Japanese knotweed is a hotly debated topic in Parliament, within the property industry and in the courts, however, a positive identification of the plant is required before any legal action is started. Although the plant has a few defining features (broad green, shield-shaped leaves and bamboo-like stems), Japanese knotweed takes on different forms throughout the seasons. Therefore, if you know what you’re looking for, it’s possible to discover an infestation at any point in the year.
This guide is intended to point property owners in the right direction when it comes to identifying this invasive plant, and takes into account how knotweed appears throughout the year, other plants that it’s often mistaken for and what risk factors are related to an infestation:
How can I tell if I have Japanese knotweed?
Japanese knotweed infestations can come in all shapes and sizes, so spotting an infestation is not always easy. In overgrown gardens, a patch of Japanese knotweed could easily be lost, especially during the winter when the plant has died back. There are, however, a handful of neighbourhood risk factors which the RICS have suggested are often linked with Japanese knotweed infestations.
- The property is close proximity to a water source, such as a canal, lake, culvert or pond.
- Nearby public or private footpaths, roads, railways, motorways, or other public land that may have been left to grow.
- Any large open public space, such as a car park, derelict homes, cleared or unused land.
- Properties close to large industrial buildings, workshops and storage depots are also considered to be risk factors.
Why is Japanese knotweed a problem?
Japanese knotweed is an invasive plant that has been recognised by the Environment Agency and government as ‘controlled waste’. Whilst the destructive potential of the plant 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.
Is there Japanese knotweed in my area?
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.
How do you identify knotweed?
Correctly identifying Japanese knotweed requires a keen eye and some key pieces of information. The plant does have some unique characteristics, however, it can also be confused with a number of other plants, which can make isolating an infestation difficult. By assessing each component of the suspected plant you can discover if you’ve found Japanese knotweed on your land or not.
What do Japanese knotweed leaves look like?
When the plant is in foliage, Japanese knotweed leaves are bright green and feature a signature shield or heart-shaped leaf. Each leaf has a pointed end and are arranged at staggered, even intervals along the stem. Leaves can be up to 14cm long, but don’t expect to see any during the winter, as the plants green foliage doesn’t last through the colder months.
What do Japanese knotweed roots look like?
Japanese knotweed roots are comprised of a network of rhizomes. These tough, woody roots can collect together in large crowns which can be difficult to remove from the ground without the help of machinery. Rhizomes are dark brown on the outside and orange on the inside, you can usually break them in your hands unless they have grown into a larger clump or ‘crown’.
How deep do Japanese knotweed roots go?
Rhizome root systems can burrow up to 3 metres deep into the ground with individual rhizomes growing up to 20cm thick, but this kind of growth only occurs if the plant is given the time and space to flourish. The veracity of this plant can make matters particularly difficult in disputes involving neighbours. A Japanese knotweed infestation can easily spread underground during the winter via its root system without even the most attentive of gardeners noticing. Only in the spring, when shoots begin to emerge, would the knotweed problem be visible above ground.
Does Japanese knotweed have red stems?
Japanese knotweed can be recognised by its red stems when it first emerges in spring, however, this colouration does not remain consistent throughout its lifespan. As the plant matures, the stem takes on a less distinctive green colour with mauve spots. It should be noted that these stems are cane-like in their appearance, growing in clearly defined segments with a hollow centre, similar to that of bamboo.
What do the flowers of Japanese knotweed look like?
Japanese knotweed flowers are often described as ‘creamy white’ and appear towards the end of summer, from late August to September. Growing in clusters up to 10cm long, they appear alongside the bright green leaves, combining to create a large vegetative mass. Related species of Japanese knotweed can be adorned with similar clusters of flowers in shades of pink and red.
Can Japanese knotweed just appear?
Japanese knotweed doesn’t appear from nowhere. Like any other plant, its origins should always be able to be traced back to an original place. Discovering the source of a Japanese knotweed infestation is almost as important as making the initial positive identification. In order to determine where the plant has come from and when it first entered your land, you may need to consider whether the plant can be found anywhere in your local vicinity (in a neighbour’s garden or on adjacent publicly owned land). If there’s no sign of any knotweed near to your land, then it’s possible that the plant may have originated from a batch of contaminated soil that was dumped on the land, or accidentally transferred from footwear or a vehicle.
What time of year does Japanese knotweed grow?
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.
What does Japanese knotweed look like in spring?
The plant can be difficult to spot until major growth above the surface begins to take place. Early spring is usually when the first Japanese knotweed shoots begin to appear, usually at the base of old canes (in the case of patches that are more than a season old). Knotweed shoots can be spotted by their bright red or pink tips. These pinkish redbuds grow to be around 1-3cm wide but do not keep this distinctive colour for long.
Once spring is well and truly underway, shoots take on a greener hue and become easier to spot due to their accelerated growth. Now growing as spear-shaped shoots, sometimes described as ‘asparagus-like, the leaves are red and rolled up, but they soon turn green as the plant grows skyward.
What does Japanese knotweed look like in summer?
Japanese knotweed is typically easier to spot during the summer as this is when the plant is most visible above ground. Growth accelerates significantly during summer making the plant much more conspicuous. The hollow stems grow up to 2cm a day and the leaves now assume their most recognisable shield shape with an alternating stem pattern. This presents a good opportunity to treat the problem with a PCA-accredited glyphosate programme, professionals are able to assess an infestation and inject each plant with their specialist equipment.
