Japanese Knotweed: How Does It Spread?

By Paolo Martini on 19th December 2019 (updated: 30th January 2020) in News

How Does Japanese Knotweed Spread?

Japanese knotweed spreads in the UK via dispersal of rhizome fragments, which comprise its extensive root system. Despite not being able to reproduce via its natural method, Japanese knotweed has been able to spread throughout most of the UK since its introduction. 

Introduced to the country via horticulturists in the late 19th-century from Japan, the plant has no controlling organisms to keep it in check. Whilst the climate in the UK is different from that of its native land, Japanese knotweed has been able to spread unchecked at the expense of native species, often commandeering large swathes of land.

Each Japanese knotweed plant is supported by an underground system comprised of stems, crowns and rhizomes which spread underground and periodically send shoots to the surface in order to fuel further growth. An entire plant can grow from a fragment of rhizome fragments as small as 10mm, therefore, if any soil beneath a Japanese knotweed patch is moved or dumped, a new crop can be expected to grow on the new site. 

Does Japanese knotweed spread by seed?

Japanese knotweed spreads by seed dispersal in its native home of Japan, however, it does not have the capacity to do this in the UK. Japanese Knotweed is a gynodioecious plant, comprising both female and hermaphrodite variants, both of which are required in order to reproduce in the usual fashion. 

Although rare cases have been reported of the plant spreading by seed dispersal, it is widely accepted that Japanese knotweed is not able to spread by seed within the UK, due to the fact that only the female of the species was imported. Whilst seeds of the plant may be produced by Japanese knotweed here, these seeds are sterile and will not lead to new growth. 

Can Japanese knotweed spread by cross-breeding?

Japanese knotweed does not spread via cross-breeding. This process can only occur when both species of plants are related closely enough and are capable of producing viable seeds. There are two species in the UK capable of hybridisation: Fallopia sachalinensis (Giant knotweed) and Fallopia japonica var. Compacta

Giant knotweed is capable of growing up to 5 metres tall, whereas compacta is smaller in stature (1 metre at its tallest). There are many plants that look like Japanese knotweed, so it’s important to brief yourself properly on how to identify the plant before jumping to any incorrect conclusions. 

Whilst hybrids, such as Fallopia bohemica are living proof that crossbreeding is possible, this is never the cause behind the spread of Japanese knotweed as hybrid seeds do not produce new growth. 

Related: How To Identify Japanese Knotweed

Can Japanese knotweed spread on shoes? 

Japanese knotweed can easily be spread by transferring from shoes or clothes, this can happen when people walk through a contaminated area. One of the most common methods of Japanese knotweed spreading is when land is redeveloped or treated in some way, leading to increased human traffic.

The plant can be easily spread when contaminated boots, vehicle tracks or tires travel off the infested site and into new territory. When any excavations or clearing takes place on infested lands, rhizome fragments can be easily broken down and then compacted into the treads of shoes, vehicles or can even stick to clothes. 

As Japanese knotweed can grow from the smallest of rhizome fragments, given enough time and space, new growth can then occur once fragments have been deposited on fresh ground. The ease with which the plant can be spread by people makes careful demarcation of contaminated grounds imperative, especially when attempting to contain and get rid of Japanese knotweed

Can birds or other animals spread Japanese knotweed in their droppings?

Some animals, such as sheep, cattle, horses, and goats can spread Japanese knotweed via their droppings. Although their droppings are unlikely to contain any rhizome fragments, there is the possibility that new growth could development from stems or canes that have passed through an animal’s digestive system. 

When animals eat Japanese knotweed they are more likely to consume young shoots, as the plant becomes mostly inedible once it matures and becomes harder, and more difficult to digest. It is also possible for these creatures to spread any Japanese knotweed plant matter that happens to be stuck to their fur or hoofs. In cases where infestations are being excavated animals can easily disturb the soil and then walk fragments from one site to another, which may lead to the plant spreading further afield.  

How far can Japanese knotweed spread?

There is no limit to how far a Japanese knotweed infestation can spread; if it is given the space and nutrients then it can grow indefinitely. Japanese knotweed spreads naturally via its underground network of roots, which are made up of rhizomes. 

