In spring, rounded pink buds form at ground level and from these reddish-purple shoots emerge. The soft shoots grow rapidly, becoming bamboo-like stems. These are characterised by reddish-purple speckling, conspicuous nodes and hollow centres. Stems usually grow about 2m (6½ft) tall, reaching their full height in late summer, by which time they may be arching over at the top.

Plants that may be confused with Japanese knotweed 

There are several similar-looking plants that are commonly confused with Japanese knotweed, these include: 
 
Fallopia baldschuanica (Russian vine) 
Leycesteria formosa  (Himalayan honeysuckle)
Houttuynia cordata (fish mint)
Persicaria microcephala (small-headed knotweed)  

Several species of Persicaria and Polygonum, including annual species Persicaria lapathifolia and P. maculosa, can also be troublesome weeds but are not as invasive. Japanese knotweed may also be confused with the shorter variety, Reynoutria japonica var. compacta, and its cultivars. Although slightly less vigorous, they are still invasive.

Japanese knotweed was introduced into the UK in the 19th century as a garden plant, valued for its ornamental qualities. However, it is now considered an invasive non-native species, due to its size, vigour and persistent strong-growing rhizomes that allow it to:  

• Outcompete native plants for space, light, moisture and nutrients. Rapid growth in spring and summer leads to dense stands of Japanese knotweed that crowd out other plants and reduce species diversity. 
• Cause damage to hard surfaces. It can exploit joints, cracks and weaknesses in hard surfaces, such as paving, tarmac and asphalt, allowing shoots to emerge and create uneven surfaces. 
• Alter and degrade the banks of watercourses. Japanese knotweed often colonises the banks of rivers, streams and ditches; this can contribute to erosion and increase the likelihood of flooding.

© Shutterstock

Japanese knotweed can grow up through joints and cracks in hard surfaces 

Here are our answers to your most common questions about dealing with Japanese knotweed:

How invasive is Japanese knotweed? 

Since its introduction in 1850, Japanese knotweed has become widespread and common across the UK. Small clumps can quickly become dense thickets, owing to its vigorous fast-growing rhizomes. Japanese knotweed regenerates from fragments of rhizome and stem, so it is often spread by digging and attempts to dispose of it. Rhizomes can remain dormant in the soil for many years, and sections as small as 1cm (⅓in) can grow into new plants. 

Almost all Japanese knotweed plants in the UK are female, so it does not spread by seed, although it can hybridise with closely related plants. 

It is tolerant of a wide range of soil types and growing conditions, and can quickly colonise various habitats, including roadsides, waste ground, spoil heaps and along rivers and streams.

Do I need to get rid of Japanese knotweed?    

It is not an offence to have Japanese knotweed growing in your garden, but it is an offence to plant or cause to grow in the wild and you should take steps to prevent it spreading. Additionally, a relevant environmental authority can order control measures if they deem such action necessary. For more information, see the Legislation covering Japanese knotweed section below.  

Due to Japanese knotweed’s invasiveness, its negative impacts on native species and the environment, and the implications it has on buying and selling properties, most gardeners want to get rid of it. 

How can I get rid of Japanese knotweed? 

Hire a professional weed control contractor:
The easiest and most effective way to remove Japanese knotweed is to hire a professional company that specialises in the removal of invasive weeds. They can provide information for mortgage purposes, including suggested treatment plans, and offer insurance-backed guarantees where required.  

The organisations below have lists of invasive weed control contractors:   
PCA (Property Care Association) Invasive Weed Control Group  
Invasive Non-Native Specialists Association  
The British Association of Landscape Industries (BALI)   
Trustmark Government Endorsed Standards   
For general advice on hiring a professional, see our guide to hiring contractors. 

Cultural control:
When tackling Japanese knotweed, cultural control methods are possible but do pose some problems: 

• Digging out the deep system of rhizomes and roots, even if feasible, creates problems over disposal, as Japanese knotweed is classed as 'controlled waste' (see information on the Environmental Protection Act 1990 below). It may be possible to destroy it on site, by allowing it to dry before burning. 
• If you choose to dig out Japanese knotweed, bear in mind it can regenerate from small sections of rhizomes, so try not to chop them up and be sure to check through the soil to remove any fragments. In spring and summer, regularly monitor the area for regrowth and dig out any remaining rhizomes.  

You should not add Japanese knotweed to your home compost, council green waste or household waste bin.  

Weedkiller control:
The RHS does not support the use of weedkillers and usually recommends that alternative control methods are used. However, when invasive plants are a threat to biodiversity, pose legal problems, and gardeners struggle to control them with cultural methods, targeted use of a regulated weedkiller is an option. 

It usually takes at least three growing seasons to eradicate Japanese knotweed using weedkiller. Professional contractors are often able to achieve complete eradication in less time.  

Things to bear in mind when considering chemical control: 

• If you choose to use a weedkiller to control Japanese knotweed, always follow the instructions on the pack to make effective and economic use of the product while minimising risks to people and the environment.  
• For home gardeners, the most effective chemical control to tackle Japanese knotweed is a glyphosate-based weedkiller (bearing in mind that, in this specific instance, the control of this plant prevents the loss of biodiversity in gardens). Garden centres and large retailers selling weedkillers have trained staff who can advise on suitable products for your needs. 
• Glyphosate-treated knotweed will often produce small-leaved, short, bushy regrowth the following spring. This is quite different in appearance to the normal plant and this regrowth should be treated to ensure complete eradication. 
• Japanese knotweed is often found on the banks of watercourses, where there are restrictions on the use of weedkillers.    
• Bear in mind that you will not get an insurance backed guarantee without using a professional company for the control of Japanese knotweed. This may be important if you plan to sell your property soon or if a neighbour is threatening litigation regarding the spread of Japanese knotweed from your property. 

For more information see:
The Environment Agency guidance on How to stop invasive non-native plants from spreading  
The Environment Agency guidance on How to stop Japanese knotweed from spreading   
The Centre for Agriculture and Biosciences International (CABI) webpage on Japanese knotweed

Wildlife and Countryside Act 1981  

Anti-Social Behaviour, Crime and Policing Act 2014 

• It is not illegal to have Japanese knotweed in your garden, but you should aim to control it to prevent it becoming a problem in your neighbourhood. If it has a "detrimental effect of a persistent or continuing nature on the quality of life of those in the locality", the legislation could be used to enforce its control and property owners may be prosecuted. 
• Where problems with Japanese knotweed occur in neighbouring gardens, you should communicate directly with your neighbours (who may already be taking action to control this difficult weed). This informal step should be taken before contacting your council to talk about action under the legislation. 

Environmental Protection Act 1990 

• If you are selling, it is your responsibility to check the garden for Japanese knotweed (bearing in mind it dies back in winter). The TA6 form asks you to confirm whether your property is affected by Japanese knotweed, where it is, and to provide a management plan for its eradication. 
• If you are buying, the presence of Japanese knotweed will be stated in the responses to the TA6 form. This often results in your mortgage lender requiring assurances that it will be eradicated before agreeing the funds. A management plan by a professional eradication company, backed by a transferable guarantee, is usually sufficient. It is most common for this plan to be provided by the seller before the purchase is completed. 
• Whether buying or selling, it is worth being pro-active and checking the property for Japanese knotweed. Disputes over the identity of a plant, the failure to disclose its presence, or the lack of a management plan can result in delays, increased costs later in the buying process, or even a possible misrepresentation claim after the sale, so this approach helps to avoid problems.