Japanese knotweed

Symptoms

This weed is especially rampant and can grow to 2.1m in just one season, overwhelming any garden plants in its way. It doesn't colonise gardens by producing seeds, instead it sprouts from very small sections of rhizomes.

Find it on: all over the garden

Time to act: early spring to autumn

Problem categories

Overview

Japanese knotweed, Fallopia japonica, was introduced to the UK by the Victorians as an ornamental plant. But because of its rampant growth, and the fact it aggressively over-runs anything in its path, it's now an offence to plant this in the wild. It spreads by sprouting from thick, woody rhizomes, and can grow 1.2m in one season. Winter is the only time of year that offers a reprieve, but for the rest of the year it's a plant that needs tough handling as soon as it appears.

Solution

Organic

Weaken the plant by cutting off the top growth every two to four weeks. Let the cut stems wither in the sun until dead before digging up and disposing of them. Do not add them to the compost heap. If you dig up any rhizomes, you must take them to a licensed waste site, as they are a 'controlled waste' under the Environment Protection Act 1990.

Chemical

Use a total weedkiller, such as glyphosate. For best results, cut away old stems in winter and spray the plant in late summer when the weed is flowering. You will need to re-apply in midsummer, then check in September and spray again if needed. Check again in spring. It can take 3 or 4 seasons to eradicate using glyphosate. Avoid spraying on a windy day and near other desirable plants.

Discuss this problem

Talkback: Japanese knotweed

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Dunstergardener 24/11/2011 at 15:27

I've got my work cut out this year then. One hedgerow is riddled with the stuff. It's going to be a tough fight to get the lot to die. Carefully digging between the roots of the hedge is a big job alone.

raylodge 24/11/2011 at 15:28

We have two large sections 4x4 metres of Japanese Knotweed in our paddock, as we graze this area a friend has advised that we cut the stems to the last leaf shoot and pour salt and oil in each stem then cover with black sheeting - is this good advice?

specialoffer 24/11/2011 at 15:29

I note from http://www.phlorum.com/identify-japanese-knotweed.html there's a cross between JK and its Giant brother that CAN set seed....? I'm a bit concerned about this:my current problem has what look like seeds on the dead flower heads?

Is it really a good idea to dig it out if a fragment of the rhizome can persist in the soil and regenerate even after 20 years? See . http://www.environment-agency.gov.uk/homeandleisure/wildlife/130079.aspx#How_to_control_Japanese_knotweed

whitevanwoman 04/05/2012 at 12:30

whitevanwoman 04/05/2012 at 12:44

Sorry hit the wrong button!
In my experience digging it out while it is still alive is a decidedly bad idea as one seventh of a gram of root can result in re-growth.

Having spent wasted time trying to spray the stuff we came up with a more radical approach.In Spring, wait until it have reached at least 75cm high, it needs to be making strong growth.Cut the top off so that you can see down the stem, which is a bit Bamboo like in that it grows in section. Straighten a metal coathanger, and poke this down through the sections as far as you can. Make up a solution of Glyphosate, from the concentrate, then using a hypodermic syringe or similar (you don't need a needle) inject the solution carefully into the stem and keep doing this until the stem is almost full. Then proceed to the next plant. Repeat this in Autumn, when the sap is retreating into the roots and again the following Spring, by which time the plant will really be struggling. Once you are certain it is dead and you may need to wait a further season, then it is safe to dig up and dispose of legally.
We have used this successfully on Knotweed which was over 1.8m high and it didn't endanger any of the surrounding trees or shrubs.

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