Japanese knotweed, Fallopia japonica, is a pervasive weed with creeping roots and bamboo-like stems, that can quickly take over its growing space. Removing Japanese knotweed can be difficult, earning it its reputation as one of the most troublesome garden weeds. Homeowners should be particularly wary of it, as the presence of Japanese knotweed in the garden can deter potential future buyers and lead to banks refusing mortgage applications.
It was introduced to Britain by Victorian plant hunters, and was prized for its exotic appearance, heart-shaped leaves and attractive flowers, which resemble those of astilbe or goatsbeard. However, its rampant growth quickly made it unsuitable for garden cultivation.
How to identify Japanese knotweed
New shoots of Japanese Knotweed
A herbaceous perennial weed, Japanese knotweed becomes dormant in winter and regrows in spring from fleshy, reddish shoots. By summer, these have developed into tall clumps of bamboo-like canes up to 2.1m high, which bear branches of alternate, heart-shaped green leaves. Plumes of cream-white flowers appear in late summer to early autumn.
From late autumn the plants become dormant and the canes die and lose their leaves. The dead canes often remain standing. Sometimes they take years to decompose.
Japanese knotweed may be confused with other plants such as Russian vine, Fallopia baldschuanica, and Himalayan honeysuckle, Leycesteria formosa. Some species of persicaria have similar-shaped leaves.
How Japanese knotweed causes problems
Japanese knotweed flowers, Getty Images
Although it rarely sets seed in Britain, Japanese knotweed can sprout from tiny sections of root. It’s extremely vigorous and can quickly spread. Its roots are known to exploit cracks in brickwork and pipework, and it can even damage roads.
It 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.
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 are 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 are 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 are fairly straightforward to manage and can be removed by the home gardener. 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.
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.
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.