Japanese Knotweed Identification

Japanese knotweed in bloom

Japanese knotweed changes dramatically in appearance throughout the year, and since it is such an invasive plant, identifying an infestation in your garden or on commercial land is the essential to ensure that it doesn't turn into something uncontrollable, and costly.   Here's how to identify Japanese knotweed throughout its growing cycle, starting in springtime.

Early Spring

In March and April, new shoots of Japanese Knotweed will begin to emerge. These shoots will look rather like asparagus and may have a red or purple tone. The green leaves and stem will unfurl as Spring progresses, sprouting up from the crown material which becomes established on the surface of the ground. The rhizome, the buried part of the plant, can extend up to three metres deep and lie dormant for many years.

Japanese knotweed in Springtime

The red leaves of new shoots sprouting from tarmac.

japanese knotweed in Spring

As the plant grows it changes from red to green.

Japanese knotweed growing from under paving stones.

Japanese knotweed will find any crevice to emerge from.

early growth of Japanese knotweed plant, looks very similar to asparagus tips

Early growth of Japanese knotweed looking very similar to asparagus tips!

Late Spring/Early Summer

As the year progresses Japanese Knotweed will start to grow rapidly in its quest for light and water. The bamboo like green stems are hollow and are speckled red or purple with red joints and knots, and can grow to over 3m in height, reaching its mature height by June. The leaves will be a lush green, shaped like a heart or a shield and can grow up to 22cm in length. The ends of the cane develop a distinctive zigzag growth appearance.

Japanese knotweed growing in garden beside greenhouse.

Japanese knotweed growing in a residential garden, and looking very much like it is on its way to invade a neighbours garden - causing problems for both homeowners.

Japanese knotweed identification - close up of Japanese knotweed leaves

Close-up of heart or shield shaped Japanese knotweed leaves, with red joints and knots.

Japanese knotweed can grow through walls, creating huge gaps and causing extensive damage to property.

Growing out of a building, here you can clearly see the speckled canes of  the Japanese knotweed plant.

Japanese knotweed infestation in front garden of residential home in London, UK

A Japanese knotweed infestation can take over the smallest garden or plot, whether it is located in deep countryside or on a busy London street.

Late Summer

In late Summer, Japanese Knotweed develops clusters of spiky stems covered with tiny white, creamy coloured flowers, bourn towards the end of the zig zag cane. The plant will produce non-viable seeds.

Residential neighbourhood overgrown with Japanese knotweed.

From the tiniest sprout in early spring, Japanese knotweed will cause devastation within just a few months.

Garage being overtaken by invasive Japanese knotweed

Growing anywhere and everywhere, no location is safe from this unwanted land invader.

japanese knotweed in late summer

In late summer the thick green foliage and creamy coloured flowers will overwhelm any location it is found, whether on commercial or residential land.

Japanese knotweed in bloom with white flowers in late August, early September

Close-up of Japanese knotweed plant in bloom.  Some may say it is a beautiful plant, and when grown in its natural habitat, it would be the predator that it is.

Autumn & Winter

In late Autumn, the leaves will fall and the canes will brown. Once the first frosts have hit, Japanese Knotweed may look like a pile of dead brown stems, again with the typical zig zag growth pattern at the end of the cane. But don't be fooled, the plant is not dead, it is storing energy deep in the rhizome ready to repeat the process again the next year.

Brown leaves of Japanese knotweed in autumn

Autumn Japanese knotweed leaves dying-back after causing another year of havoc.

bare Japanese knotweed canes in autumn/winter

Now in winter the knotweed canes are devoid of leaves.

Japanese knotweed rhizome close-up, this can lie dormant underground for year and should be removed to ensure no regrowth is possible.

The rhizmone can lie dormant underground for years, so removal is the best option to stop regrowth and property damage.

extensive damage is caused by invasive Japanese knotweed to building as seen in this picture where the plant has literally pushed the wall apart

The longer Japanese knotweed goes untreated, the worse the final outcome will be, as can be seen in this picture.

What’s Next? Contact us today

Howard Downer


Howard Downer, AKA Dr. Knotweed, has over 20 years of experience as an Environmental Consultant and is regarded by his peers as one of the most knowledgeable people in the Japanese knotweed industry.

Follow Dr. Knotweed to hear about the latest developments regarding Japanese knotweed and the implications of infestation.

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