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Japanese Knotweed Identification Guide

In its native region Japanese Knotweed is controlled by a range of natural pests and diseases including a weevil and rust fungus. Here in the UK, without these restrictions, it is thriving and its vigorous growth is a problem to our wildlife and capable of damaging buildings and hard surfaces. Once it is established in the built environment it can be particularly difficult to control.

One of the things to note is that Japanese Knotweed looks and acts very different, depending on what season we are in. Here is our guide to help you recognise Japanese Knotweed in every season.

In spring, April to May, the weed grows extremely fast and it can look very different from one month to the next. Growing from small shoots (100-200mm) to anything up to 3 metres in height, the new shoots will be red/ purple in colour and look like asparagus spears. During its growth, leaves will sprout along the cane and start to unroll. In late spring when the canes are tall, they are hollow like a bamboo with purple speckles.

In summer, June to August, the weed will have reached its final height of approximately 2-3 metres and will stay at this height for the duration of the season. The plant will be a dense cluster of bamboo like stems with green leaves that have a distinct heart shape with a pointed tip. In late summer, August into autumn small white flowers will bloom.

Moving into autumn, September to November, Japanese Knotweed will look similar to that in late summer, bamboo like tall stems, dense green foliage and small white flower blooms.  However the leaves will begin to turn yellow and wilt.  The bamboo like stems will also turn darker brown.  During late autumn the canes will begin to die off and the plant becomes dormant.

During winter, December to February, the canes will begin to die off and lose their leaves as the plant becomes dormant.  The canes may remain standing or may fall over and can take up to 3 years to decompose.  Quite often, you will see canes from previous years at a different stage of decomposition and evidence of new shoots underneath the recent growth fall.

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