The Grumpy Gardener – Dreaded Knotweed

The Grumpy Gardener – June

Hints & Tips With Mike Alexander
In his regular column for French News Online, professional gardener Mike Alexander tells of the horrors of trying to root out a Japanese invader that bamboozles many into thinking it is bamboo when in fact its a plague of Biblical proportion.


Ten years ago I started work for a new client. On day one she showed me around her garden paying particular attention to a patch of what she referred to as “unusual bamboo”.

Knotweed NOT what you want in your garden (Credit: Mike Alexander)

Immediately my heart sank. The plant I was proudly being shown was not a bamboo at all, it was in fact Fallopia japonica or Japanese knotweed. Explaining to a client that their beautiful exotic looking plant is in fact one of the most invasive plant species in Europe is not something one relishes doing, least of all on one’s first day. 

In France knotweed is called Renouée du Japon. It was introduced to Europe in the early 1800s as a garden exotic and it certainly is an impressive looking plant which can grow to heights of two metres or more. The problem is that it thrives here and once established is extremely difficult to remove. 

Although it does not reproduce sexually in Europe it reproduces from underground stems called rhizomes. These rhizomes are so pernicious that digging out is almost an impossibility in the domestic garden. Each rhizome can extend for up to 7 metres horizontally and descend to a depth of three meters. Good luck digging that up with a garden spade and a fork. The smallest piece left in the ground will simply take a deep breath and continue its unrelenting quest to conquer the world. 

To put the problem into perspective lets look to the UK. Estimates to clear the weed from London’s Olympic park (built for the recent games) ran to over 70 million pounds and DEFRA, the country’s environmental agency, predicts the cost of eradicating it from the UK would run to 1.56 billion pounds. The plant has been declared an invasive species in much of the US, Canada and most of Europe. Not bad for a pretty little herbaceous perennial brought in to beautify our gardens! 

At the moment tests are being conducted in France and the UK with biological controls such as the release of various insects that it is hoped will feed exclusively on this Nipponic invader. In some cases experiments with a sea water spray are also showing some promising results. 

All this was very interesting, but of little help in dealing with my bulging patch of the hungry “bamboo”. I had little choice but to launch a chemical attack which is still the route most likely to get results.

Renouée du Japon – a garden nightmare (Credit: Mike Alexander)

I first cut the plants to the ground and burned all the stems. I didn’t need to wait long before new shoots had reached a height of about ninety centimeters. At this stage I sprayed thoroughly with a glyphosate-based herbicide. Once the tops had died I then cut them down and burned again. After that I made a weekly attack on small shoots as they appeared. Painting them with small amounts of undiluted glyphosate mixed with chain saw oil so that it would adhere better to the leaves. The whole process was painstakingly laborious and it took more then three years before I could say that I had finally won.

If you have this plant in your garden I suggest you take aggressive action now. If not you should consider selling your house and moving to Japan where the plant is considerably more benign.

Previously – click an image below
To read February's gardening tips article - click this image
Is a world without bees possible? Read about this crisis that will affect us all - click here
Prune for Results
A World Without Bees?
What you should have done in January!
Prune your roses - click here
Fruit Tree Pruning
Wars of the Roses
Grumpy Gardener April - It's War Out There. Click to view
Click to read this article
Prune When Finished
Herald of Spring…
click to read Grumpy's july  tips
You’ve got to be quick!
Un-thirsty Lavender
To read this August 2011 article - click here
To read this August 2011 article - click here
Grasp the Nettle
Star Jasmine – Madrid
To read this August 2011 article - click here
To read this August 2011 article - click here
Jihad – on Bunnies Ears
Autumn Arrives
Designer chic or neccessity? - click here for full story
To read this article - click here
Designer Chic?
Gravity – not to be ignored!
If winter comes, can Spring be far behind? Grumpy contemplates the winter garden
If winter comes, can Spring be far behind? Grumpy contemplates the winter garden
If Winter Comes….
Hottest chilli in the world
Dogs and Daffodils... full story - click this image
The iris and Madame la Guillotine... full story - click this image
Dog Days…
Fleur de Lys
Hedge You Bets - how do YOU like your hedges... full story - click this image
To read a previous article
– click an image –
Hedge Your Bets

Grumpy Gardener

Our Grumpy Gardener has been gardening professionally in France for more years than he cares to remember and before that in Africa and the UK. Today he happily shares his expertise with French News Online readers. Your gardening questions are welcome and while they may not be individually answered, they may form the basis of future monthly columns.

Writer: Mike Alexander

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14 Responses to The Grumpy Gardener – Dreaded Knotweed

  1. Lafayette May 29, 2013 at 6:13 am

    Why does the plant defy glysophate, which supposedly will follow throughout its ramifications?

  2. Mike May 29, 2013 at 9:04 am

    Hi Lafayette
    you are correct in that glyphosate is carried from the leaves to the roots. In this case the sheer length of the rhizome and the fact that each segment is capable of regenerating means that it takes many treatments to finally kill this plant.

  3. anniedm778 May 30, 2013 at 1:01 pm

    Here in the Southern US is an invasive vine called ‘Kudzu’. It’s heartbreaking to drive through the Southern states and see this plant completely covering acres and acres.

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