How to Identify Japanese Knotweed

Japanese knotweed (Reynoutria japonica) is a perennial that has thick underground stems, known as rhizomes. It forms a dense network of these that grow out in all directions, taking up water and nutrients from the soil as they go. They can penetrate deep into the ground, up to 3m, and form dense canopies of growth that block out sunlight for native plants below. This means that native species are unable to compete for resources and can die out. It also smothers and erodes the surrounding environment and damages habitats such as riverbanks, hedgerows and even roadsides.

The appearance of knotweed changes depending on the season, and it can be difficult to recognise if you have never seen it before. In the spring it has red shoots that look like asparagus and can grow at a rate of 10cm a day. By late summer the plant can reach a height of 2-3 metres. The leaves are shield or shovel-shaped and have a zig-zag pattern to them, with creamy-white flowers appearing towards the end of summer.

A mature plant can appear to be dead in the winter months, with brown desiccated stems lining pathways. But it is important to know that these stems are still alive, just dormant. The stems break easily and can be pulled out of the ground, but this will not kill the plant, which has a very long life span and is capable of growing back from its rhizomes.

During the late summer and early autumn Japanese knotweed produces panicles of small cream-coloured flowers, clustered in dense groups on thin spikes. These are attractive to wildlife, including insects, birds and bees, and provide an essential source of late-season pollen.

Although it is not considered to be a threat to livestock, it can be dangerous to dogs if eaten and may cause stomach upsets. It is recommended that animals are kept away from areas of Japanese knotweed to minimise risk.

It is extremely difficult to eradicate Japanese knotweed once it has established itself, and this can be especially challenging for homeowners. However, there are a number of ways that people can help to identify Japanese knotweed and encourage others to report sightings.

The easiest way to identify it is by its colour and growth habit. It grows very quickly and its green canes look very similar to bamboo. Its flowers are the most distinctive feature and can be very easy to spot, especially when in bloom. It can also be identified by the orange tinge of its rhizomes, which are particularly obvious when they are cut or damaged. As a result of its invasive nature it is often concealed in gardens, for example under landscaping fabric or covered with soil, aggregates and bark mulch. It is very important to make the effort to try and identify Japanese knotweed as much as possible, so that it can be reported to the correct authorities and controlled. The best way to do this is to use the free PictureThis smartphone app, which is an online plant encyclopaedia and identifier that can spot suspected Japanese knotweed within seconds.