Japanese Knotweed Guide

By Paolo Martini on 22nd February 2021 (updated: 31st March 2021)

Japanese knotweed (also known as Fallopia japonica or Polygonum cuspidatum) is a perennial herbaceous invasive plant that is a native species of Japan. Since being imported to botanic gardens in Britain during the Victorian era Japanese knotweed has spread throughout the UK and has been labelled as ‘controlled waste’ by the government. This plant grows aggressively during the summer and has the potential to grow through temporary structures (such as garden sheds or greenhouses) and can even rupture concrete.

What Does Japanese Knotweed Do?

Whilst the destructive potential of Japanese knotweed has been exaggerated by some journalists and removal companies, the plant can nevertheless prove to be a nuisance for homeowners, landlords and farmers alike. The plant dies back during the colder months of the year, but Japanese knotweed can grow at an alarming rate during the summer, leading to large patches of thick growth that can stifle other plants and quickly overrun an outdoor space.

Unfortunately, for those who find the plant on their land, getting rid of Japanese knotweed is not a simple process. Depending on how large the infestation is, the roots (rhizomes) of the Japanese knotweed can be submerged several metres underground. It might seem like a simple task to remove the foliage above ground but to properly eradicate the plant the entire rhizome system must be excavated. Due to its status as an invasive plant, there are also serious legal implications to having Japanese knotweed which can make resolving disputes with neighbours and treating the problem even more difficult.

Learn More: What Does Japanese Knotweed Do?

Is Japanese Knotweed Poisonous?

Japanese knotweed can prove to be an expensive, time-consuming problem to deal with, however, it is not dangerous to humans. Japanese knotweed is not poisonous, in fact, it’s edible and can be consumed easiest when it’s first shooting in spring.

Japanese knotweed has been discovered all over the UK and is often grouped along canals, motorways and nearby areas that have been heavily redeveloped. Since the plant’s arrival in the UK in the 19th century, Japanese knotweed has been steadily disseminated throughout the country via unwitting gardeners and careless construction firms. Thanks to a public appeal made by the Environment Agency, we are now able to pinpoint which areas in the country have been affected by the plant and the results are not for the faint-hearted. Look at our Japanese knotweed map to find out if it has a foothold in your area yet.

Learn More: Is Japanese Knotweed Poisonous?

How Does Japanese Knotweed Spread?

Despite it flowering throughout the warmer months, this is not how knotweed reproduces in the UK. This invasive plant is such a problem because of its deeply embedded root system which is constituted of ‘rhizomes’. An entire infestation can manifest itself from the smallest of rhizome fragments. Construction sites, canals and railway lines are common sources of large infestations, however, unknowing gardeners attempting to dig out the root can also exacerbate the problem.

Japanese knotweed does not grow throughout the year, like most plants it has a seasonal life cycle, developing through a number of phases in response to the changing environmental conditions. In order to correctly recognise Japanese knotweed, it’s useful to have a reference point to better understand these various stages of this plant’s life cycle.

Learn More: How Does Japanese Knotweed Spread?

Dormant Japanese Knotweed

Japanese knotweed is a rhizomatous plant, meaning that it has a modified stem system that grows underground (much like a tree’s roots), which can render it dormant for extended periods, making it practically invisible to the naked eye from above the system. Unlike a tree’s roots, however, each rhizome is capable of producing new nodes and sending shoots up to the surface to create a new plant.

Learn More: Dormant Japanese Knotweed

How To Identify Japanese Knotweed

Japanese knotweed enters several recognisable phases throughout the year, whilst treatment can take place at any time it’s typically easier to spot during the summer, as this is when the plant is most visible above ground. You’ll first spot small red-purple asparagus-like shoots growing in spring, these can grow up to 10cm a day during the summer and often grow in large patches.

The leaves are the most distinguishable feature throughout the year. These are broad and shield-shaped with a distinctive alternating stem pattern, unfortunately, there are other common plants that look like Japanese knotweeed. Both Lilac and Dogwood share similar shaped leaves. Look for small white creamy flowers and bamboo-like canes to be certain that you have a Japanese knotweed infestation, or hire a professional to survey your property.

Learn More: How To Identify Japanese Knotweed

Getting Rid Of Japanese Knotweed

There are many methods of removing Japanese knotweed from your land. Which one you choose will depend on the size of the infestation that you’re dealing with, how quickly you want to clear your land and if there’s any other plant life or animals that are living in the area.

Herbicide treatment is ideal for large areas that are heavily infested and do not contain any living things that are in need of protection. Stem injection of concentrated Glyphosate herbicide is a targeted solution that is more time consuming, but also more effective. Digging out the infestation with machinery is costly, but can often be the only solution for large construction sites.

The use of a root barrier is intended to halt the spread of knotweed, rather than remove it entirely, whereas screening methods can be used to sift the soil and only remove the contaminated soil. Finally, incineration is one of the most effective methods of destroying the plant, but you must ensure that you are burning the entire infestation in order to prevent the problem returning.

Learn More: Getting Rid Of Japanese Knotweed

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. 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.