Identifying Japanese Knotweed
Japanese knotweed is usually easy to identify. Stems: 2-3m tall, green with red/purple speckles, hollow, and typically form dense and often extensive stands (although small clumps of only a few short stems do occur). The arching branches have a characteristic zig-zag pattern.
Leaves: shield-shaped, green, up to 12cm long, with a flat base and pointed tip. They are hairless, only slightly longer than wide, and are arranged alternately along the branches.
Flowers: abundant clusters of tiny creamy flowers are produced along the outer parts of most branches from August to October.
Seeds: since fertile male flowers are very rare on British plants, seed is produced only rarely from hybridization with other exotic knotweeds, and rarely survives (although climate change could change this).
As the aerial parts of the plant die in late autumn, the leaves turn brown (along with the rest of the plant) and can remain attached for many weeks. Even when the leaves have dropped, the zig-zag branches and tall bamboo-like stems are easy to identify. The dead canes may remain standing for several years. The dead parts of the plant decompose slowly, leaving a deep litter under large stands. New red/purple shoots with furled leaves appear in March, and grow up to 40mm per day. They are produced from tough spreading rhizomes (underground stems), which are thick and woody, and snap like a carrot when fresh revealing a yellow/orange centre. They can reach out 7m or more from the parent plant, and up to 3m deep.
