(Fallopiajaponica) Japanese knotweed is a noxious, invasive, deep-rooted perennial weed that quickly spreads to produce dense areas of growth. Its roots can grow through concrete!
What is Japanese knotweed?
Japanese knotweed is a strong, fast growing, clump-forming perennial. It was originally introduced into the UK as an ornamental plant, thanks to its stems that resemble bamboos and its masses of flowers.
Once it is established in the garden, it becomes very difficult to control, because of its strong, deep penetrating rhizomes (creeping underground stems).
In accordance with The Anti-social Behaviour, Crime and Policing Act 2014, you must take action to remove and prevent the spread of Japanese knotweed.
Digging out the rhizomes creates a problem over disposal, since Japanese knotweed is classed as “controlled waste” under the Environmental Protection Act 1990. And under the provisions of the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild, so it is illegal to simply dump it anywhere. Topsoil movement has caused much of its spread throughout the UK, for instance.
The Anti-social Behaviour, Crime and Policing Act 2014 includes Japanese knotweed. Allowing Japanese knotweed to become established in your garden could see you prosecuted as an act of anti-social behaviour.
Where does it grow?
- Gravel paths
- Waste or uncultivated ground
In spring, bamboo-like green stems with purplish speckles appear from below the ground and grow to around 2.1m (7ft) high. The leaves are heart shaped and up to 15cm (6in) in length. Creamy-white, 15cm (6in) long, flower tassels are produced in late summer and early autumn.
The stems turn brown in autumn and the plant usually dies back to ground level in winter, but they can remain standing over winter. This may lull you into believing it has gone away, but it will grow back with a vengeance the following year.
How to control Japanese knotweed
As with most pernicious perennial weeds, never allow it to become established. This will make it more difficult to fully control. Early identification and eradication is very important to prevent it spreading even further and becoming more of a problem.
There are a number of weed control options available to treat Japanese knotweed. In addition to traditional weedkillers there are now also a range of more natural alternatives.
You can start by digging out plants, but the rhizomes are very tough and deep and you may have to use a pickaxe or similar. No matter how careful you are, pieces of rhizome are sure to remain and new growth will appear from these.
As it is illegal to dump the rhizomes, once dug up allow them to thoroughly dry out and then burn them.
Then, repeatedly destroy any regrowth as it appears to exhaust the energy remaining in the rhizomes. This could take several years to give full control.
Covering the soil with weed-control membrane (landscape fabric) or even thick black polythene to exclude light will have no effect on Japanese knotweed, since the stems can grow through concrete.
In lawns, regular mowing throughout the year, and possibly together with the use of lawn weedkillers, may weaken it and keep it under control.
For best results, spray with a systemic weedkiller. A systemic weedkiller, which is absorbed by the leaves, then moves down to the roots to kill them.
To ensure the weedkiller works effectively:
- Spray the plants when they’re growing actively; this is mainly from May to September. The most effective time to spray is during flowering in late summer.
- The larger the leaf area present, the greater the amount of weedkiller that can be absorbed and move down to the roots. So don’t bother spraying when the growth first emerges through the soil – wait until the stems are around 60-90cm (2-3ft) high.
- Use a fine spray to thoroughly coat the leaves in small droplets.
- During the summer, spray in the evening to prevent the spray evaporating and to give maximum time for the spray to be absorbed. In spring or if overnight dew is forecast, spray earlier in the day to allow the spray to dry before dew falls.
- One application of weedkiller is highly unlikely to completely kill it. You will need to spray once, allow it to die down, and then spray the regrowth again. Three or four applications a year, over several years, may be needed to completely kill it, depending on how extensive the root system is.
One of the most effective Japanese knotweed treatments is to cut back the stems and apply the weedkiller inside the cut stems. Roundup Tree Stump & Root Killer is the only garden weedkiller that carries detailed instructions and an application pipette for this method.
A gel, which is smeared onto, and sticks to, the leaves, may be a better option than a liquid spray.
Most contact weedkillers are total weedkillers – that is, it will damage or kill any plants whose leaves it is sprayed on. Make sure you keep the spray off wanted plants – including lawns – and, if necessary protect plants by covering with polythene or similar when spraying.
Use weedkillers safely. Always read the label and product information before use.
For severe infestations, it may be better to use a professional contractor specialised in Japanese knotweed control. Your council may be able to help you with details of local contractors, or there are several recommended companies listed on the internet.
Japanese knotweed grows rampantly, especially on waste ground. As it spreads by its underground rhizomes, it will be coming into your garden from a neighbouring garden or surrounding waste ground. Unless you can stop this source, it will just keep coming back.
If it’s coming from a neighbour, have a quiet word and try and encourage them to control it in their garden – especially as they may be prosecuted under The Anti-social Behaviour, Crime and Policing Act 2014!
As the roots can go down several feet and will grow through most things, putting a vertical physical barrier in the soil on your boundary is highly unlikely to prevent it coming in.
- Systemic weedkillers
- Lawn weedkillers