With Japanese Knotweed identification, the different seasons bring out different characteristics of the plant.
The growing season for Japanese knotweed is usally from April to October but with mild winters and warm damp summers experts have seen the growing season extended. Early signs of growth are seen in mid march and new shoots have even been identified as late as november. Therefore, Japanese knotweed treatment and removal programmes have to be reviewed annually to ensure the methods used are effective.
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.