Feature: What does Japanese knotweed look like?
A beginner’s guide to identifying the UK’s most invasive and problematic plant
Awareness of Japanese knotweed is growing in the UK. Environet’s annual survey with YouGov shows that 82% of adults in the UK are aware of knotweed in 2022, up from 77% the previous year, but correctly identifying the plant is another matter altogether.
Japanese knotweed (Reynoutria Japonica) is a non-native, invasive plant that was imported to the UK in Victorian times. Popular for its beauty and durability, it was planted in nurseries and gardens across the country and has since spread widely across the UK, largely through the movement of soil and plant material. It is now thought to be present in every 10 sq km of our green and pleasant land. A pest species that spreads through its extensive and powerful underground root, or rhizome, system, knotweed causes damage to homes and gardens by pushing up through paths, tarmac driveways, cracks in concrete, drains and even cavity walls.
So, what does it look like and how can you tell if it’s growing on or near your property?
Knotweed can be tricky to spot because its appearance changes dramatically depending on the season, so to identify it successfully you need to know what to look out for at different times of year.
How to identify knotweed in spring…
As soon as the soil temperature begins to warm in early spring, reaching around 4°c, knotweed emerges from hibernation. Thick, red or purple asparagus-like stems pierce the ground and grow rapidly, sprouting bright lime green leaves as they get taller before forming hard canes similar to bamboo. By May, the plant is covered in heart, or shovel-shaped leaves which emerge alternately from purple-speckled stems in a zig-zag pattern.
… in summer…
During the summer months of June, July and August Japanese knotweed is in full growth, with mature plants reaching up to 3 metres in height. Canes are hard, connected to distinct crowns in the ground, and leaves become richer and darker green in colour. In late July or early August, dense clusters of creamy white flowers appear, hanging from the upper leaf axials.
… in autumn…
A perennial plant, knotweed begins to die back in autumn as the temperature cools and the light fades. During October and November the leaves turn yellow, then brown, and begin to fall, while the stems turn brown and brittle.
…and in winter
In the winter months, knotweed becomes more difficult to spot. All above-ground growth disappears, leaving just the dead brown canes standing with the distinctive crowns still visible in the ground. Yet the underground rhizome system is very much alive, replenished by the summer growth season and waiting to emerge when spring arrives.
Check out Environet’s Identification Guide if you’re worried about a plant in your garden and if in doubt, just email a photo to [email protected] for a completely free ID service. But rest assured, around half the photos received every week turn out not to be knotweed, which is commonly mistaken for ivy, bindweed, peony, lilac, houttuynia and Russian vine.