Japanese Knotweed Identification
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Japanese knotweed (Fallopia japonica), is an invasive herbaceous perennial (a plant that can live more than one year). Since it was introduced as an ornamental plant in the 19th Century from Japan, it has spread across Ireland, particularly along watercourses, transport routes and waste grounds where its movement is unrestricted. Japanese knotweed can:
Seriously damage houses, buildings, hard surfaces and infrastructure growing through cracked concrete, tarmac and other hard surfaces in some cases.
Threaten native plants and animals by forming dense thickets.
Block routes used by wildlife to disperse.
Riverside Japanese knotweed damages flood defence structures and reduces the capacity of channels to carry flood water.
Hinder walkers and the public who visit rivers and lakes to fish.
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 Ireland 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).
You can download an identification sheet from the following link: Japanese Knotweed (Fallopia japonica)