Willow Grass, Water knotweed, Longroot smartweed, Water smartweed
Polygonum amphibium

Family: Polygonaceae

What it is like

Polygonum amphibium is a PERENNIAL growing to 0.3 m (1ft). See above for USDA hardiness. It is hardy to UK zone 5. It is in flower from July to September. The species is hermaphrodite (has both male and female organs) and is pollinated by Insects. Suitable for: light (sandy), medium (loamy) and heavy (clay) soils. Suitable pH: mildly acid, neutral and basic (mildly alkaline) soils. It can grow in semi-shade (light woodland) or no shade. It prefers wet soil and can grow in water.

Height (m): 0.3

Where it is found

Lakes, ponds, slow-flowing rivers and canals, also on banks by the river.

Most of Europe, including Britain, temperate Asia, N. America and S. Africa.

Conservation Status:

Countries/locations it is found in

How it is used

Food

Rating: 1

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

Seed: includes nuts, cereals, peas and beans.

Medicine

Rating: 2

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. A poultice of the fresh roots has been applied directly to the mouth to treat blisters.

Astringent: Produces contraction in living tissue, reducing the flow of secretions and discharges of blood, mucus, diarrhoea etc.

Depurative: Eliminates toxins and purifies the system, especially the blood.

Pectoral: Relieves respiratory diseases, a remedy for chest diseases.

Poultice: A moist, usually warm or hot, mass of plant material applied to the skin in the treatment of burns etc.

Skin: Plants used in miscellaneous treatments for the skin.

Other

Rating: 0

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

Tannin: An astringent substance obtaied from plants, it is used medicinally, as a dye and mordant, stabilizer in pesticide etc.

How it is grown

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. Their stems root at the nodes wherever they come into contact with the soil.

Propagating it: 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.

Best place to grow: Pond; Bog Garden;

Habit: Perennial

Hardiness: 4-8

Growth:

Soil: Light (sandy), medium, heavy (clay)

Shade: Semi-shade, no shade

Moisture: Wet, water

Things to keep in mind

Although no specific mention has been made for this species, there have been reports that some members of this genus can cause photosensitivity in susceptible people. Many species also contain oxalic acid (the distinctive lemony flavour of sorrel) - whilst not toxic this substance can bind up other minerals making them unavailable to the body and leading to mineral deficiency. Having said that, a number of common foods such as sorrel and rhubarb contain oxalic acid and the leaves of most members of this genus are nutritious and beneficial to eat in moderate quantities. Cooking the leaves will reduce their content of oxalic acid. People with a tendency to rheumatism, arthritis, gout, kidney stones or hyperacidity should take especial caution if including this plant in their diet since it can aggravate their condition.

Its other names

Local names

Synonyms

Persicaria amphibia.