Water knotweed (Polygonum amphibium)

At a glance

Lifecycle: Perennial (hardy)
Exposure: Full sun
Frost limit: −28.9 °C

Cultivation

A water or marsh plant growing in water up to 3 metres deep or in boggy soil. This species is hardy to about -25°c. Plants seem to be immune to the predations of rabbits. The leaves are often attacked by the larvae of the water-lily beetle. Plants can either be aquatic with floating ovate-oblong leaf-blades or a leggy marsh plant with lanceolate leaves.

Propagation

Seed - sow spring in a cold frame. Germination is usually free and easy. When they are large enough to handle, prick the seedlings out into individual pots and plant them out in the summer if they have reached sufficient size. If not, overwinter them in a cold frame and plant them out the following spring after the last expected frosts. Division in spring or autumn. Very easy, larger divisions can be planted out direct into their permanent positions. We have found that it is better to pot up the smaller divisions and grow them on in light shade in a cold frame until they are well established before planting them out in late spring or early summer.

Edible uses

Leaves - raw or cooked. The young shoots are eaten in the spring. Seed - cooked. It is rather small and fiddly to utilize.

Medicinal uses

The whole plant, but especially the root, is astringent, depurative, skin. An infusion of the leaves and stems has been used to treat stomach pains and children with diarrhoea. The root has been eaten raw, or an infusion of the dried, pounded roots used, in the treatment of chest colds.

Material uses

Tannin is obtained from the plant. No more details, but it is likely to be from the root.