A beginner’s guide to identifying the UK’s most invasive plant
Awareness of invasive plants is growing in the UK but what does Japanese knotweed look like, recognised as the worst of them all?
In Environet’s annual survey with YouGov, 82 per cent of adults in the UK are aware of knotweed, up from 77 per cent the previous year. However, identifying the plant is another matter altogether.
Japanese knotweed ( Reynoutria japonica) is a non-native, invasive plant introduced in Victorian times. Popular for its beauty and durability, it was planted in gardens and has since spread widely across the UK. It is thought to be present in every 10 sq km of the country.
It is such a pest because it spreads through an extensive and powerful underground root, or rhizome, system. Japanese knotweed causes damage to homes and gardens by pushing up through paths, tarmac, concrete, drains and even cavity walls. Read about a legal ruling concerning Japanese knotweed here.
Knotweed can be tricky to spot because its appearance changes dramatically depending on the season. To identify it successfully you need to know what to look out for at different times of the year.
What does Japanese knotweed look like in spring?
As soon as the soil temperature reaches around 4°C, knotweed emerges. Thick, red or purple asparagus-like stems pierce the ground and grow rapidly. Bright lime green leaves appear as they get taller before forming hard canes similar to bamboo. By May, look out for heart, or shovel-shaped leaves which emerge alternately from purple-speckled stems in a zig-zag pattern.
…in summer
During June, July and August, Japanese knotweed is in full growth, with mature plants reaching up to 3 metres tall. Canes are hard and connected to distinct crowns. 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.
What does Japanese knotweed look like in autumn?
A perennial, knotweed begins to die back 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
During winter, knotweed becomes more difficult to spot. All above-ground growth disappears, leaving just the dead brown canes with the crowns visible in the ground. The underground rhizome system is very much alive, waiting to emerge when spring arrives.
Check out Environet’s Identification Guide to name a plant in your garden. If in doubt, email a photo to [email protected] for a free ID service. However, around half the photos received every week turn out not to be Japanese knotweed, which is commonly mistaken for ivy, bindweed, peony, lilac, houttuynia and Russian vine.
For more information, visit www.environetuk.com. The company also deals with invasive bamboo – read about my clumping v running bamboo here.
