Monday 14 October 2019

Getting rid of Japanese knotweed quickly and cheaply(?)

The most expensive plant in your garden
Everything is relative. A year would be ‘quickly’. As to ‘cheaply’ I mean compared to a second mortgage to pay a contractor to eliminate JKW (I will call it) before you can sell your house.

It is in the news again relative to invaded railway properties and creeping into adjacent gardens.
Of course it is over-hyped. Worse plant dangers can threaten your home. Think of trees near your house when you have a clay soil. Things like brambles and bamboo if left for years can be a real problem.
They just do not get the publicity to be cast as evil alien invaders.

It is a perfect storm. A whole industry exists to extract huge payments to eliminate it. In unconscious cahoots with surveyors who believe the hype and who consider homes are seriously threatened. (They are when house agents put the fear of god into clients - both sellers and buyers)

It is not that that JKW is inconsequential. It’s just that with a little forward planning by homeowners a year or two in advance of selling they can easily eliminate it with glyphosate. Good gardeners will not have it all.

Here again another perfect storm creates a problem. Apart from irrational fears of glyphosate safety being high in the public domain, pesticide regulations get in the way of law abiding citizens who are led to believe that using professional glyphosate is illegal. There is a kind of conspiracy that keeps amateur gardeners and professionals apart. And certainly there are good reasons that the general public should not use particular chemicals.
Professional suppliers do not want to bother themselves with amateur’s foibles and very small purchases. The likes of garden centres want to trade in products with very high margins. Price differences are at least tenfold and for those suckers who buy ‘ready to use’ diluted spray the difference in cost might be a factor of hundreds. Legislation regarding amateur gardeners is a very grey area which suits our authorities fine.
I know very few gardeners who do not use professional glyphosate. With a little effort it is easily acquired. In my opinion it is one of the safest pesticides ever invented. (For the uninitiated ’pesticide’ is the generic word for all ‘plant protection’ products)
Even if you don’t take my word for its safety the professional product is only different to amateur products in its strength. When applied appropriately diluted to the weeds it is for all practical purposes the same. The good news is that if you choose to control with stuff from the garden centre it is equally effective. It’s just that it will cost you very much more. This will be as nothing compared with tens of thousands knocked off the value of your home.

Peter eliminated it with a single injection
My new post today was inspired by a letter to the Times describing how the householder eliminated JKW in a season with a cutting back technique that exposed the root system to the flow back of weedkiller to the roots via the hollow stems. I have not done this myself on JKW but have good reason to believe it works very well.

My reasons for predicting success

Equisetum hyemale - just as aggressive as the common marestail
I have used the technique very successfully indeed on mares tail, equisetum. Not actually the common weed species itself but on an equally deep rooted ‘impossible to kill’ relation Equisetum hyemale. It was taking over a six square metre patch in Bolton Percy cemetery. I did not want my own memorial to be a huge blot on the landscape and decided it must go. Not only did it completely go for ever after one single treatment, even untreated outlying spreading growth was completely killed as it sucked up the glyphosate from its deep root metres below

Cut back by hedge trimmer and ready to drench

I once read of a professional gardener who used the technique on the feared equisetum weed itself and although at the time I did not believe him when he claimed it worked very well I think he was not fibbing.

Six weeks later
Six months later - at this point it was mulched over

I described in my JKW control opus how Peter Williams eliminated JKW in one go by injection. This is rather fiddly but works extremely well. It succeeds by exactly the same principle as trickle back down hollow stems and subsequent root translocation.

Peter cobbled together this natty injector
My main post on JKW control has now accumulated almost a hundred comments. Although I was then unaware that the cutting back technique was a viable option several readers report great success with it.
I was also rewarded with many grateful thanks for pointing the way to eliminating JKW on their property by conventional spraying

How and when
Although I have not carried out the technique on JKW myself I go on general principles
JKW should be growing strongly when you first treat it. No use zapping when growth first appears in Spring. Little use either if you have recently been scrabbling out roots.
Perhaps optimum is to cut new 18 inch shoots to perhaps an inch to the ground (although it might start very much higher). If you have previously embarked on a spraying regime new growth might be very much shorter.
Immediately give a heavy spray For larger areas use a knapsack sprayer, for smaller areas any bog standard hand sprayer. Although a rose on a watering can is generally inappropriate to apply glyphosate to leaves it is suitable here. You want to apply enough spray to visibly penetrate down into the hollow stems.
I would dilute commercial ‘Roundup’ (any professional version of glyphosate) at perhaps one in ten dilution. That would equate to amateur product concentrate at one in one, two or three (product strengths vary).
I do not expect complete success with a first application. You might miss new growths and it might not be the optimum time. (You can start the procedure right through to late Summer). If there is new growth let it grow strongly before you repeat treatment.

With the more usual spraying techniques I describe in previous posts it takes at least three years of repeated glyphosate sprays to eliminate strong established JKW. With this new approach I expect it to be very much quicker
If you do not declare JKW as a house vendor you will not escape your responsibility to any new householder unless JKW is completely and permanently eliminated. No use in court if it was merely dormant

My post that details control of JKW by spraying has been read twenty five thousand times and includes in ‘comments’ reader’s  tips and questions

Although this post takes a different direction it includes many of the above observations

My original post on marestail/horsetail

Use my search box to read detailed dedicated articles on controlling epilobium (some misidentify as willow herb), ground elder, hairy bitter cress, hymalayan balsam and brambles  - which is the worst weed of all.


  1. My mum has got a problem with some bushes in her garden which are spreading definitely too fast. She can't keep up to trim them because of the lack of time so probably she will have to get rid of some of them. When I read 'knotweed' firstly I thought about the one I have in my yard but then I realised that they differ. Mine, pink knotweed (from ), as the name suggests, had pink flowers. They were blooming in summer but they weren't so agressive in growth. I appreciate the tips that you have shown. My mum has to make a look at them and apply them in real life - to declutter her garden again!

  2. Yes there are ssveral very excellent knotweeds that are quite innocent plants that will cause you no problems - I have a few myself that I am very fond of albeit one does need to be kept in check
    You have no need to worry about P. capitatum.It is a shame that JKW and Russian vine give polygonums a bad name
    I do not recomend the method against hollow stemmed plants for general shrubs in the garden

  3. You can understand why the Victorians introduced this plant - it is quite striking; there are well contained examples at Myddleton House Gardens in Enfield where it was planted by EA Bowles.
    (It was good to meet Peter at the HPS Group Secs meeting last week ... )

    1. He mentioned you Neil
      I agree it is a very handsome plant!


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