Japanese knotweed: How to identify and remove it

• Invasive garden plants
• Prevent Japanese knotweed from spreading (gov.uk)

What is Japanese knotweed?

Japanese knotweed, Fallopia japonica, is a rampant plant that is considered an invasive weed. Its creeping roots and tall, bamboo-like stems can quickly take over a space. The roots of Japanese knotweed can grow 1m deep, making them extremely difficult to dig out, and the plant can grow through cracks in brickwork and pipework. It is against the law to allow Japanese knotweed on your land to spread onto other people's property or into the wild.

What does Japanese knotweed look like?

A herbaceous perennial weed, Japanese knotweed grows in large clumps of tall, bamboo-like canes with purple spots, up to 2.1m high. From these stems alternate, light green heart-shaped leaves appear, followed by hanging clusters of creamy white flowers. The leaves are 14cm in length and the flowers grow to 15cm in length. From late autumn the plants become dormant and the canes die and lose their leaves. The dead canes often remain standing – sometimes they can take years to decompose.

In spring, look out for fleshy, reddish shoots growing on their own or among dead bamboo-like canes. New canes, leaves and flowers grow from these shoots.

Japanese knotweed may be confused with other plants, including Russian vine, Fallopia baldschuanica. While the flowers and leaf shape are similar, Russian vine has narrower, almost-arrow-shaped leaves and a much more scrambling habit than Japanese knotweed. The lack of tall stems is the most obvious identifying factor. Himalayan honeysuckle, Leycesteria formosa, is also confused with Japanese knotweed. However, on closer inspection you will notice that Himalayan honeysuckle has opposite leaves (the leaves emerge at the same point on either side of the stem), not alternate leaves. The long, purple and white flowers are very different, too. Some species of persicaria have similar-shaped leaves, but they tend to be more narrow and don't have the bamboo-like growth of Japanese knotweed.

How does Japanese knotweed cause problems?

Japanese knotweed can sprout from tiny sections of root. It’s extremely vigorous and can quickly spread around your garden and into other gardens. Its roots are known to exploit cracks in brickwork and pipework, and it can even damage roads. If you spot Japanese knotweed growing near your house you should eradicate it immediately, as it could potentially damage the foundations of your home.

Japanese knotweed can sometimes grow from seed, too. This means that if Japanese knotweed is growing in a neighbouring garden it will very likely come into your garden, either by its spreading roots or by seed. Keeping an eye out for early signs of growth can therefore help you remove Japanese knotweed quickly, and keep further outbreaks under control.

Japanese knotweed causes problems in gardens owing to its invasive nature and its habit of out-competing all plants growing nearby. However, it’s also a problem in the wild. Sadly Japanese knotweed has escaped and/or been dumped in the wild, where it quickly spreads and threatens natural ecosystems. It is a common sight along railway banks and at train stations, and along canal towpaths. It's thought the wind caused by passing traffic helps makes it easier for Japanese knotweed seed to spread.

Japanese knotweed and the law

Schedule 9 of the Wildlife and Countryside Act 1981 makes it an offence to cause Japanese knotweed to grow in the wild, either by means of fly-tipping or allowing the plant to escape the confines of your garden.

While it’s not illegal to grow Japanese knotweed in your garden, it is illegal to allow it to spread into wild areas or neighbouring gardens, therefore it’s best practice to eradicate or at least control its spread. What’s more, if you're selling your property then you will need to remove it. Since 2013, homeowners selling their property are required to check the garden for Japanese knotweed and declare its presence, as well as provide details of a management plan on its eradication.

If you're buying a property and Japanese knotweed has been found to be growing there, your mortgage lender may require assurances that a management plan for its eradication is in place. Regardless of whether it has been declared or not, it’s important to check for yourself, to avoid any problems later on.

Under the Environmental Protection Act 1990, Japanese knotweed is classed as 'controlled waste'. This means it’s illegal to add it to home-compost or council-run garden waste bins. You are permitted to burn the waste (ideally after allowing it to dry). Alternatively, you will have dispose of it at a licensed landfill site.

Contact The Environment Agency if you notice that Japanese knotweed has been dumped illegally in the wild.

How to remove Japanese knotweed

Small clumps of Japanese knotweed are fairly straightforward to manage and can be removed by the home gardener by digging or spraying with weedkiller. However, we recommend you hire a qualified, professional company to control large clumps. The advantage of using professionals is that they draw up risk reports and offer treatment plans with a guarantee on its complete eradication, which is usually accepted by mortgage lenders.

Organic methods of removing Japanese knotweed

Digging the plant out of the ground can cause more problems in the long run, owing to its ability to regenerate from small pieces of root and the issues around its disposal. It is possible to gradually weaken the plant by removing all leaves as soon as they grow, which stops the plant photosynthesising. However, this method can take many years to have an effect – you will need to check the plant at least once a week and remove new leaf buds as and when you see them.

Chemical controls of removing Japanese knotweed

A glyphosate-based weedkiller is the best option here, though bear in mind it can take several applications, over up to four seasons, to completely eradicate Japanese knotweed. It’s best applied to cut canes so the weedkiller can thoroughly penetrate the plant and roots. Some brands have specific instructions on how to control Japanese knotweed.

Make sure you follow the instructions to ensure the most effective control, while minimising risks to yourself, pets and wildlife.

Glyphosate-treated knotweed will often regrow the following spring, albeit much less vigorously. It’s important to administer a second application to this growth.

Other invasive plants to look out for

• Himalayan balsam (Impatiens glandulifera)
• Giant hogweed (Heracleum mantegazzianum)
• Gunnera (Gunnera tinctoria, Gunnera manicata, Gunnera x cryptica)
• Milkweed (Asclepias syriaca)
• Three-cornered leek (Allium triquetrum)
• Cotoneasters (Cotoneaster horizontalis, Cotoneaster integrifolius, Cotoneaster simonsii, Cotoneaster bullatus, Cotoneaster microphyllus)

View the full list of invasive plants listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales.