About Japanese Knotweed

So what does Japanese knotweed look like? Like any plant Japanese Knotweed looks different depending on the time of year. Typically the growing season is from April to October, however mild winters and damp summers have extended the growing season from as early as mid-March to as late as November. This means that there is more time for the Japanese knotweed to take root and compromise your property or site.

The most defining characteristics of Japanese knotweed identification are its spade-shaped (as in the playing card shape)leaves and bamboo-like stems.

Japanese knotweed in spring


In early spring, Japanese knotweed shoots can be compared to asparagus spears with reddish/purple speckling and can reach up to 10 feet (3 metres) in height. These spears, which grow in dense clumps known as “stands” then develop into green bamboo canes with some purple speckling. As they grow the leaves on the hollow canes begin to unroll to reveal red lined, lime green spade-shaped leaves that will garden as the season progresses.

Japanese knotweed in summer


Established Japanese knotweed is easy to identify in the summer; it should be more than 1.6 metres high, with cane-like stems, which will often present in clumps. The plant will resemble Dogwood or Lilac at first glance but on closer inspection you should see the typical lime to mid-green spade shaped leaves present on branches which normally exhibit a zigzag shape between each leaf node.

New infestation or infestations that have been disturbed are generally smaller in height and consist of anything from a single stem to lots of shoots peppered across a garden or open space. Young leaves have a reddish tinge as they unfurl and is often evident in lawns where regular cutting stops knotweed developing.

Japanese knotweed in autumn


Like many seasonal plants, autumn is the time that Japanese Knotweed slows down. It will have done the majority of its growing in the summer and will have reached between 2 and 3 metres in height. The leaves will start to turn yellow, wilt and fall, exposing the bamboo-like ‘stand’ of stems, which also start to change to a darker colour of dark brown/red.

Japanese knotweed is very efficient in being able to store energy and in late summer it will take in a lot of energy through photosynthesis and store it in its rhizome (root) system in readiness for spring.

Japanese knotweed in winter


Winter is the time that Japanese knotweed becomes dormant. All of the leaves will have fallen and created a dense litter on the floor. The hollow canes turn dark brown/red and remain standing, but they also start twisting and collapsing on top of one another. It can take anything up to 3 years for the dead canes to fully decompose. Don’t be fooled with this plant’s appearance in winter. Although it may look dead, it is just waiting for the weather to warm; underneath the leaf litter are next year’s shoots just waiting to emerge.

Contact PBA Knotweed Solutions


Japanese Knotweed is a dangerous weed if left untreated. Not only can it invade your garden, it can invade your home and cause dangerous structural damage.  For more information on how you can control and identify Japanese knotweed, call a member of the PBA Knotweed Solutions team on 0203 174 2187 or email us at [email protected].