The plants in this group have only one thing in common – their roots do not need anchorage in soil. In form and behaviour they vary considerably and some are more interesting than ornamental. Azolla, Lemna minor and Lemna gibba form surface carpets of tiny individual plants that should be introduced only into pools whose size makes a few sweeps with a pond net a practicable method of controlling excessive growth. They, together with hydrocharis, trapa, and eichhornia, stay on the surface and discourage algae by cutting down the amount of sunlight that penetrates into the water. Stratiotes and Lemna trisulca contribute little in this respect since they appear at the surface only briefly. Azolla caroliniana. Fairy Floating Moss forms a crinkled surface carpet of mossy green, beautifully tinted with lilac and red in autumn. A fern and an annual, it is reproduced from spores shed in the water. Severe winters may kill it off completely. Eichhornia (Water Hyacinth). A beautiful tropical with bulbous-stemmed shining leaves and spikes of lovely lavender flowers marked with gold and blue. In Britain it may be left in the outdoor pool only in frost-free months and seldom flowers well. Overwintering indoors is essential, and by no means always successful.

Hydrocharis morsus-ranae (Frog-bit). Forms clusters of green leaves like a miniature lily and bears small white flowers. The plant disintegrates in autumn, dropping buds to overwinter on the bottom and rise to the surface in early summer to form new plants. The best – and safest – of the hardy floaters. Lemna gibba and L. minor. Duckweeds that make bright green sheets on the surface. Good fish food, perhaps, but do not expect fish to control their prodigious rate of growth.

Lemna trisulca (Ivy-leaved Duckweed). Remains submerged most of the time, is a good fish food and valuable water-clearer, and is not likely to make itself a nuisance.

Stratiotes aloides. The well-named Water Soldier: it forms a rosette of saw-edged bayonet leaves and looks like the top of a pineapple. It sits on the bottom mostly, but surfaces July/August to raise three-petalled white flowers.

Trapa natans (Water Chestnut). An annual, relying on the production of nut-like seeds for regeneration: in Britain they rarely ripen so it is a difficult plant to keep. A pity, since the platters of overlapping triangular bronzy-green leaves are most attractive. Utricularia vulgaris. A botanical curiosity of small ornamental and no practical value. A tangle of thin stems and thinner leaves bears tiny bladders said to be capable of trapping aquatic insects but more used, probably, as buoyancy chambers. Small yellow snapdragon flowers lift their heads several inches above the surface.

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