How to identify Japanese Knotweed at every stage of its life


Rapidly spreading in warm weather due to its ability to grow up to 10cm a day, Japanese Knotweed is one of the most invasive plant species in the UK. Commonly distinguished by its large size, this invasive plant can reach a total height of four metres, and its roots can burrow 3 metres deep and up to seven horizontally. However, knowing how to identify Japanese can still be tricky as young Knotweed shoots have similar leaves to many woody shrubs, and it has the same hollow stems as Lesser Knotweed and other knotweed species.

 

Although native to Japan, China and Korea, Japanese Knotweed was first introduced to the UK through the Kew Botanic Gardens in 1850. Since then, it has spread rapidly across the country, invading around 10% of our rivers by the year 2010. One of the reasons for this is because the plant was introduced to the UK without psyllids, the insects that keep its growth under control.

 

Despite recent psyllid release programmes to ease the spread of this invasive species, its presence can still cause big issues for home and business owners. Because of its large size, deep roots, and the basic fact that it spreads rapidly, Japanese Knotweed causes structural damage to pavements and buildings by creeping into bricks and cavity walls. It also negatively affects native plants, insects and aquatic life, which is why you can be fined if you don’t prevent the spread of knotweed into the wild.

 

Unsure if this invasive plant is growing on your premises? Our guide to identifying Japanese Knotweed at every stage of its life can help…

 

Identifying Japanese Knotweed in Spring

 

Japanese knotweed shoot in the spring

 

Spring is the season that Japanese Knotweed begins to sprout, and it’s also the time that it encounters its fastest growing stage (in fact, it can grow up to 10 feet tall by the end of the season).

 

Although the new shoots will originally look similar to asparagus spears, Japanese Knotweed’s distinguishing feature is the red/purple tinge of its stems.

 

Japanese Knotweed shoots also have rolled up leaves with the same purple/red tinge as the stems. As the season progresses and canes shoot out of the ground, these leaves will gradually open up and turn light green in colour.

 

Identifying Japanese Knotweed in Summer

 

Japanese knotweed in the summer fully grown

Image courtesy of https://www.designingbuildings.co.uk/ 

As the freshly sprouted canes grow taller throughout spring, they’ll become hollower and start to look more like bamboo shoots than asparagus by the time summer is in full swing. However, the stems will have distinguishable purple speckles, and the leaves will begin to grow in a distinctive zig-zag shape.

 

One of the features that Japanese Knotweed is most easily identified by is its leaves, which are green and flat with pronounced mid-ribs and veins, as well as being a unique heart/shovel shape. In summer, you can also distinguish this invasive plant species by looking out for its clusters of small creamy-white flowers. These will typically have bloomed by late August.

 

Identifying Japanese Knotweed in Autumn

 

Japanese knotweed in Autumn

Image courtesy of https://www.designingbuildings.co.uk/.

At the beginning of autumn, Japanese Knotweed will still have clusters of flowers and a dense covering of heart/shovel shaped leaves. Although the flowers are still a creamy-white colour and the leaves are still a healthy green at the beginning of autumn, they’ll turn yellow and wilt as the season progresses.

 

At this point, the plant will also be around 2-3 metres in height.

 

Towards the end of autumn, the hollow bamboo-like canes will change from the reddish-brown colour they were in the summer months, to a darker shade of brown. As we approach the end of autumn (in September and October), the yellow leaves will also begin to wilt.

 

Identifying Japanese Knotweed in Winter

 

Japanese knotweed in the winter

 

Japanese Knotweed goes dormant in winter, and you’ll be able to tell when this happens because the canes will be very hollow and dark brown in colour. All of the wilted yellow leaves will fall off the plant, and as the canes become more brittle, they’ll also begin to snap and collapse under each other, leaving a pile of debris in the plant’s place. You’ll usually find decomposed canes from previous years beneath it, as well as fresh new shoots.

 

Identifying Japanese Knotweed is probably the most difficult in winter due to its distinctive leaves and flowers no longer being present. At this point, the broken canes will most similarly resemble bamboo.

 

Removal of Japanese Knotweed

 

When it comes to removal and the safe disposal of Japanese Knotweed, it’s better to leave it to the professionals. It’s not easy to control and completely remove this invasive weed from your grounds by yourself. This is because it requires the use of herbicides that aren’t sold easily over the counter, says Pol Bishop from Fantastic Gardeners. And since the process of removing Japanese Knotweed is cumbersome, there are companies that specialise in its control and complete removal from both residential and commercial properties.

 

Before hiring such a company, you should:

 

Inform Natural England that there’s Japanese Knotweed growing on your property and apply for permission to treat it if the area is protected.

Get a permission for treatment from the Environment Agency if your premises or property is located near water sources e.g. rivers, ponds, lakes.

Find a licensed company that holds a certificate of competence for working with and storing herbicides, such as PHS Greenleaf.

Check if the company can safely dispose of the Japanese Knotweed for you. If it doesn’t offer disposal services, check with your local council to find out how and where to dispose of it without breaking the law.

 

Need help removing Japanese Knotweed from your business premises? The experts at phs Greenleaf will treat and remove the invasive plant with approved herbicides, while carrying out necessary Risk Assessments. Contact us for more information.

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