This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Though it is unknown in any ancient deposits in Britain this ubiquitous plant is known in S. Sweden. At the present time it is found in the N. Temperate and Arctic regions. It is found everywhere in Great Britain except in Cardigan and Merioneth, as far north as the Shetlands, and in Ireland and the Channel Islands.
As the specific name implies, this plant grows both in water and on land, being by nature really aquatic or a hydrophyte, but taking, when so driven, to a terrestrial habitat, when it passes through a hygro-philous state, growing in damp situations, to a sometimes quite dry habitat, where it becomes a xerophyte. When aquatic it is smooth, when terrestrial it is extremely hirsute, and seldom flowers. It is found in rivers, brooks, ponds, pools, and when found on land its occurrence denotes the former existence of some pool or ditch, since drained.
When growing in water it is prostrate or floating, the flower-stalks, however, being erect; but on land the whole plant, from a creeping, fibrous, long root, is erect. The root is reddish-brown. The leaves are stalked, heart-shaped, lance-shaped, hairy or smooth, rigid, sometimes spotted, alternate and spreading. The ocreae or membranous stipules are large, closely pressed, blunt.
The flowers are apetalous, with no corolla, bright-pink, in solitary, terminal racemes, dense, on hairy flower-stalks, which are paired. The calyx is pedicellate, 5-partite, with oval, blunt segments. The seeds are glossy and flattened at the margin. There are 5 stamens and 2 styles.
Amphibious Knotgrass is 1-2 ft. high. The flowers bloom in June till August. The plant is perennial, and freely propagated by roots.
The stigma is projecting one-fifth, allowing ants to get honey without pollinating it. The honey is plentiful, situated in 5 yellow glands at the base of the ovary, and the flowers conspicuous. The stamens are short, and ripen before the stigma, the former being proterandrous. When growing in water it is protected from ants or flying insects, so that the stem is smooth. When growing on land it is densely hairy with sticky glands, so that small insects cannot crawl up the stems, an adaptation for the promotion of cross-pollination.
There are two forms of flowers; in the one case the pistil is long and the stamens are short, and there are other flowers with a short pistil and long stamens. Pistillate flowers also occur.
Photo Flatters & Garnett - Amphibious Knotgrass (Polygonum Amphibium, L.)
The fruit is partly enclosed in a perianth, and may be blown to a distance to fall in the water and be thus dispersed by it. If aquatic it needs no special soil, if on land it is usually a clay-loving plant addicted to a clay soil.
A fungus, Puccinia polygoni, attacks the leaves, and it is galled by Cecidomyia persicariae, causing the leaves to curl. A beetle, Rhinonchus inconspectus, and a Homopterous insect, Aphalara calthae, feed upon it.
The second Latin name refers to its amphibious habitat. It is called Arsmart, Flatter-dock, Willow Grass, Ground Willow, Lake-weed, Red Shank, Ruckles, Willow-weed. Ruckles was the name given to a pond weed said to be dangerous to bathers by impeding their swimming - probably this one. The name Willow Grass is given to the land form because the leaves resemble those of a willow.
It is very difficult to eradicate from garden ground, in which it sometimes occurs.
Essential Specific Characters: 268. Polygonum amphibium, L. - Rootstock creeping, stem erect or submerged, with floating leaves, aquatic form glabrous, terrestrial hirsute, leaves oblong, cordate below, flowers pink, in terminal spikes, solitary.