Water turns green because the mineral salts it contains, the sunlight it absorbs and the carbon dioxide given off by fish and pond animals, are just what the minute spores of green algae need to exist and multiply. When present in their millions, these tiny organisms clutter the water to such an extent that everything else in the pool is invisible. Changing the water is no more than a temporary solution because the process will immediately begin all over again, and in any case water-lilies require undisturbed water. The pond looks hopelessly polluted when vast amounts of algae are present, although, in fact,they are not harmful to fish who will happily feed off these organisms. Algae are simply unsightly.

The answer to clearing a pond of algae, or at least to radically reducing their number, is to starve them of their needs. Water weeds (or oxygenating plants, as aquarists call them) will fulfil this function. Actually to describe underwater plants as oxygenators is really only to describe half their function, and not necessarily the most important one. Oxygen is absorbed by water through surface contact with the surrounding air and plants add to that process. But they also absorb the carbon dioxide and mineral salts essential to algae. Your new pond, freshly filled with water, will have a crystal clarity. But as you will discover, that state will not last. Quickly or slowly, the water will discolour until it becomes a murky green. The process is inevitable. But no less inevitable is the process by which this state is reversed. Without changing the water, the original clarity can be regained permanently. Having built your pond, the next step is the judicious planting of underwater plants.

And once the oxygenating plants have appropriated all the available food, the algae, in the unequal struggle for survival, simply die, and the pond becomes crystal clear. Do bear in mind, however, that between filling the pond with water and the establishment of oxygenating plants, algae will thrive. A ‘pea soup’ period has to be endured in the confidence that if you have a sufficient number of oxygenating plants they will win in the end, and the algae will be reduced, as one writer so brightly put it: ‘to a fine shower of corpses’.

And with only temporary lapses, the water should remain permanently clear. The lapses are most likely to occur in spring after a sudden spell of warm, sunny weather. The algae will immediately start into growth and conditions will be favourable to them. During the dormant winter period there will have been a build up of waste animal matter. The pond will be in a eutrophic or nutrient-rich condition. And this state will remain until the oxygenating plants are again growing actively. Then the pond will return to a comparatively oligotrophic or mineral-deficient condition and the water will clear. And it is surprising how quickly the change occurs. One day you may be glumly looking at the ‘soup’ and the next into water almost as transparent as a mountain stream.

It is impossible to say what amount of oxygenators any particular pond needs. The factors are far too complex and variable. They would include the amount of sunlight the pool receives, the depth and temperature of the water, its mineral and animal content and much more. But it can be said that all underwater plants will carry out the dual function of removing carbon dioxide and adding oxygen to the water. Their presence is absolutely essential to clear water. As a rule of thumb — and it is no more than that — if a third of the total volume of the pool appears to contain oxygenators, then that should be sufficient. But there is no reason why the pond should not contain more. The propagation of many oxygenators is very simple. All that is required is to snip off the top few centi-metres of a stem. Stick this in the soil and it will develop quite happily. Alternatively, a whole plant with its roots can be weighted down with a piece of lead and simply dropped into the pond to root naturally on the bottom. Strips cut from an empty tooth-paste tube make good weights. All underwater plants reproduce vegetatively and they can often be divided up after only a couple of months. Instead of laying down a layer of soil over the whole base of the pond, it is now common practice to grow plants in baskets. When the pond is made of concrete either way is possible. Liner pools require the use of baskets because stones contained in a layer of soil might puncture the liner when walked upon or when soil is being removed. Oxygenators grown in baskets will very soon creep over the sides and root themselves in any debris on the bottom. So when you want to reduce their number, all you have to do is remove the baskets, take out all the surplus weed in the bottom of the pond and then return the baskets. Most oxygenators will quickly take over a pond, so baskets are a useful way of keeping them in check.

