Japanese knotweed flowers belong to a tall invasive perennial weed that is widespread throughout the UK.  It is mostly found along the roadsides, railways, in woods or near properties. The knotweed was initially introduced to the country as an ornamental plant.

The Japanese knotweed flowers occur in clusters and look extremely pleasant. It is the beautiful flowers that made the plant irresistible among property developers and homeowners in the 19th century. The plant was soon found in urban areas near expensive and exquisite properties.

Japanese knotweed flowers growing during summer and their distinctive characteristics identified

What time of the year does the Japanese knotweed grow?

The Japanese knotweed does not grow throughout the year. It has a seasonal life like most of the plants of the same family. It grows in phases depending on the climate and season.

It is therefore crucial that you understand how the plant looks at different times of the year as this will help you identify the plant quickly.

Japanese knotweed flowers in full bloom and ready to populate the local proximity
Japanese knotweed flowers are in full bloom and ready to populate the local proximity

What do the flowers of Japanese knotweed look like?

It is not easy to spot the plant until there is significant growth in spring. The first shoot appears in early spring. At first, the stalks are reddish but later turn green towards the end of spring. The leaves are red but will become green as the plant grows taller.

This weed is easier to identify during the summer. This is the time that the plant is more visible. There is fast growth during summer; the stems can grow up to 2cm in height each day.

From the end of August towards early September, the Japanese knotweed will have elongated clusters of creamy white flowers. They grow in clusters of about 10 cm long and 0.5cm wide. The clusters and the bright green leaves form dense foliage as summer comes to the end.

The weed has both male and female organs. However, the organs are just vestigial, and the flowers are unisex. During the last weeks of summer, the weed develops winged fruit seeds measuring about 2.5mm long, the seeds are small and shiny.

The flowers of the Japanese knotweed bloom and spread during late summer
The flowers of the Japanese knotweed bloom and spread during late summer

There are several varieties of sub-species that be easily mistaken for Japanese knotweed. Here are the plants that have similar features to the Japanese knotweed but also have some essential differences that may help when identifying the Japanese knotweed.

  1. Giant knotweed (Fallopian sachalinensis)

The Giant knotweed is often mistaken for Japanese knotweed.  It grows up to 4 meters in height; the leaves are dark green and feature creamy white flowers. The plant is, in fact, a large variant of the Japanese knotweed.

The Giant knotweed has several similar features to Japanese knotweed making it so identical thus the confusion. The only feature that can easily differentiate the two is that the Giant knotweed is taller and generally more massive than the Japanese knotweed.

  • Himalayan knotweed (persicaria wallichii)

The Himalayan knotweed can grow taller than the Japanese knotweed. Its stems are also hollow which leads to more confusion between the two plants. This knotweed, just like the Japanese knotweed, is a fast-growing invasive plant that is found near walkways, railways or properties.

The leaves are also arranged opposite each other on the stem-like those of Japanese knotweed. The leaves are narrower but longer than those of the Japanese knotweed. This plant has large white-pinkish flowers; which narrowly differentiates it from the Japanese knotweed.

  • Lesser knotweed ( Persicaria campanulata)

The lesser knotweed, just like the Japanese knotweed, was initially planted as an ornamental plant. The leaves are alternately arranged like those of Japanese knotweed.

The stems are also hollow while the shape is much larger and features pronounced veins than those of the Japanese knotweed. The lesser knotweed is much shorter; it only grows up to 1.5 meters. The flowers are pinkish in colour and bell-shaped.

  • Russian Vine (Fallopia  baldschuanica)

Russian vine is the first growing invasive plant.  The plant is similar to the Japanese knotweed in several ways.  The Russian vine is a climbing plant with dense green leaves. The plant relies on the support of other plants to grow taller toward the sun.

The flowers are white and small like those of the Japanese knotweed.

  • Hedge Bindweed( Calystegia Sepium)

The Hedge Bindweed is one of the most common plants mistaken for Japanese knotweed. This climbing plant has heart-shaped leaves like those of  Japanese knotweed. Its flowers are white and trumpet-like, they begin to grow in early summer.

The plant’s flowers differ slightly from those of the Japanese knotweed.

  • Bohemian Knotweed(Fallopiax bohemica)

This plant has similar fast growth as that of the Japanese knotweed. However, its leaves are longer and lack the truncate base. The Bohemian knotweed florets are about 1-1.5 mm long. The flowers are white- pinkish.

  • Dwarf Japanese knotweed (Fallopia Japonica var.compacta)

Just like its name, this plant can only grow up to 1.7 meters long. The leaves have crinkled edges and a leathery feel. The weed’s leaves come in different shapes. The Dwarf knotweed’s flowers are either pink or white and appear in the late summer.

The Japanese knotweed flowers in late summer and its panicles cover the ground extensively
The Japanese knotweed flowers in late summer and its panicles cover the ground extensively

The impact of Japanese knotweed

The Japanese knotweed spreads through seeds that are transported by water, soil, animals, humans and wind. It also spreads by rhizomes fragments and crowns or shoots.

The weed spreads so fast forming dense foliage within a short time. These thickets kill native vegetation, thus reducing their diversity. Additionally, the plant displaces wildlife from their native habitat and also alters the existing ecosystem. The knotweed infestation is challenging to get rid of.

Identifying Japanese knotweed flowers in summer as they begin to grow


Having proper identification can be hectic when you are unaware of the plant’s distinct features and changes through the seasons. We hope that this article helped you identify the Japanese knotweed flowers easily.

If you find a plant in your garden whose flowers resemble those of Japanese knotweed, it is worth seeking advice from an environmental expert. A knotweed specialist will help you get rid of the weed completely.

Want to know more?

Knotweed Removal aims to provide the most up-to-date information, help and advice for YOU to make informed decisions. If you are unsure or uncertain about how to proceed, please reach out to us and we will gladly come back and advise you as best we can.

Governmental advice can be found here and the UK law covering the removal of Japanese Knotweed as stated under the Wildlife and Countryside Act 1981 can be found here.

The best means to contact us is via our email – hello@knotweedremoval.tips

Do not forget we have a library of blogs covering many areas relevant to Japanese Knotweed, our free downloadable How-to Guides and Product Reviews on the latest methods being employed to eradicate or remove Japanese Knotweed.

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