Japanese Knotweed

There are a number of ways to identify Japanese Knotweed, although, like any other plant, its shape, size and colouring can vary dramatically from its description. This sheet acts as a guide to identifying Japanese Knotweed.

If you suspect that you have identified the plant, take photographs or leaf samples and please contact us for it to be verified by an expert.

LEAVES
Colour
Young shoots have curled-up red/purple or light green leaves with red/purple colouring. ‘Lush green’ when mature, healthy and in growing season. Yellowing/ browning of leaves in late autumn/early winter.

Shape
Can be described as heart or shield shaped; broadly oval to somewhat triangular.
Size Up to 22cm in length and roughly 2/3 as wide.

FLOWERS
Appearance
80mm-120mm long clusters of small green-white flowers, blooming from late summer to early autumn.

STEMS
Appearance
Bamboo-like, regular, swollen and often red joints, nodes or knots.

Colour
Red and asparagus-like when a young shoot. Stems develop throughout the growing season from being supple and green to brown and brittle. Stems sometimes have characteristic red/ purple specks or spots.

Size
Up to 4cm in diameter. Stems grow up to and over 3m in height.

Structure
Hollow stems with periodic nodes. Snap a stem off to test this.

RHIZOME
Do not disturb the rhizome!
The rhizome is the concentrated energy source which allows the plant to re-grow each spring and resides under-ground. Disturbance can slow the eradication process and may spread the plant.

SEASONAL APPEARANCES
General Points
Plants can produce new shoots throughout the year, even as the dormancy stage approaches. The following should be taken as general seasonal appearances, not as a strict rule:

Spring: Young red/purple shoots appear, turning green as they age. Some plants leaf initially, but others can grow up to 50cm as straight red or green ‘rods’ before leaves begin to appear.

Summer/early autumn: Supple green stems with flowers and large leaves.

Late autumn/winter: Stems turn brittle and brown. The flowers die and the leaves turn brown-yellow, eventually falling off.

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