How to Identify Japanese Knotweed: Complete Guide

How to Identify Japanese Knotweed: Complete Guide.

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When it comes to Japanese knotweed infestations, you need to act quickly. The first step is to confirm that you are indeed dealing with Japanese knotweed instead of another similar plant like Russian vine, bindweed, dogwood or lilac.

There are distinctive features you use to identify Japanese knotweed. Chief among these are the bamboo-like stem, cream-white flowers and shovel-shaped leaves. However, some of these features won’t be available throughout the year. As the year moves along and the seasons change, Japanese knotweed changes, too.

Therefore, to know how to identify Japanese knotweed, you must understand both the general morphology of this plant and the different forms it takes during each of the four seasons of the year. Let’s get started.

How to Identify Japanese Knotweed: Plant Morphology

Four main physical features make up the structure of Japanese knotweed. These four are the leaves, stem, flowers and roots. Here is how to identify each one of them.

How to Identify Japanese Knotweed Leaves

The leaves of Japanese knotweed are lush green, pointy-tipped, and shaped like shovels. They also have small purple or red dots. Each leaf starts life rolled up and bright green. Then, it unfurls, darkens and gets bigger until reaching a width of 10cm and a length of 15-20 cm upon maturity. The leaves are arranged in a staggered manner on alternating sides of the stem, creating a zig-zag pattern.

How to Identify Japanese Knotweed Stems

Japanese knotweed stems look like bamboo. They have the same structure and hollow centre. Each stem starts life looking like a red or purple shoot of asparagus. Then, they mature, becoming tough, pale green and bamboo-like. 

Mature Japanese knotweed stems have speckles of purple and a fleshy interior. They also have leaves and flowers. However, they will shed those leaves and flowers around autumn. They will also start browning and wilting, becoming straw-coloured or red-brown. Finally, they become pale and brittle as winter draws near and begins.

How to Identify Japanese Knotweed Flowers

Japanese knotweed flowers are cream-white. They grow as clusters on panicles. Each flower grows on a thin spike and can reach a maximum diameter of 0.5cm. Each cluster of flowers is about 10 cm long.

The flowers are late bloomers. They won’t come out until late summer or early autumn. This allows Japanese knotweed to attract the attention of pollinators like bees while other plants have already shed their flowers.

Fortunately, the actions of these pollinators can’t spread the infestation because the UK only has the female variety of Japanese knotweed. Therefore, there is no way to fertilise those Japanese knotweed seeds. 

However, there are cases of hybridisation. An example is the bohemian knotweed, a hybrid of the Japanese and the giant knotweed. These hybrids can spread through seedlings. Japanese knotweed, on the other hand, can only spread through fragments of rhizomes and stems. 

How to Identify Japanese Knotweed Roots

Japanese knotweed roots are woody, tough and thick, about 20 cm in diameter. Colour-wise, they are dark brown on the outside and yellow or orange on the inside. The root of each plant is a network of rhizomes that can spread as wide as 2-7m and as deep as 3m.

The rhizomes feel and snap like carrots when fresh. They also contain a lot of energy and are the primary means by which Japanese knotweed spreads in the UK. A single fragment of rhizome can start or restart an infestation of Japanese knotweed. 

How to Identify Japanese Knotweed Throughout the Year

Now that you know how to identify Japanese knotweed, here is how it looks during the four different seasons of the year.  

Identifying Japanese Knotweed in Spring

The growing season starts in spring. So, Japanese knotweed will come out of dormancy during this period. Active growth restarts with new shoots springing forth. We have already discussed what these new shoots look like. They are red or purple and like asparagus.

Japanese knotweed shoots grow in groups, forming dense thickets. They can reach an incredible growth rate of 10cm per day during spring. Leaves will show up over time. They will be shovel-shaped, bright green, curled and staggered, with nodes on alternating sides of the stem.

Identifying Japanese Knotweed in Summer

The plant will mature around this period, reaching its peak height of 2-3.5cm. The stems will become green and tough, leaves will unfurl and darken, and flowers will bloom. Spring is the time of year when Japanese knotweed is at its full maturity and peak stage. 

The plant has dense foliage and forms a striking and highly conspicuous visual appeal. Therefore, you see the charm that made the Victorians import Japanese knotweed into the UK as an ornamental plant. Fortunately, this conspicuousness also makes it easier to identify Japanese knotweed. 

Identifying Japanese Knotweed in Autumn

The growing season slows down during autumn. Plants will also start to die back in preparation for winter. Japanese knotweed does the same. Seeds fall off, flowers die back, stems go brown, and leaves turn yellow and brittle.

At the end of everything, those brown, brittle and dead-looking stems will be the only things that remain. However, remember that Japanese knotweed is a late bloomer. So, expect these changes to start late.   

Identifying Japanese Knotweed in Winter

Japanese knotweed is a perennial plant. So it doesn’t die in winter. It only becomes dormant, waiting out the cold and returning in spring. Therefore, all you will see are brittle, dead-looking stems. Don’t let this fool you. The plant is not dead. It is merely staying dormant until winter is over. Check the root to find out, or wait until spring. 

