Japanese Knotweed Identification

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 sprouting through tarmac
Young Japanese knotweed stems
Japanese knotweed growing on rough ground
Japanese knotweed early growth looking rather like 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.
Close up of Japanese knotweed leaves
Japanese knotweed growing through gap in wall
Japanese knotweed infestation in London garden.

Late Summer

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

Residential neighbourhood overgrown with Japanese knotweed.
Garage sprouting Japanese knotweed!
derelict property with invasive Japanese knotweed in garden.
Japanese knotweed with white flowers

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 do be aware, it is not dead, it is storing energy deep in the rhizome ready to repeat the process again the next year.

Japanese knotweed canes in autumn
Brown leaves of winter Japanese knotweed.
Japanese knotweed rhizome close-up
Japanese knotweed damage to external wall of building.

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Howard Downer

THE EXPERT DR. KNOTWEED

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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