Japanese Knotweed Identification Guide

Japanese Knotweed FAQs

Your Guide to Recognising Japanese Knotweed in Every Season

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 differently, depending on what season we are in.

Here is our guide to help you recognise Japanese Knotweed in every season.

Nimrod treating a site, eradicating Japanese Knotweed. Sign says Warning, Do Not Enter, Area Contaminated with Japanese Knotweed


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.

Free Identification Service

Fast, Free Identification Service for Invasive Weeds

Unsure if you’ve spotted Japanese Knotweed or another invasive weed? Let our experts help with a quick and easy identification service. Follow these simple steps:

  1. Snap a Clear Picture Take a clear picture of the weed you’re concerned about.
  2. Send it Over Fill in the form provided and upload your picture. Feel free to include any additional information about your situation in the enquiry field.
  3. Receive Expert Advice Our team will promptly review the image and get in touch with you. We’ll confirm if it’s Japanese Knotweed or another type of invasive weed, and explain your options and the best solution.

Benefit from a fast response, precise identification, and practical advice – all without obligation. Take the guesswork out of weed identification with Nimrod Environmental.