How to identify Japanese Knotweed

In the early spring red/purple shoots appear from the ground and grow rapidly forming canes.  As the canes grow the leaves gradually unfurl and turn green.

The plants are fully grown by early summer and mature canes are hollow with a distinctive purple speckle and form dense stands up to 3 metres high.  The plant flowers in late summer and these consist of clusters of spiky stems covered in tiny creamy-white flowers.  These provide a good source of nectar for insects.  The seeds are rarely fertile and in Britain the plant spreads mainly by vegetative means.

The canes can arise from the rhizome which grows underground, from an existing crown, where previous growth has taken place, or from a cut stem.  During the late autumn/winter the leaves fall and the canes die and turn brown.  The canes remain standing throughout the winter and can often still be seen in new stands in the following spring and summer.

The rhizome is the underground part of the plant.  It is knotty with a leathery dark brown bark and when fresh snaps like a carrot.  Under the bark it is orange or yellow.  Inside the rhizome is a dark orange/brown central core or sometimes it is hollow with an orange, yellow or creamy outer ring, although this is variable.  Young rhizome is very soft and white.  The ‘knots’ are nodes, spaced at 1-2cm intervals where there are often small white fibrous roots or buds emerging.  Each of these ‘knots’ can potentially become a new plant if the rhizome is cut up (e.g. through digging).


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Hybrids and related species