What Is Japanese Knotweed and What to Do if You Find One in Your Garden?

Japanese knotweed is one of the most invasive perennials, known for its ability to cause severe damage to properties, no matter if residential or commercial. Sometimes, Japanese knotweed identification can be difficult since there are four more species of it and other Japanese knotweed look-alikes.

In this article, we’ll cover how to identify Japanese knotweed, plants mistaken for Japanese knotweed and what to do if you find one in your garden.

What does Japanese knotweed look like?

Japanese knotweed produces a lot of foliage and vigorously growing stems, which makes the perennial easy to identify, but only in the growing season. Below, we’ve listed detailed characteristics of the plant.

Japanese knotweed leaves

This invasive plant’s leaves are typically broad, heart-shaped, and have a distinct appearance.

1. Shape – The leaves are broadly oval or heart-shaped. They have a flat, smooth surface, and their edges are usually slightly lobed.
2. Size – Japanese knotweed leaves are relatively large, measuring around 6 to 8 inches (15 to 20 centimetres) in length.
3. Arrangement – The leaves are arranged alternately along the stem, meaning they are not directly opposite each other but instead staggered along the stem.
4. Colour – The leaves are typically medium to dark green in colour. However, they can turn yellow during the fall season before eventually withering.
5. Veins – The leaves have prominent veins that run from the base to the tip of the leaf. The veins are usually a lighter shade of green compared to the rest of the leaf.

Japanese knotweed stem

The stem of Japanese knotweed is one of its most distinctive features.

1. Appearance – Japanese knotweed stems are hollow, bamboo-like, and segmented. They are reddish-brown and often mottled with dark brown or black spots.
2. Growth habit – The stems of Japanese knotweed grow upright and can reach heights of up to 10 feet (3 meters) or more. They are known for their rapid growth and can spread vigorously, forming dense thickets.
3. Structure – The stem is sturdy and can have a diameter of 0.5 to 2 inches (1 to 5 centimetres). It has distinct nodes (joints) where the leaves attach and internodes (gaps) between the nodes.
4. Leaf attachment – Japanese knotweed leaves are attached to the stem in an alternate pattern, meaning they are positioned one after the other along the stem and not directly opposite each other.

Japanese knotweed flower

Japanese knotweed produces small, clustered flowers during its blooming period.

1. Appearance – Japanese knotweed flowers are small, typically measuring around 5-6 mm in diameter. They have a unique, frothy or plume-like appearance due to the arrangement of the flowers in elongated clusters called panicles. Each panicle consists of numerous individual flowers.
2. Colour – The flowers of Japanese knotweed are usually white or cream-coloured. While white is the most common colour, some variations can exhibit a slightly pink or pale green tinge.
3. Structure – Each Japanese knotweed flower consists of five petal-like sepals, which are often mistaken for petals. These sepals are typically white or cream-coloured and are arranged in a star-like pattern. The flowers do not have petals. Inside the flower, there are numerous stamens (male reproductive structures) and a single pistil (female reproductive structure).
4. Blooming season – In the UK, Japanese knotweed flowers typically bloom in the summer months until August or early September. The exact timing may vary based on factors such as temperature, sunlight, and local conditions.
5. Pollination – Japanese knotweed flowers are pollinated by insects, particularly bees, butterflies, and other nectar-seeking insects. The flowers produce nectar as a reward to attract these pollinators. During pollination, pollen is transferred from the stamens to the pistil, allowing fertilization and seed production to occur.
6. Fruiting – The flowers develop into small, winged fruits containing seeds after successful pollination. These fruits are dark brown or black, and each one usually contains a single seed. The winged structure aids in seed dispersal, as they can be carried by wind or water to new locations.

Japanese knotweed roots

The roots of that invasive plant are the biggest problem, as they grow extremely fast.

1. Rhizomatous growth – Japanese knotweed spreads primarily through an underground network of rhizomes, which are thick, woody, and fleshy underground stems. The rhizomes can extend horizontally and vertically, enabling the plant to reproduce and spread rapidly. They can grow several meters from the parent plant and give rise to new shoots.
2. Depth and density – Japanese knotweed roots can penetrate deep into the ground, with some rhizomes reaching depths of 3 meters or more. The roots form dense networks, often growing in close proximity to one another. This dense and interconnected root system contributes to the plant’s ability to spread and survive in various soil conditions.
3. Regeneration ability – Japanese knotweed roots have remarkable regenerative capabilities. Even small fragments of rhizomes or roots left in the soil can give rise to new plants. This makes the plant highly resilient and difficult to eradicate once established.

