Identification

Identifying knotweed could save you money

Japanese knotweed is a highly invasive, non native plant. It is a vigorous, perennial that can cause damage to hard surfaces and engineered structures. As a consequence, it has far reaching economic implications for many industries.

Red buds begin to emerge from the base of the plant (the crown) during February/March. They develop into shoots, maintaining a reddish appearance with furled leaves. Maturing stems are hollow, somewhat like bamboo. They are green with characteristic red speckles. The canes grow rapidly through the spring and summer, reaching a height of 2-3 metres.

In spring, the newly expanded leaves are a yellowish green, becoming darker as they mature through summer. They alternate along the stem forming a zigzag pattern.

The plant flowers towards the end of the growing season (August/October), producing clusters of small, creamy white flowers at the points where the leaves join the stem.

Towards the end of autumn, leaves begin to turn a yellowy brown and eventually drop, revealing feathered branches from the remains of the flowers. Hollow canes shed the darker outer skin exposing a lighter, smooth cane that is quite brittle.

In addition to the common Japanese knotweed (Fallopia japonica) there is a smaller compact variety Fallopia japonica var. compacta, reaching a height of 1 metre and a Giant knotweed (Fallopia sachalinensis), growing up to 5 metres. A hybrid between Japanese knotweed and Giant knotweed, Fallopia x bohemica is also found in the UK.

There are slight differences in leaf shape and size that distinguish between the species but generally leaves are shovel shaped. The base of the leaf, joining the stalk, is straight on a Japanese knotweed leaf whereas on a Giant knotweed leaf it is lobed, forming a heart shape. The hybrid is in between with a slightly lobed base. Other introduced members of the Polygonaceae family are often mistaken for Japanese knotweed. The most common being Himalayan knotweed (Persicaria wallichii) with elongated leaves.

In terms of best practice, there are no differences in the management of all the above species. Treat all species in the same way.

Spread

Japanese knotweed develops an extensive network of underground shoots called rhizomes. Rhizomes tend to grow laterally, spreading out from the crown of the plant. Research indicates that lateral extension from the parent plant can reach 7 metres and that it can grow to a depth of 3 metres. However, this can vary considerably and will depend on the nature and history of the site and age of the stand.

Appearance

The outer surface of rhizome is dark brown in appearance. It snaps easily, like a carrot, if fresh, to reveal a bright orange inside with a different shade core. Older rhizome can be very woody and may have a hollow centre with an orange to cream outer ring. Reddish, pink buds and white, hair like, roots are found along the rhizome during the growing season. New rhizome growth is white and very brittle.

The appearance of rhizome does not differ between species except that Giant knotweed is generally larger.

Rhizome fragments smaller than a fingernail can regenerate into a new plant. Always check imported materials for knotweed rhizome

Rhizome can remain dormant for many years but can be stimulated to grow by disturbance.

Lack of regrowth does not indicate eradication. The rhizome may be dormant as a result of treatment.

Sometimes if canes have blown down or have been cut, it can be difficult to spot knotweed during the winter months. However, if you look carefully there are usually canes and a deep layer of plant litter on the ground from previous years’ growth, due to the plant’s slow rate of decomposition. If a site has been scraped it may be necessary to look for rhizome material to determine if knotweed is present.

Cornwall Council operates a mapping database for knotweed in Cornwall. By recording your knotweed sightings you can help inform future control strategies.

The Environment Agency’s Knotweed Code of Practice provides guidance for managing Japanese knotweed and includes information on knotweed identification and waste disposal and is available on the Environment Agency website