Japanese knotweed - what do I do?

[IlanaK]

We moved here two months ago. The garden is over 100 ft and the rear quarter was totally overgrown and had both ground elder and japanese knotweed. We just finished having the garden totally landscaped. The rear has been covered with that dark weed suppressant membrane and has a thick layer of gravel on one part and bark chip on another. The gardeners did dig up the weeds before they laid the membrane.

I am already noticing shoots of japanese knotweed breaking through the membrane and out of the gravel.

What on earth do I do? The gardeners said nothing would grow through the membrane.

Get the gardener back pronto. If the deal was to get rid of the knotweed he hasn't fulfilled his contractual obligation.

Japanese knotweed grows though anything apparently, even a thick layer of concrete. I think there is loads of info out the if you Google, but as far as I know the best method of getting rid of it is to apply weed killer to the leaves in the autumn. This is because it sucks nutrients from the leaves down to the root at the end of its growing season, so if you apply at just the right time the weed killer will be sucked down into the roots.

It was not part of what we asked him to do as such. I wouldn't have known japanese knotweed if it bit me on the behind. He pointed it out for me. I did ask if the weeds (pointing generally at the ground elder and knotweed) would be suppressed by the membrane and he said yes.

Ooh, naughty man - he lied

The membrane probably won't even suppress the ground elder either - they are both hateful things.

I have heard that the best way to kill the knotweed is to inject it with weedkiller into the stem. I haven't tried it myself but I understand that you can buy a syringe from the chemist and use that to suck up some strong glyphophate which you then inject directly into the centre of the stem.

I thought that every bit of knotweed root had to be dug out and blitzed with chemicals to try and get rid of it.

I think it has to be burned or buried in landfill doesn't it? I'd call environmental health or your local council.

I saw a pic yesterday of Japanese knotweed growing through tarmac - very common I think.

Bloody knotweed, we've got it poking through everywhere.

I treated last year and the patches that have been killed off competely are the ones that I treated twice (once in the early summer and then treating the regrowth in the autumn). This isn't the recommended way of treating though.

Have still got it growing in 3 different places, but mainly bonsai growth so hopefully it's on it's way out!

It's not nice stuff to have around, best of luck with getting shot of yours.

I reckon that's an implied term then. What's the point in laying membrane (and getting you to pay for it) if it wasn't with the intention of getting rid of the knotweed.

Chemicals are the best way to get rid but you have to repeat the application. The Council are the best people to advise.

JK is unbelievably hard to eradicate and bran is right - you really need to get it at the right time of year. Right now it is definitely going to start growing again. But it shouldn't be able to break through membrane already - unless they've put it down in strips and there's not enough overlap.

PS It's illegal to dispose of it in compost waste or similar. If you do dig it out then it needs to be labelled as hazardous waste and disposed of appropriately. I had a bloke from the council come out and look at ours to confirm what it was and recommend how we sort it out.

did you see that artivcle last week about the guy who was refused a mortagage as the house had japanese knowtweed?

see here

yes, I saw that too.

Ohh this has been the bane of my life for the past few years. The back 30 feet of my garden was 6 foot high knotweed when we bought the house.
I initially cut it down and squirted roundup down the stem for the first couple of years, now we only get the occasional shoot - I go round with the roundup once a week and it is under control.
Wish it would bugger off completely though

wuglet, that's good to hear, as we've mainly got bonsai growth now I was hoping that regular patrols with my roundup gun would keep things in check. How soon do you spray with the roundup? Do you wait until the leaves open up or spray when it's still a shoot?

I have shoots but just keep pulling them up for now as they are in the veg patch. When we finish in the autumn will be moving stuff around so will be able to weed kill it then

I spray as soon as I see it but tend not to get any visible effect until it has a lot of leaves.
I just enjoy squirting it and shouting "die weed die!!"
Mine is all bonsai at the back but there is one rogue bit 50 feet nearer to the house which thankfully has not spread - just 1 plant that keeps coming back every year.
Basically every time I hang washing out I take the roundup with me and spray any bits I can see - even if it is a dead cane! ATM I think I have maybe 3 or 4 bonsai ones and the one big one. "big" has changed definition - it is about 18inches!

My DH (horticulturalist) says the only foolproof way of getting rid of it is to dig it out. And to dig out 3 times as much soil as it occupies - ie if there's 1 square meter of it, you need to dig out 3 square meters.

Also, it is a notifiable weed, so you can't just bin it as other people have already said. It needs to be taken to a licensed tip.

If you choose to attack it with weedkiller, you should inject with roundup every month during the year. It'll take between 3-5 years to start to get rid of it.

Weed suppressing membrane is about a million miles off the mark - although there is a membrane called root barrier membrane which is different see here providing it's installed correctly.

We had clumps of this in our garden when we moved in.

Whenever it appears DH paints each individual leaf with roundup.

This year we have had 4 shoots appear. Compared to the original clumps this is brilliant.
They have been killed and I am hoping the end is in sight.
I think the trick is to be vigilant and treat every single shoot as soon as you spot it.

DH is just tackling some that has invaded from next door . He found out it is hazardous waste and has to be specially bagged and taken to the toxic waste dump. And apparently cutting a stem then syringing in some weedkiller is the best way to kill it without spreading further.

There is also a statutory duty to deal with it apparently also...

Thankfully I've never had to deal with Japanese knotweed, it sounds like a nightmare (Ground elder is bad enough). Your question did bring this to mind.

Let's hope the trial is successful and expanded...

I had a clump of it growing in my veg patch. I started spraying it with round up before the growing season, cut the stems and sprayed Roundup directly onto the cut stems and into the hollow bits, re-spraying every week. It SEEMS to have worked, the clump is completely brown and when I dug some up the other day the roots were definitely dead, but I'm not committing to it until the end of the year!

Just buy a HUGE amount of Roundup and keep at it, it will die off eventually, but you'll have to be vigilant for a good few years yet!

Our local council will spray knotweed for you. Their contractors treat masses of the stuff growing on highway verges etc but will treat on private land for a charge (it was £2.50 per sq m a couple of years ago). Definitely worth contacting your council to see what they can do.

To self treat it I would say ready mixed spray on glyphosate is not strong enough to work. Best method is to cut it off and pour glyphosate into the hollow cut stems. Add food colouring to it so you can see which ones you have done. If any grows back the next spring spray the leaves, then cut any more regrowth down and fill stems in the autumn. Repeat for 3 years and it should work, but depends on how long its been there and how deep the roots are. Concentrated glyphosate is difficult to get hold of now so much better to get a contractor to do it if you can.