How to identify Japanese knotweed by season

Understanding the seasonal characteristics of Japanese knotweed is crucial in identifying this persistent plant throughout the year. By observing its distinct features during different seasons, you can confidently recognise and address the presence of Japanese knotweed.

Identifying Japanese knotweed in Spring

Look for reddish-purple shoots emerging from the ground. These shoots are often one of spring’s first signs of Japanese knotweed. They multiply rapidly and reach heights of up to 10 centimetres within a week.

As the shoots continue to grow, they develop into distinctive reddish-purple stems. These stems have a hollow centre and can feature raised nodes, giving them a segmented appearance. The colouration and unique stem structure are key identifiers of Japanese knotweed.

The plant’s leaves appear a vibrant green and have a characteristic zigzag pattern. They have a pointed tip and a flat base, with parallel and prominent veins.

During the early stages of spring, Japanese knotweed growth may seem slow compared to other plants. However, once established, it can quickly grow several centimetres per day, forming dense stands of vegetation.

In the spring season, Japanese knotweed is primarily focused on vegetative growth. Flowers usually appear later in the summer or early fall, so you won’t typically find them during the spring months.

While not visible above ground, Japanese knotweed’s extensive underground rhizome system plays a crucial role in its identification. Digging around the plant’s base may reveal the presence of interconnected rhizomes.

Identifying Japanese knotweed in Summer

By summer, Japanese knotweed can reach its full height, typically ranging from 1.5 to 3 meters (5 to 10 feet). The tall, dense stands of knotweed can be an indicator of its presence.

The leaves continue to grow during summer and reach lengths of 10 to 20 centimetres (4 to 8 inches). The leaves remain heart-shaped and have a distinct zigzag pattern along the stem. Their colour is usually deep green.

Look for stems with reddish-purple speckles or spots, especially near the base of the plant.

This invasive plant produces small, creamy-white flowers in clusters in late summer. However, not all Japanese knotweed plants will produce flowers yearly.

Japanese knotweed stands out as summer progresses due to its vigorous growth and dense foliage. It can easily overpower and shade out other plants in the vicinity.

Identifying Japanese knotweed in Autumn

By autumn, Japanese knotweed typically reaches its full height, ranging from 1.5 to 3 meters (5 to 10 feet). Look for tall, thick stems that may have started to turn brown or reddish in colour as the foliage begins to die back.

The leaves undergo a noticeable colour change in autumn. They transition from the vibrant green of summer to a yellowish-green or yellow colour. This change can help distinguish Japanese knotweed from surrounding plants.

The clusters of small, creamy-white flowers of Japanese knotweed may still be present. However, the flowers and panicles often begin to wither and turn brown. These dried clusters remain on the plant, adding to its distinctive appearance.

Japanese knotweed sheds its leaves as autumn progresses, and the stems become more prominent. The stems are hollow, bamboo-like, and have a segmented appearance due to raised nodes. These stems can persist through the winter.

Even in autumn, the extensive underground rhizome system of the plant remains intact.

Identifying Japanese knotweed in Winter

Identifying this invasive perennial in winter can be challenging as the plant goes through dormancy. However, a few characteristics remain to consider when attempting to identify it during this season.

One of the most prominent features of Japanese knotweed in winter is its persistent, woody stems. These stems are hollow, bamboo-like, and have a segmented appearance due to raised nodes. They can be greyish-brown or reddish-brown and remain upright throughout the winter months.

Look for dried clusters or remnants of the small, creamy-white flowers that were present in summer and autumn. These clusters may still be visible on the plant, albeit dried and withered.

Japanese knotweed loses its leaves during winter, and the plant appears leafless.

Familiarize yourself with the locations where Japanese knotweed was present during the growing season. Even in winter, the plant may leave behind noticeable signs such as persistent stems, rhizomes, or patches of dead vegetation that can help identify its presence.

Plants mistaken for Japanese knotweed in UK

Numerous plants look like Japanese knotweed, and many homeowners get suspicious when they notice a sudden appearance of an unknown shrub-like plant in their garden. Hence, you must get familiar with Japanese knotweed look-alikes and rule out every possibility before panicking.

