According to recent estimates, Japanese Knotweed has devalued the UK housing market by £11.8 billion as the invasive plant spreads across the country, making homes more difficult to sell.
Although Japanese Knotweed is a much-contested topic in the property business, and the courts, identification of the plant is essential before any legal action is initiated. Despite a few distinctive features (large green, shield-shaped leaves and bamboo-like stalks), Japanese Knotweed changes form throughout the seasons. As a result, if you know what you’re looking for, you can find an infestation at any time of year.
This guide is meant to lead property owners in the right direction when it comes to detecting this invasive plant. It considers how Japanese Knotweed appears throughout the year, other plants that it is sometimes mistaken for, and risk factors associated with an infestation.
How to identify Japanese Knotweed
Japanese Knotweed Shoots
Japanese Knotweed shoots resemble canes and develop in distinct segments with a hollow middle. When recognising early Japanese Knotweed shoots, check for the following characteristics:
- – They can grow to be up to 2-3 metres tall.
- – The inside of the stem is hollow.
- – In the spring, it is crimson.
- – In the summer, it is green.
- – Similar to bamboo stalks in appearance.
Japanese Knotweed Leaves
Japanese Knotweed leaves are bright green while in foliage and have a distinctive shield or heart-shaped leaf. Each leaf has a pointy tip and is spaced at regular intervals throughout the stalk. Leaves can grow to be up to 20 cm long but don’t expect to see any throughout the winter because the plant’s green foliage doesn’t stay that long.
Knotweed leaves are meant to give energy to the plant’s rhizome system and promote future growth. The following are some of the primary characteristics of Knotweed leaves to look for:
- – Heart-shaped with a pointy tip
- – Growth pattern zig-zag
- – Lush Green Colour
- – Up to 20 cm in length
Japanese Knotweed Flowers
Japanese Knotweed is distinguished by its creamy-white flowers (panicles), which bloom in bunches from late August to mid-September. Japanese Knotweed blossoms can grow up to 10 cm long and 0.5cm broad, providing a thick look that often obscures the characteristic stems. A blossoming Japanese Knotweed plant indicates that it is well entrenched and may be difficult to remove. Identifying and treating the plant before it blooms in late summer can help prevent a longer-term infestation.
Japanese Knotweed Height
During the early summer, Japanese Knotweed can be recognised by its hollow stems with purple speckles that can grow up to 3 metres tall. The less common Giant Knotweed can reach a height of 5 metres, while the hybrid Fallopia x bohemica can reach a height of 4 metres. Dwarf Knotweed rarely grows taller than one metre.
Plants Similar To Japanese Knotweed
The most common species of Japanese Knotweed found in the UK is Reynoutria Japonica/Fallopia Japonica, but it has been reported to hybridise with related species. Giant Knotweed (Fallopia sachalinensis/Reynoutria sachalinensis) is distributed throughout the United Kingdom but is less prevalent than Japanese Knotweed. The dwarf form of Fallopia japonica var. ‘Compacta’ is substantially lower in stature.
Some commonly mistaken plants that may have similar features are:
- – Russian Vine
- – Leycesteria Formosa (Commonly known as “Pheasant Berry/Himalayan Honeysuckle)
- – Broad-leaved dock
- – Lilac
- – Bindweed
- – Bamboo
- – Houttuynia
Identifying Japanese Knotweed throughout the year
Identifying Japanese Knotweed in winter and autumn is more difficult since it loses its distinctive heart-shaped leaves and delicate white blossoms. During the winter and autumn months, Japanese Knotweed becomes nothing more than a cluster of brittle stems above ground, yet the root system continues to extend below ground.
When the growing season begins, which is usually in March or April, Japanese Knotweed can grow up to 10 cm every day. Through its branches, it will produce heart-shaped, medium-sized leaves that are a lighter green shade. From late August to October, delicate branches of little white blooms, stacked like foxgloves, appear.
Japanese Knotweed in Spring
Japanese Knotweed grows the fastest in the spring and begins to sprout new shoots, making it much easier to identify. There are various characteristics to look for when identifying Japanese Knotweed in the spring:
- – New shoots appear as red/purple asparagus spears.
- – The leaves are folded up and dark green or red.
- – Canes can grow up to 3 metres tall. Shoots have beautiful red/pink tips and can grow up to 1-3 cm broad.
Japanese Knotweed in Summer
During the summer, Japanese Knotweed grows at an exponential rate, with some plants reaching heights of 2-3 metres at their peak. At this time of year, it can also create dense clumps of foliage, making it easier to spot throughout the mid-Summer months.
Japanese Knotweed in Autumn
The plant drops seed cases as autumn passes, and the traditionally bright green leaves turn a striking yellow. If the weed is left to its own devices, the leaves and blooms will wilt and fall off. Leaves fall off the branches, which become brown, then pale straw as winter progresses, and take on a more stiff, woody structure.
Japanese Knotweed in Winter
Knotweed appears to ‘die back’ throughout the winter, but it is dangerous to think that the problem has simply disappeared. All that remains above ground in the middle of winter is a collection of pale, dry canes, with the Japanese Knotweed latent beneath the surface, waiting for warm weather to sprout and spread further. The plant is still alive and well as it enters this slumber phase.
Having trouble identifying a potential Japanese Knotweed infestation?
We offer a free Japanese Knotweed identification service at TP Knotweed Solutions. Send us photographs of your suspected infestation, and one of our Japanese Knotweed experts will respond within 24 hours to assist you in identifying your infestation.