By mid-summer knotweed plants are close to reaching their full height (between 1.9-3m). The stems are now less green and are rigid with more pronounced dark purple speckles. Clear nodes are visible on these stems giving them the appearance of bamboo, they can also be snapped easily in the hand. It’s not until late summer that Japanese knotweed flowers; multiple bunches of creamy white flowers appear amidst the leaves, but the seeds that are produced from this process rarely lead to new growths.
What does Japanese knotweed look like in autumn?
As Autumn progresses, the characteristically bright yellow leaves turn a vivid yellow and the plant sheds seed cases. If the weed is left to its own devices the leaves and flowers will eventually begin to fade and fall back. Leaves fall off the stems which start turning brown and then a pale straw colour as winter progresses, also taking on a more rigid, woody form.
Does Japanese knotweed die back in winter?
Knotweed appears to ‘die back’ during winter, but it’s unwise to assume that the problem is simply gone. In the middle of winter, all that remains above ground is a collection of pale, dry canes with the rhizomes lying dormant beneath the surface waiting for warm weather to sprout and spread further. Whilst the plant enters this dormant phase it is still very much alive.
What plants look like Japanese knotweed?
There are a variety of sub-species and cross-breeds that can easily be confused with this plant. The following plants share a number of features with Japanese knotweed but have a few crucial differences which may aid in confirming, or ruling out a positive identification of the plant.
Russian Vine (Fallopia baldschuanica)
Commonly known by gardeners as ‘mile-a-minute’, Russian vine is a fast-growing relative to Japanese knotweed which also shares some physical traits which often lead to its misidentification. Whilst its flowering structure and masses of green leaves are arguably a red herring, the key difference here is that the vine is a climbing plant which relies on other structures, be it plants or buildings, to grow upwards, as opposed to Japanese knotweed which supports itself.
Hedge Bindweed (Calystegia sepium)
Another frequent case of mistaken identity occurs with this climbing plant. Bindweed’s leaves share Japanese knotweed’s heart-shaped leaves which can lead to false alarms amongst homeowners. Another key distinguisher is its large pink or white trumpet flowers which start growing in early summer, these are unmistakably different to knotweed’s small, creamy white flowers.
Giant knotweed (Fallopia sachalinensis)
Giant knotweed looks like Japanese knotweed because it is simply a larger variant of the plant. Giant specimens can grow up to 4 metres tall and can be distinguished by the green hue to the usually creamy white flowers. Unfortunately, Giant knotweed shares many of the same undesirable features with its close relative, to the point where it can almost be considered to be the same plant.
Himalayan knotweed (Persicaria wallichii)
Himalayan knotweed grows taller than the average Japanese knotweed plant at up to 3 metres, however, its stems are hollow which often leads to the two being mistaken for one another. Also introduced in the late 19th century, Himalayan knotweed is also very fast-growing and can also be found growing near watercourses. However, this variety can be easily identified from two features. Leaves are arranged opposite one another along the steam (as opposed to alternating), they’re also narrower in shape and feature a faint pink rib. The plant disseminates via exploding seed pod, which develop after its large pink flowers (which also sets it apart from Japanese knotweed).
Lesser knotweed (Persicaria campanulata)
This common ornamental species is a close relative to the aforementioned Himalayan variant and also shares a hollow stem structure. Its leaves are alternately arranged, like Japanese knotweed, but that is where the similarities end. The shape of the leaves are much longer than Japanese knotweed and feature more pronounced veins. True to its name, Lesser knotweed is also shorter in stature growing to 1.5 metres at most. Its flowers are also pink and bell-shaped.
Is Japanese knotweed a shrub?
Japanese knotweed is technically a herbaceous perennial plant – not a shrub. The plant is often mistaken for a shrub due to the large amounts of foliage it produces. Common shrubs such as lilac, poplar and dogwood also feature similar-shaped leaves, which can lead to misidentifications. A defining trait that separates Japanese knotweed from shrubs is its hollow bamboo-like stems. Most shrubs features woody stems that can be stripped away, whereas Japanese knotweed stems will snap, rather than split, if bent.
Is Japanese knotweed bamboo?
Bamboos is one of the fastest-growing plants in the world, so it’s unsurprising that it’s often confused with the similarly speedy Japanese knotweed, especially when you consider the likeness between the way the plants grow. Like Japanese knotweed, bamboo grows in nodes from difficult to eradicate rhizomes. At closer inspection there are some key differences: although shoots look similar, Japanese knotweed will snap when bent, whereas bamboo will not yield easily. The leaves are also very different, growing up to 50cm long, as opposed to Japanese knotweed’s smaller heart-shaped leaves.
How do I know if I have Japanese knotweed?
Getting a positive identification of Japanese knotweed can be difficult if you’re unaware of the seasonal changes the plant goes through, or the numerous copycats that it can be mistaken for.
If you have any plant matter on your land that resembles these descriptions or images then it’s worth taking photos and sending them to us using the form on the right. We could help you recover the costs of treatment, especially if you’ve bought a property with Japanese knotweed or if it has entered your property from a neighbouring property.
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