Under the surface of the ground, roots can grow as far as 7 metres horizontally and up to 3 metres deep from each Japanese knotweed shoot. Due to the speed and ease with which Japanese knotweed is capable of spreading, the plant has been labelled as invasive by the UK government.

How far has Japanese knotweed spread in the UK? 

Since its introduction in the late 19th-century, Japanese knotweed has spread to large swathes of the UK, to the point where there are few regions of the country that have not been affected by it in some way. A Government-approved scheme has been created to track the spread of this invasive plant.

The Environment Agency, the Scottish Environment Protection Agency and Natural Resources Wales have collaborated to locate incidences of invasive plants across the UK, including Japanese knotweed. The crowd-sourcing campaign led to Japanese knotweed infestations being reported across the country and displayed on a map, revealing the extent to which the plan has dispersed across the UK.

Related: Japanese Knotweed UK Map: Is there Japanese knotweed in my area?

How fast does Japanese knotweed spread?

During the summer Japanese knotweed has been reported to grow as quickly as 10cm a day, however, there is no research to support how fast it spreads underneath the ground. The spread of Japanese knotweed can be increased by unwitting breakage or disturbance of the ground.

Any stems, crowns or rhizomes that are cut and left to sit in the ground can potentially grow new shoots and lead to an expedited spread of Japanese knotweed. 

How easily does Japanese knotweed spread?

Japanese knotweed has been labelled as an invasive plant by the government because of the ease with which it can spread both above and below the ground. The plant has proved to be resistant to many common forms of weed removal. 

Gardeners who have attempted to dig up, hack away or burn their Japanese knotweed infestations have found that any gains they have been made are short-lived, as new growth has appeared once more, even from the ashes of bonfires. Public waterways and railway lines have acted as conduits for the spread of Japanese knotweed across the UK, whilst abandoned construction or industrial sites are also common locations for the plant to thrive. 

How to stop Japanese knotweed from spreading

There are a few methods that have been proven to stop Japanese knotweed from spreading further, however many of these techniques require professional experience and equipment in order to be successful. Getting rid of Japanese knotweed usually involves a combination of methods sometimes including herbicidal spray, excavation, burning and burial using layers of material to prevent any rhizomes from successfully surfacing shoots. 

There are dozens of DIY methods that have been suggested to decrease the spread of Japanese knotweed, with many gardeners eager to try any cost-effective method of treatment. The only way for a property owner to guarantee that their Japanese knotweed infestation will be efficiently treated is by opting to use a PCA-accredited treatment firm that can offer an insurance backed certificate with their work. This will ensure that treatment of the land will continue, even in the case of the firm closing for business. 

Is it possible to stop Japanese knotweed spreading from a neighbour’s garden?

The best way to stop Japanese knotweed spreading from a neighbour’s garden is by opening up a line of communication with them, so that they are aware of the infestation and can move forward with treating it. A Japanese knotweed management plan can then be developed in order to tackle the problem. 

The legal implications of allowing Japanese knotweed to spread into a neighbouring property can lead to large compensation claims, with this being said there are still cases where neighbours are unwilling to treat their infestation, even if it has manifested on their land. In these cases, it is possible to install a root barrier defence along the border of the land you wish to protect, however despite the high cost of this preventative measure, they are not guaranteed to work.

Allowing Japanese knotweed to spread

It is against the law to allow Japanese knotweed to spread into neighbouring land, regardless of if it’s publicly or privately owned. The Wildlife and Countryside Act 1981 is just one of a handful of laws that set out the government’s stance on this invasive plant and demonstrates how seriously it deals with those who negligently cause it to further spread.

It is worth noting that businesses and organisations are also open to legal action, so there is recourse for property owners who have discovered Japanese knotweed entering from council land.

Related: Japanese Knotweed Law FAQs & Legal Advice

If you’ve discovered Japanese knotweed on your land and are unsure of how to proceed, we may be able to point you in the right direction. We can help find a PCA-accredited removal firm to get rid of the problem, and even pursue legal action to recover the costs of the treatment. Call us today on 0151 242 9050, or send us a message using the contact form.

Paolo Martini

Paolo Martini is the lead solicitor for Knotweed Help and has over 30 years of experience in the field of Civil Litigation and is an expert on the legal issues faced by individuals dealing with Japanese knotweed on their land. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.