be a result of water shortage as such, which might seem to be the most obvious cause given the dry weather we've experienced recently - many plants that are growing in moist soil all along the canal are wilting. One possibility is that the mild conditions of early Spring this year have stimulated an early, rapid growth of tender knotweed stems which have subsequently experienced catastrophic chilling shock injury during the recent, unseasonal cold conditions. It's what happens to your tender half hardy seedlings when you plant them out too early.
I haven't seen this reported elsewhere, nor am I aware of any information on it in the literature, but I suspect it has occurred on a wide scale throughout the UK. Comments welcome.

15 comments:

Valley Naturalist said...


Interesting observation. I visited a proposed development site in Cwmbran last Friday that has extensive JK - all was wilting. I assumed it had been sprayed but seeing your blog I'm know not so sure.

Steve Williams (valleynaturalist.blogspot.com)

Jamie Bevan said...

I've also noticed this a lot around Swansea recently, Charles. I just assumed it had been sprayed. I was actually doing some Knotweed control behind my garden yesterday, trying to stop it encroaching in. I left one wilting patch as I thought it had already been sprayed by the college grounds people. Sounds like I have to go back at it - damn!

ian tew said...

Blimey, good observation, I'd observed the same all around Swansea and thought the council was doing an efficient job spraying like they have at all the roadside "conservation" wild flower patches. Even though it's sadly successful here maybe it's environmental requirements are on the edge sometimes. I get the idea that this is the area of maximum success for it but maybe that's because I live here? Would be interesting to know where it's not happening, altitude/aspect/distance from the sea etc. Oh, I feel a student project coming on.....!

Ian Morgan said...

I have n`t had a methodical look around Llanelli in SE Carms, but it`s a `mixed bag` here, with some wilting and some ok. I`ve seen Jap knotweed hit very hard before (esp in more upland/inland Carms) by sharp spring frosts, such as the one we had on last Tuesday night, when puddles were turned into ice here, even in coastal Llanelli. Bear in mind that lots of plants were quite advanced after the mild winter, so a late frost will have `clobbered` them.
However, it seems that your observations partly or wholly pre-date that unseasonably cold night and you`ve already considered drought/spraying.

Anonymous said...

No sign of any wilting Jin my part of north Kent, and we've had frosts as well, in fact I found a new stand of KW recently. Nothing to do with this release of a bug that attacks it that I've heard of?

Charles Hipkin said...

Thank you all for the comments. It's clear that some stands of Japanese Knotweed have not suffered any wilting, even in South Wales. I drove past a colony near Onllwyn (where it was really cold last week) that looked OK yesterday. Fascinating! Is this just a local phenomenon? It would be nice to know the nationwide picture. With those that have been hit, there has been no sign of recovery following the relatively mild, wet weather we've had recently. In some plants, tissue in the apical parts of the shoots are very soft, or even mushy, indicating sever damage. Just to clarify, I first noticed this in a large patch of JK near Ystalyfera on Wednesday April 26th. Ian (Morgan), I think that ties in with the cold snap. If it is a physiological response to cold shock and if this response varies in different populations, then that is very interesting, particularly since we are told that JK has very low genetic diversity in Britain (frequently spoken of as one single clone). As for other possibilities, e.g. disease or bug attack, I don't know, but it certainly should be investigated.

PBA Solutions said...

We're seeing the same thing happening in the south (Hampshire). Three stands in different locations were showing wilted stems and orange/brown leaves. A few stems completely shattered. Was thinking it might be fungal attack at first but no brown spotting on leaves. No evidence of aphalara itadori population (and don't actually know what the signs of psyllid attack look like).
Interestingly, on one of the sites there are five large stands, four of which are exposed, one under a tree canopy. No evidence of this sort of damage within the stand that's under cover so frost damage would appear to be the most likely culprit. We'll just have to wait and see...

Barry Stewart said...

One week on and as I drove along the JK-infested road between Dunvant and Waunarlwydd yesterday, I too was fooled into thinking how effive JK control had been (even though I'd already read this post). All stands were affected, with not just wilted shoots, but all the leaves were completely brown. Interestingly JK shoots of young plants, which appeared in my wife's school in Newton after soil disturbance last year, were all fine yesterday.

Sophie Hocking said...

Interesting - I've seen the same thing in Gorseinon and thought it was a result of spraying. But also have seen v. young shoots coping fine elsewhere, although these came from extensive stands that haven't been treated for years.
It would be interesting to know if stands exhibiting wilting have been treated in past growing seasons. Is it possible that plants with depleted rhizome reserves following herbicide treatment (or other stressors) could exhibit severe responses to late frost whereas large knotweed stands left untreated are less impacted?

Anonymous said...

Hi, something similar is happening in County Limerick in Ireland. Again, I thought a local group was spraying, but it turns out not. We have had (by Irish standards) a fairly prolonged dry spell. I wondered if it was that.

Rosemary Mason said...

In November 2016 I noticed that Swansea City and County Council had asked Monsanto's Contractor Complete Weed Control (that they had been using to try to eradicate Japanese Knotweed in various parts of the County, to banish weeds from the pavements of 1500 km of the City streets in spring and again in Autumn. I asked for an FOI and they had sprayed 3,000 km pavements with 518 kg Dakar Pro, a professional preparation of Roundup) but they hadn't completed 26 wards. I asked again in April 2017 and was alarmed to find they had already sprayed 169 kg in March 2017 and they were planning to have three sprayings in 2017. I have tried to stop it, but to no avail. Monsanto bought glyphosate as a chelator of minerals used on boilers (it extracts minerals such as Zn, Manganese, Copper etc) before they marketed it as a herbicide. They say that glyphosate only poisons plants, fungi and bacteria, not humans and animals...but humans can only absorb nutrients via the gut micro biome, trillions of bacteria in the gut. So we and all the wildlife in the area are being poisoned by Roundup...that's what has happened to out nature reserve on which Barry did some moth counts.

Martin Bell said...

I've noticed a couple of stands up in Quakers Yard area that appear to be wilting but others that look normal. I've also heard of wilting at Tir founder fields in Aberdare

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Jonathan Upton said...

And wilting observed in Landimore..

Martyn Hnatiuk MartynH said...

Large areas of JK along the Llynfi valley river system are all wilting, but in the wooded glades where they have taken root seem unaffected so far. Many of these areas are not visited by anyone and are out of the way. Also of note in late 2016 I noticed the same thing happening to Himalayan Balsaam, but that seemed to be fungal related. I haven't noticed any insects there that are unusual, I.E. defra were thinking of introducing some kind of beetle to combat the spread of JK.