Identification through the seasons

The information below gives a brief explanation of how the appearance of Japanese Knotweed changes throughout the year – it can be most difficult to identify and therefore easily missed during the winter months.

Knotweed in Spring

During March to April, small red/purple shoots (similar to asparagus) emerge from the ground.  As time passes, the leaves begin to open, and they can be red or green with red veins.

Knotweed in Summer

This is when the plant grows most quickly – it can easily achieve 10cm per day in the right conditions.  The bamboo like green and purple speckled hollow stems will grow in tightly formed stands.  The shield shaped leaf with its flat base grows alternatively from zig-zag stems.

Knotweed late Summer

During September to October, the plant begins to flower.  Small creamy white flowers, growing on 100mm long stalks, hang down in clusters.  Even though the plant does produce seeds there are only female plants here so the plant does not spread through the seeds.   You will see the bees enjoying the plant which takes them away from pollinating the native plants that need them.

Knotweed in Winter

After the plant has flowered, the leaves begin to turn yellow, and the plant begins to die back for the winter don’t be fooled though it will almost certainly still be very much alive below the ground.  The leaves fall off, and the stems turn brown and become dry and brittle.  These dead stems can take up to 3 years to break down.

What is Japanese Knotweed?

Japanese Knotweed (Fallopia Japonica) is one of the most invasive Non-native species found in the UK.  It is a large clump forming plant that can grow to almost 3m in height.

As the name suggests, it is native to Japan, and was introduced here in the mid ninteenth century as an ornamental garden plant. Just before the turn of the century, many gardeners’ perceptions changed, and it quickly picked up a reputation as a harmful species, starting to ‘escape’ and spread in the wild.

How can I identify Japanese Knotweed?

Japanese Knotweed is a perennial plant that produces tall canes during the summer growing season.  The canes are easily identifiable, whilst they can look a lot like bamboo they have distinctive purple flecks along the green stem.

The branches and leaves grow in an alternate zig-zag pattern from nodes along the canes. The canes die back during the winter, and turn into a brittle hollow dry winter cane.  It can take up to 3 years for the winter canes to break down naturally.

Why is Japanese Knotweed a problem?

It is an incredibly fast growing and strong plant
It can grow 10cm per day and get to its full height of almost 3m in as little as 10 weeks. The speed of its growth, and dense leaf canopy area, makes it difficult for other plants to grow beneath the Knotweed, and very quickly you will find that there is only bare earth beneath the Knotweed plant.

This bare earth is particularly a problem on watercourses where, during floods, or high tides, the river bank can be eroded away, taking fragments of the plant and its roots and shoots with it, resulting in a the risk of spreading of the Knotweed further downstream.

Even though the plant appears to die back in winter the plant continues to grow underground
In winter the plant continues to grow underground from a continuously growing stem (a rhizome) that sends our roots and shoots from its nodes. These form a dense crown, from which the canes grow. Studies have shown that these stems can spread up to 7m away from the parent crown, and to a depth of 4m – the rhizome will become dormant through winter and start growing again as the weather gets warmer.

Although it spreads much slower, the rhizome can extend in any direction, and will tend to follow the path of least resistance. This can cause problems to homeowners, and it can have a destructive effect on building foundations and drains.

Knotweed can rejuvenate from very small pieces of plant material
Although most of the Knotweed found in the UK is because of rhizome or crown material. A finger nail size piece of rhizome weighing 0.7g can regenerate quickly into a new knotweed plant so to avoid accidental spread of the species you should always ensure you clean the grips in your shoes and your tools if you have been walking or working near to Knotweed.
Japanese Knotweed has an incredibly sweet tasting nectar
This nectar attracts bees, this doesn’t encourage the spread of Knotweed as there are only female plants in the UK, however,  it can be an issue as it takes the bees away from other neighboring plants and flowers. Having a detrimental effect on pollination of native species. Here at Knotweed Support we love the bees so we avoid treating, where possible, during the main flowering period so the bees can fill their hives ready for winter.
There is the risk that Japanese Knotweed could cause structural damage
If you’re trying to sell your property, finding Knotweed can be an issue, but don’t worry we can help to solve this. There is the risk that Japanese Knotweed could cause structural damage to your property as it has been known to undermine and grow through concreted pathways, patios and in some cases foundations affecting the strength and integrity of the structures. Find out more about how it could affect your sale or mortgage here >>

Other Invasive Species to consider


As well as helping clients deal with Japanese Knotweed, we also treat other invasive species and general nuisance weeds. Find more information on some of the other invasive species that we can help you with at Other Invasive Species.  

If you have an overgrown garden area or green space surrounding your property, please feel free to get in touch.  We can assist with all of these problems.


Get in touch and we can help

We’ll help you to identify, tackle and eliminate the invasive species that you have found on your property.