These are placed first because, though having little ornamental value, SOP are the most important functional plants in the pool. They consume mineral salts dissolved in the water and are thus a powerful factor in controlling algae. For fish they provide food, shelter, a spawning medium, a refuge for fry, and oxygen. All plants produce oxygen, of course, under the influence of light, but those which have their foliage on or above the water surface release it into the air. SOP are invaluable because they produce oxygen abundantly, and directly, into the water.
Most are planted as bunches of unrooted. In soil, and not, as is often thought, ‘just dropped in’. Usual stocking rate is 1 bunch for every 2 sq. ft. of water surface area in pools up to 100 sq. ft.; in larger pools 1 for every 4 sq. ft. will be enough; beyond 500 sq. ft. the rate can drop to 1 bunch for every 5 to 6 sq. ft.
These produce splendidin succession from June to October. Their floating shade fish and, by off light at the surface, help to control algae and keep the water clear. Varieties vary in vigour of growth from pygmies suitable for bowls to giants with leaves like soup plates that tolerate, but do not need, 3 ft. of water. All, in fact, will flourish within a range of 10 to 15 in. of water depth. Lilies vary proportionately in surface spread. Ignoring the very tiny and the very large growers, a rough guide to planting numbers is one for every 20 to 25 sq. ft. of surface area.
This is a group of plants whichin deeper water than marginal plants and have leaves and flowers in some cases on the surface and in others lifted well above it: they are untidily categorised as deep marginals though their place is not confined to the margins, but the term seems to be widely accepted and I cannot invent a better one. These plants are both ornamental and functional, in the same way as .
The plants that grow in shallow water in containers on a shelf and lift theirand leaves above water – for example water irises and bulrushes – are known as marginals. In functional terms a pool can do without them, but many have highly decorative flowers and leaves. Pool perimeter rather than surface area is relevant here. As a general guide about one third of the pool’s perimeter in feet should be planted with marginals along the shelf.