Reynoutria japonica Seeds | Japanese Knotweed

Small, glossy, triangular brown Japanese Knotweed seeds, on a plain background — Japanese Knotweed (*Reynoutria japonica*): Polygonaceae, native to Japan, Korea and northern China and now invasive across Europe and North America.

LATIN NAME: Reynoutria japonica

COMMON NAME: Japanese Knotweed

Plant Profile

Japanese Knotweed is a vigorous herbaceous perennial that dies to the ground each winter and returns in spring as a cluster of thick, bamboo-like shoots, speckled red and growing several centimetres a day. By late summer it stands two to three metres tall, with broad, heart-shaped leaves and sprays of small, creamy-white flowers that bees work heavily.

It comes from the volcanic slopes of Japan, Korea and northern China, where it is kept in check by competition and by herbivores. Elsewhere it is one of the most notorious invasive plants in the world, spreading by fragments of rhizome that can travel in soil, in water and on machinery.

Science & Biotechnology

In Britain and much of Europe the plant is a single male-sterile clone, so it sets little or no viable seed and spreads almost entirely by rhizome. Where both sexes occur, as in parts of Japan, seed is fertile, and hybrids with Giant Knotweed, Reynoutria × bohemica, are common and even more vigorous than either parent.

Its rhizome contains resveratrol and its glucoside piceid, together with emodin and other anthraquinones, and the root is a major Chinese herb, Hu Zhang. The same compounds have made the plant a subject in pharmacology, though the garden problem it creates has nothing to do with its chemistry and everything to do with its growth rate.

Uses & Benefits

Young shoots are edible in spring, when they are still tender and less than twenty centimetres tall, and they are cooked like asparagus in Japan, where the plant is itadori. The root is used in Chinese medicine, and beekeepers value the late-summer flow of nectar from its flowers.

Beyond those uses, the plant is a liability: it damages paving, walls and foundations, it shades out native vegetation, and its rhizome can regenerate from a fragment the size of a fingernail. In many countries its disposal is regulated, and it should never be moved from a garden without professional advice. The flowers are also a late-summer nectar source at a time when few other plants are in bloom, which is why hives are sometimes placed near a stand.

Growing from Seed

Seed is rarely the route in cultivation, since the widespread clone is male-sterile; where fertile seed is available, sow it in spring in a pot of moist compost at 20 °C, cover it lightly, and expect rapid germination. Seedlings grow fast and should be potted on before they crowd the tray.

If you grow it at all, grow it in a large container on a hard surface, well away from drains, walls and boundaries, and never let it reach open ground. The rhizome will crack a pot within a few years, so plan for repotting or for removal. In most gardens the honest advice is to grow something else.

Our Take

An edible, medicinal, bee-friendly plant that is also one of the worst invaders in the temperate world. Grow it only in a container, and only if you are prepared to keep it there.

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