Japanese knotweed

in : Problems: Weeds

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
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.
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.

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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