For example, bindweed has heart-shaped leaves and looks almost the same as Japanese knotweed, except that it is a climbing plant that will wrap around your trees and walls. Another good example of a plant that looks like Japanese knotweed is the broadleaf dock. The difference here is in the stems, as the broadleaf dock’s stems are shorter.

Below you can find more plants commonly mistaken for Japanese knotweed:

What damage can Japanese knotweed cause?

Since it’s not poisonous, Japanese knotweed does not pose a threat to people or animals – it cannot physically harm you. However, Japanese knotweed dangers are more financial and concern many homeowners, property sellers and potential buyers.

If homeowners discover Japanese knotweed in their garden, they’ll have to face the fact that their home’s value will depreciate significantly. And when it comes to potential property buyers, they may decide to spend a lot of money on a survey to find out if a Japanese knotweed infestation exists. Unfortunately, if they discover such a problem, there is no legal recourse for them, and no one will recover all the money spent on the survey.

You might be wondering why is all that? What does Japanese knotweed do so much? Well, you may be shocked to learn that Japanese knotweed can and will displace building foundations, patios, deckings, flood defences, garden sheds, greenhouses, fences and everything that got in its way if not removed promptly.

Japanese knotweed can even break into houses in extreme infestation cases since it can grow through concrete. That happens rarely and is usually a result of a very severe infestation and weak building construction, but you should still be concerned if you find Japanese knotweed in your garden. That’s because even if it doesn’t break through your floor, there is a significant risk that the plant may damage or destroy some of the brickwork or your patio.

History knows cases in which homeowners have been advised to demolish their entire homes since the foundations got severely damaged and the plant already found its way through floorboards and skirting boards.

What to do if you find Japanese knotweed?

It’s more about what not to do than what to do if you find Japanese knotweed in your garden. For instance, don’t be tempted to cut, pull or trim the weed since that will lead to it spreading uncontrollably. That also applies to the area around the weed. In addition, it might not be illegal to have Japanese knotweed in your garden, but it is absolutely illegal to throw Japanese knotweed cuttings or stems in your general green waste bin.

Never try to kill Japanese knotweed with some domestic weed killers – that won’t help. On the contrary, it will make the Japanese knotweed removal process more complicated. That’s because when treated professionally, the plant should be in the best of health and condition.

Don’t hide the infestation from your neighbours, either, since this may land you a fine. Yes, it is your responsibility to notify all of your neighbours as there is a big chance that their gardens will also need an inspection. Remember that Japanese knotweed is a community problem, and you’ll be glad to know if there is Japanese knotweed in the neighbours’ garden.

Now, let’s talk about what to do about Japanese knotweed. After you’ve informed your neighbours, the second thing you have to do is to call professional Japanese knotweed control and removal experts. 

They’ll permanently eradicate the invasive plant from your property by eliminating even the smallest rhizome. The professionals will arrange an on-site survey and identify how severe the infestation is. Then, you’ll be provided with an eradication plan and responsible disposal of the plant.

Remember, you’re not allowed to bury, burn or throw away Japanese knotweed without the permission of your local council and the Environment Agency.

What to do if your neighbour has Japanese knotweed?

Noticing Japanese knotweed in your neighbour’s garden can be stressful, as it won’t take long to appear in yours. In such a case, you have one option – to converse with your neighbours, asking them to treat their land before Japanese knotweed has the time to spread to neighbouring ones. You cannot do much with Japanese knotweed in neighbours’ gardens since having the plant is not illegal.

However, if your neighbours don’t take action on time and the invasive plant spreads to your property, you have the right to file a claim against them.

On this note, a Japanese knotweed infestation within 7 metres of your property can decrease your house’s value, and once it reaches your garden, it can lead to various types of damage and costly repairs. So, what to do if neighbours have Japanese knotweed but refuse to treat it? Well, your very best option is to get a Community Protection Notice, which will force your neighbours to take action and eradicate the invasive plant. 

However, it’s good to remember that, in some cases, it’s hard to estimate where the infestation started. If you share a garden with your neighbours and the borders are not precisely defined, then the best course of action for all parties is to reach a mutual agreement on sharing all Japanese knotweed removal expenses equally.

The bottom line

By itself, Japanese knotweed might not be dangerous but it requires careful planning, in terms of its proper management. Getting rid of one can be a very complicated, time-consuming and expensive job, especially if the infestation is severe.

The best solution you have is to contact a professional Japanese knotweed removal company and let them guide you through the process of eliminating the problem for good.