Japanese knotweed (Fallopia japonica) is a highly invasive weed that grows tall and spreads quickly. It inhibits the growth of other plants and is required to be controlled by law. To help prospective home buyers, sellers, and landowners, we’ve put together this guide to help you determine whether or not you have Japanese knotweed.
Why is it hard to tell if you have Japanese knotweed?
Japanese knotweed looks a lot like a number of other plants, making it somewhat difficult to recognise if you don’t know exactly what you’re looking for.
Most commonly, Japanese knotweed is confused with:
Himalayan Honeysuckle (Leycesteria formosa)
Russian Vine (Fallopia baldschuanica)
Bindweed (Convolvulus arvensis)
Himalayan Balsa (Impatiens glandulifera)
Broad-leaved Dock (Rumex obtusifolius)
Himalayan Knotweed (Persicaria wallichii)
Each of these have certain characteristics that are similar to those of Japanese knotweed, from broad, heart-shaped leaves to hollow bamboo-like stems. See our page on plants that look like knotweed to learn more about these similar plants.
It is because there are so many similar-looking plants that many people struggle to recognise if they have Japanese knotweed. That, and because it can be found in hard to reach, hidden places such as within deep, dense shrubbery, means it can go unchecked for a significant time. Japanese knotweed also dies right back in winter, so it can be completely undetected for months.
What does Japanese knotweed look like?
The main features of Japanese knotweed’s appearance include:
The shoots look similar to bamboo or asparagus, with red flecks
When in full bloom, the average height of the plant is two to three meters
Small white/cream flowers that grow along the length of the stems
Flowers are in bloom from August to October
DOWNLOAD OUR GUIDE
Japanese Knotweed Characteristics Throughout the Seasons
Our handy guide can be printed out and used in your garden, or when on the road.
There are several other kinds of Knotweed known to exist in the UK, though none are quite so widespread in the UK as Japanese Knotweed. The most common are:
Giant Knotweed (Reynoutria sachalinensis)
Also originating from Japan, Giant Knotweed has been less adaptable to UK and European climes than Japanese Knotweed and does not thrive in shady conditions.
Dwarf Japanese Knotweed (Reynoutria japonica var. ‘Compacta’)
The dwarf version of Japanese Knotweed is not too prevalent in the UK but can be found more commonly across Europe. It presents problems when it comes into contact with other knotweeds to form more hybrids.
Himalayan Knotweed (Persicaria wallichii)
Although this variant is one of the least common in the UK, it is known to be spreading in the southwest. It’s particularly prosperous in wet conditions, and quickly thrives after flooding.
Bohemica Hybrid Knotweed (Reynoutria japonica x bohemica)
A hybrid of Japanese and Giant knotweed, Bohemica is also known to be spreading rapidly in the UK and is very resistant to herbicides.
How to recognise Japanese Knotweed
Recognising Japanese knotweed can take some investigation, not only because the plant looks so similar to other species, but also because it can grow in hard to reach, hidden areas.
It’s a perennial plant which means it will survive year after year but takes on different characteristics throughout the seasons. In the winter it completely dies back, dormant above ground but with rhizomes below the earth that are very much active and potentially spreading. For further details on what this invasive species looks like throughout the seasons, you can learn about how to identify Japanese Knotweed when you know the signs.
In the winter, one effective way to tell if you have knotweed is by using highly trained sniffer dogs. It’s possible to undertake dog detection surveys in which the dogs will search in all the hard-to-reach areas of an outside space, to sniff out the presence of Japanese knotweed rhizomes. Find out more about Japanese knotweed dog detection services.
Why it’s important to recognise Japanese Knotweed
The Environment Agency have declared that Japanese knotweed is ‘the UK’s most aggressive and invasive plant”. It’s extremely quick to grow (up to 10cm per day in the summer) and stops other plants from surviving.
In addition to its negative effects on other flora, Japanese knotweed is also notorious for the problems it can cause to walls, drainage and building foundations. The rhizomes (underground roots) of the plant penetrate deep into the earth and spread at a rapid pace. In fact, they can grow three metres downwards, and seven metres across. This means they will cause damage to a variety of surfaces, both below and above the ground.
The law now requires that Japanese knotweed is controlled in the UK. While it’s not illegal to have it growing on your property, measures must be taken to ensure that Japanese knotweed does not negatively impact the neighbours/neighbourhood. Because of its invasive nature, this can be difficult to achieve without expert assistance.
Another consideration regarding the importance of recognising Japanese knotweed is the effect it can have when buying or selling a property. Today, if Japanese knotweed is discovered in a garden, a mortgage lender might refuse to loan funds before it has been eradicated. Whether you’re a buyer or seller, the presence of Japanese knotweed within a property should be determined.
Japanese knotweed is thought to exist in every 10sq km of the UK. This reveals just how prevalent this invasive plant is, regardless of where in the country you live. Anyone who thinks that Japanese knotweed might be present on their property should call in the experts to set about controlling it.
The team here at Japanese Knotweed Specialists have all the experience and know-how needed not only to recognise the plant, but also to eradicate it. Get in touch for further advice and information. If you’re unsure if you have Japanese knotweed growing in your garden, feel free to send in a photo of the plant in question, and we will get back to you with a no obligation answer.