This group of aquatic plants takes its nourishment from dissolved salts in the water, and while some remain in character all the year round, others form winter buds or turons which sink to the bottom of the pool in autumn and reappear in spring. Floating plants shade water and so inhibit algae. They also provide nurseries for young fish and shelter for countless tiny creatures which laterthe fry. Some, however, can become a nuisance and the lemnas (duckweeds) should never be introduced to outdoor pools or, in warm climates, salvinas or water (eichhornias).
Among the hardiest are the frogbits ( Hydrocharis morsus-ranae). When selecting oxygenators it is important to choose sorts which really are efficient in this respect and some are undoubtedly better than others. Among the most outstanding are the elodeas, particularly Elodea callitrichoides and E. canadensis (syn. Anacharis canadensis), the Canadian pondweed. Both have small, deep green, narrowon branching . Elodea crispa (more correctly Lagarosiphon major) resembles a giant elodea with curly and it too is a splendid oxygenator.
The starworts take their name from the fact that the floating leaves of the spring starwort, Callitriche palustris (C. verna), form starry masses at the water surface. They are pale green in colour and help to provide shade early in the season. The autumn starwort, C. hermaphroditica (C. autumnalis), with finer leaves, is more active in the fall and remains submerged.
Crassula (Tillaea) recurva is a succulent plant with needle-fine leaves and minute white; all the milfoils ( Myriophyllum sp.) are good; also certain of the pondweeds or potamogetons, particularly Potamogeton crispus, the curled pondweed, and P. densus or frog’s lettuce. These are small plants with leafy, rather brittle . Caution should be exercised with certain other potamogetons and particularly with P. natans. This will rapidly take over a small pool and can become an obnoxious weed. For deep water both the hornwort (Ceratophyllum demersum) and the stonewort (Chara hispida) are useful; they have brittle stems with whorls of bristly needle-like leaves.
Among the prettiest of the flowering oxygenators is Hottonia palustris, the water violet. This has finely cut, fan-shaped foliage and, in spring, 6-in, spikes of pale, primrose-like flowers above the water. Later the plant forms winter buds and disappears until the following spring.
Ranunculus aquatilis, the water crowfoot, is another spring bloomer; its white, buttercup-like flowers starring the surface of lakes andin March and April. It forms long trails and has two kinds of leaves; the submerged sorts, fan shaped and finely cut; the floating kinds, smooth and resembling small buttercup leaves. This species will also grow in running water.
Utricularia vulgaris is the bladderwort and is a plant which does best in small shallow pools. The 4 to 6-in, emergent spikes of antirrhinum-like flowers are rich yellow and in character in late summer. Utricularias have small bladder-type contraptions on the submerged stems which trap tiny insects for food.