A picture of a Japanese Knotweed

Japanese Knotweed

Reynoutria japonica

Also known as

Giant Knotweed, False Bamboo, Mexican Bamboo, Himalayan Fleece Vine, Monkeyweed, Monkey Fungus, Hancock's Curse, American Bamboo, Fleeceflower, Knotweed

Fallopia-japonica(Blaetter) by Migas (CC-BY-SA-3.0)

Full Sun
Easy care
Light watering
Frost Hardy

H4

RHS hardiness

-10°C

Minimum temperature

Expected size

Height
Spread

4m

Max

1m

1m

Min

50cm

2 years to reach maturity

Flowering

  • spring
  • summer
  • autumn
  • winter

This plant has no fragrance

More images of Japanese Knotweed

A photo of Japanese Knotweed
A close up of some white cream Reynoutria japonica Fallopia japonica flowers
A close up of some white cream Reynoutria japonica Fallopia japonica flowers
A green Reynoutria japonica Fallopia japonica plant in a garden
A close up of some white Reynoutria japonica Fallopia japonica flowers and green leaves

Japanese Knotweed Overview

Japanese knotweed is a prolific, invasive weed that dies back to ground level during the winter and produces shoots rapidly in early spring. It frequently overgrows and smothers other plants, outcompeting many native species in their natural environment for space, light and water. Very difficult to remove from the soil due to its vigorous, spreading rhizomes. Historically it is well known as the species Fallopia japonica, however it is now thought to be better placed within the genus Reynoutria and has been renamed to R. japonica. For a long time, it was thought there were no natural predators of this species, however recent trials investigating a plant louse called Aphalara itadori have been promising. This small insect has been shown to inhibit Japanese knotweed growth without impacting native species. A. itadori has been licensed by the UK Government for use as a biological control method in England. This species produces large, upright and hollow stems with distinct nodes that are very similar in appearance to those of bamboo, but much softer and with large-leaved foliage. Stems are also marked with distinct purple speckles. Leaves are arranged oppositely, measure approximately 10-15cm long and form on a characteristically zig-zagging stem. Leaves are heart to shield-shaped, with notable flat bases and pointed tips. A crown of rhizomes may be visible at the base of the plant, these are bright orange inside and produce white shoots. Capable of growing over 4m in height, in a single season, these readily return from rhizome fragments when cut down. Flowers are small and usually cream-white in colour, they measure between 6-15cm in length and are produced late in the summer to early autumn.

How to harvest Japanese Knotweed

Not usually harvested.

How to propagate Japanese Knotweed

Rhizomes

Has invasive rhizomes that are very hard to remove.

Other uses of Japanese Knotweed

Medicinal, fodder, ornamental.

Edible

Young shoots are edible and a good source of Vitamin A and taste similar to rhubarb. However, make sure the plant is not growing on soil with any contaminants in it as the plant will absorb these.