Why Identifying Japanese Knotweed Early Is Crucial

Now that you know how to identify Japanese knotweed, let’s talk about why you must act quickly. The reason is simple. By identifying and dealing with Japanese knotweed early, you can curb a very stubborn and destructive infestation before it gets a solid foothold on your property.  

Once the infestation gets a foothold, it is extremely difficult and expensive to eradicate. Japanese knotweeds are that resilient. They can grow anywhere, under any condition. Even if the temperature, soil and moisture levels are too severe for other plants, the Japanese knotweed will survive. 

There is also the matter of growth rate. As stated earlier, Japanese knotweed can grow as quickly as 10cm per day. Therefore, the plant will mature and start flowering before you know it. Finally, just a fragment of rhizome or stem is enough to start an infestation and overrun a property. 

These qualities allow Japanese knotweed to outcompete other plants and survive situations and treatments that would kill most plants. Let’s use herbicide treatment as an example. It’s difficult to eradicate Japanese knotweed using glyphosate alone. 

Sure, the knotweeds will die, but that is not the end. They could return because a fragment of the rhizome is likely to survive. This fragment can remain potent for up to 20 years. Japanese knotweed plants that sprout from this situation might be deformed. These deformed varieties are called bonsai knotweed.

Either way, it’s better and easier to identify and fight Japanese knotweed as early as possible. If not, the infestation will overrun your property. Structures could also be damaged because Japanese knotweed can grow through concrete. 

Japanese knotweed can knock down the value of a property by up to 15%. However, this is just the beginning. The problem will spread to neighbouring properties. It could also get you in trouble with the authorities for breaking regulations under Schedule 9 of the Wildlife and Countryside Act (1981).

The neighbours can sue, while the authorities can penalise and force you to treat the infestations. These are all issues you can avoid if you identify and remove Japanese knotweed early.

How to Treat Japanese Knotweed

Identification is just the beginning. There is no point in learning how to identify Japanese knotweed if you don’t also learn how to fight it. Therefore, we can’t end this article without taking this final detour.

Fortunately, you can hire a professional to handle the job. Whoever you hire can help confirm if what you have is indeed a Japanese knotweed problem. Then, they can conduct tests to determine the severity of the problem and suggest means of treatment.

Japanese knotweed removal costs could cost between £1,000 to £20,000 depending on the treatment method and the size of the affected area. Here are some methods of treating Japanese knotweed in the UK.

Herbicide Treatment

This involves treating the infestation with a glyphosate-based herbicide. The herbicide is either injected or sprayed on the plant. The treatment is systemic, with multiple sessions over a few years. Herbicide treatment is the cheapest but not the most effective way to remove Japanese knotweed.   


This involves excavating and burying Japanese knotweed about 5 to 10 metres deep. The burial could be onsite or offsite. For this treatment to work, you must get all traces of Japanese knotweed, including fragments of seedlings, stems and rhizomes.

To achieve this, you must remove the plant with its root. Then, screen and sift the affected soil. Another option is to excavate and bury the soil, too. For this second method, you must remove soil to about 3 metres deep and 7 metres wide on all sides.


This involves burning Japanese knotweed materials. You have to collect and incinerate every trace. Then, you can dispose of or bury the ashes.

Root barriers

This usually includes some degree of excavation. The root barrier will prevent new growths from sprouting through. 


There are two keys to identifying Japanese knotweed. The first key is to know what the stems, leaves, flowers, and roots look like. The second is to know what the plant itself looks like at different periods of the year. We have covered both keys. 

However, even with everything we have discussed, you may still need help identifying Japanese knotweed. So, consider contacting a professional at some point. Start by sending them a picture of the knotweed on your property. Ask them if your suspicions are right and wrong. Then, ask them to recommend a way forward.

Do note that the stakes are high. So, you need to act fast and hire reputable and experienced professionals. Reach out to us right away. Let us help you find quotes from reliable Japanese knotweed specialists in your area.


Where Is Japanese Knotweed From?

Japanese knotweed is from East Asian countries like Taiwan, Korea, China and Japan. The Victorians brought it into the UK as an ornamental plant. 

What Plants Resemble and Are Often Mistaken for Japanese Knotweed?

Plants that resemble and are often mistaken for Japanese knotweed include Russian vine, bindweed, dogwood, lilac, Himalayan honeysuckle, Ivy, Houttuynia, buckwheat and horsetail.

Why Is Japanese Knotweed So Difficult to Remove?

Japanese knotweed is difficult to remove because of its fast growth rate, resilience and fertility. It can reach a growth rate of 10cm per day, grow in adverse conditions and regenerate from just a fragment of rhizome. It can also survive dormancy for up to 20 years.  

When Is the Easiest Time of Year to Identify Japanese Knotweed?

Late spring and early summer are the easiest times to identify Japanese knotweed. The plant is in peak maturity during these periods. So, the stems, leaves and flowers are easy to recognise.   

When Is the Best Time of the Year to Treat Japanese Knotweed?

Late summer and autumn are the best time of the year to treat Japanese knotweed. There are three reasons for this. Firstly, there is no rain to wash away the chemicals before they can do their work. Secondly, active growth is slowing down. So, the plant is no longer growing rapidly.

Thirdly, the plant unwittingly pulls the chemical deep into its body while trying to store energy in preparation for winter.  

Author: Eleanor

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