Floating Aquatic Plants
Floating aquatic plants are useful additions to the pool, not only being decorative, but reducing the amount of direct sunlight falling into the water, thereby making life difficult for water-discolouring algae. Like submerged aquatics they are capable of absorbing plant foods directly from the water thus providing stiff competition for the more primitive slimes and algae.
Floating plants can be loosely divided into carpeting kinds and individual free-floating varieties. The carpeting species, like the bright green native duckweed which infests ditches, ponds and waterways is familiar to all. One need not, therefore, stress the dangers inherent in introducing such species to our garden ponds, even though the enthusiastickeeper would doubtless defend their inclusion as being useful for providing green food for the fish. Only Lemna trisulca (ivy-leafed duckweed) is worthy of a place in the ornamental garden, for this has attractive, dark green, crispy foliage of well-restrained growth. It produces minute, greenish of little significance.
The azollas have no flowers at all as they are really aquatic ferns. Two species are commonly cultivated, Azolla caroliniana and A.filiculoides. Both are so similar that only a trained eye can separate them. Indeed, in many cases the species are sold mixed together and as they are both as hardy as one another, their correct identities would seem to be of little consequence.
Both form thick mats of lacy, bluish-green foliage which, in bright sunshine and at the approach of autumn, take on rich crimson tints. As winter starts to bite, the handsome congested fronds become brittle and decompose. However, warm spring sunshine revives the over-wintering bodies which start into growth again towards the end of May. This is often rather late in the season to be effective against algae, so I like to keep a handful growing throughout the winter months. A jam jar of water with a littlein the bottom which is stood on the kitchen window-sill will provide an advanced battalion to put out on the pool towards the end of April.
Similar precautions should be taken with Hydrocharis morsusranae (frogbit), for this retreats into tiny turions for the winter and is very slow to break into growth the following spring. Hydrocharis is a charming plant with rosettes of small kidney-shaped leaves and attractive, white, three-petalled blossoms. These are produced regularly throughout the summer, and together with its modest habit, make this a most desirable acquisition for the tub or sink garden.
Stratiotes aloides (water soldier) is a close relative of the frogbit, although it really looks nothing at all like it. Only the flowers bear any resemblance. Delicate creamy-white blossoms, which on the male plant are borne in clusters in a pinkish papery spathe, and singly in the axils of the leaves on female specimens. Both sexes are of the same general aspect, looking very much like floating pineapple tops with their stiff rosettes of dark green, spiny foliage and reproducing freely from in the same way as strawberry plants.
Eichhornia crassipes (water hyacinth) increases in the same manner. However, with this gorgeous, free-floating aquatic the young plants are of much greater importance, as they not only increase the species numerically, but ensure its safe passage through the winter. Eichhornia crumbles at the first touch of frost and must, therefore, be given protection. Unfortunately, old plants do not over-winter successfully. Planting young stock in a pan or tray of very wet mud and keeping it frost-free in a light window or greenhouse is the only recommended course of action. Although a plant or two floated in a tropical aquarium would doubtless survive as well.
All this fuss for a plant which can only be enjoyed in the pool from June until September may seem rather tiresome. But it is really worthwhile, for the blossoms are like lovely exotic blue and lilac orchids. They are borne on short, stiff spikes and arise from amongst clusters of dark green leaves with grossly inflated bases. These look like small green balloons and being full of tiny air pockets enable the plant to float successfully.
The various utricularias are not quite so sophisticated, although they do hold the dubious distinction of being one of the few carnivorous aquatic plants. At first glance one would think them to be akin to submerged oxygenating plants like myriophyllum and ceratophyllum, having very similar much-divided filigree foliage. However, lurking amongst this seemingly inoffensive green tangle are small bladders which ensnare and ingest all manner of aquatic insect life.
Utricularias do not float on the surface of the water, but just beneath it, only their flower spikes venturing above. In most species these appear during late summer and look superficially like those of an antirrhinum, but more sparsely distributed along the flower spikes. In our native Utricularia vulgaris (greater bladderwort) they are rich golden yellow, while in other less common species, like U. neglecta and U. inflata, they may vary between bright yellow and pale primrose.
Finally, no mention of floating plants would be complete without Trapa natans (water chestnut). Although strictly speaking, an annual, this handsome plant with its neat rosettes of dark green, rhomboidal, floating leaves and striking creamy-white blossoms, is usually self-perpetuating. As summer fades it produces liberal quantities of hardy, black, spiny nuts which fall to the bottom of the pool for the winter, yielding fresh young plants the next spring. It is useful to collect some of these over-wintering nuts and store them in a jar of water so that plants can be started off earlier in the season. But be sure not to allow the nuts to dry out as they rapidly lose their viability.