Many ponds are oxygenated and purified by one or more of the Elodeas. The North American Water Thyme or Ditch Moss, Elodea canadensis (Anacharis) is an excellent purifier but also a most vigorous one especially outside its native habitat. It may have been recorded in Ireland in the 1830’s and reached England in the early 1840’s. But the most celebrated transference of the plant occurred in 1847-8, when the Curator of the Botanic Gardens at Cambridge, England, received specimens of the plant from a Professor Babington and placed them in a tributary of the River Cam. Within a few years the plant had monopolized huge stretches of the Cam, making swimming and boating all but impossible, and actually raising the level of the river. The weed became known as ‘Babington’s Curse’. Its prodigious ability to spread was carried out purely vegetatively, as only female strands of the plant had been imported. E. canadensis caused the same problem in other places where it was planted or transferred. Then after some years its vigour, for some reason, declined and the plant became a civil enough member of the plant community. When introduced to ‘new’ water it does seem to regain some of its lost vigour and may not settle down for some years. When I first used the plant I found my pond was very quickly taken over. Then after about five years its rate of reproduction seemed to slow down. If your pond is small enough to be swept clean of the surplus by hand, there is no real problem. The surplus makes a fine fertilizer.

For large ponds, you could use Elodea densa (Egeria densa) which is also an excellent oxy-genator while being less prolific. The two plants are very similar in appearance and they are not easy to distinguish. The easiest way of identifying them is through their flowers. Densa throws up conspicuous three-petalled white flowers above the surface, whereas if canadensis flowers at all, it will have tiny pink or white flowers, floating on the surface on the ends of long strands. The leaves of canadensis are produced in whorls of three or four and each leaf is usually minutely toothed. (You may need a magnifying lens to make out the teeth.) Leaves of densa are toothless or nearly so and produced in crowded whorls. And there are usually four leaves to each whorl. In a mature plant the leaves will be 2.5 cm. Long at least, whereas the leaves of canadensis should be a great deal less than that. Elodea callitriclwides is larger than the other two, having leaves over 3 cm. Long in whorls of three or occasionally two. Elodea crispa (Lagaro-siphon major), which has alternate leaves about 3 cm. Long, is more suited to the aquarium than the outdoor pond as it is not fully hardy. Incidentally, when identifying these plants, bear in mind that considerable variation occurs from locality to locality. The length of leaves, for example, can vary greatly.

If you are collecting your underwater plants from the wild rather than from a stockist, wash them well before introducing into the pond if you have fry about. There may be parasites attached to the leaves. Adult fish will usually feed off any insects that come into the pond in this way. Many underwater plants are notoriously difficult to identify even for the trained botanist (and let it be said that classification of underwater plants is neither exhaustive nor always clear), but unless you have a huge pond in which rampant weeds are going to cause a problem, strict identification is not necessary. All plants that produce underwater leaves (as opposed to those whose leaves begin under water only to rise above it later on), will function as water purifiers. If you are collecting ‘blind’ however, concentrate on still water. Some oxygenators can adapt themselves to both running and still water, but others are only capable of surviving in one situation. And in-troduce several different species into your pond, not only for the sake of variety, but also because some species will probably do better than others under the conditions particular to your pond.

The Callitriches comprise a large genus that one is certain to find in the wild. The Common Water-Starwort, Callitriche stagnalis, is to be found throughout much of Europe. It is well suited to cold water but not to warm, hence its absence from Mediterranean areas and from indoor aquariums. It has a preference for shallow water and will continue to grow during the winter — a useful feature. So too will the Autumnal Starwort, C. allium-nalis, which, however, is not fussy about temperatures. It also has the advantage of not breaking the surface, a feature shared by the Hornworts, Ceraiophyllum demersum and sub-mersum. If you have a deep pool, demersum is a particularly good choice as it will fill whatever depth is provided for it. The Willow Moss, Fontinalis antipyretica, is another excellent oxygenator and a beautiful one into the bargain. It has long foliage, not unlike seaweed, but of a rich, vivid green. Obtain a whole plant with the roots intact if you can. It is more likely to be found in running water, attached to a rock or stone, than in still water. As the name anti-pyretica might imply, it is non-combustible and was once used as a form of fire insulation be-tween chimneys and walls.

If you like the idea of including an oxygenator with attractive flowers held well above the surface, then one can hardly do better than to obtain the Water Violet, Hottonia palus-tris, which produces whorls of light purple or sometimes white flowers. In the American form, Hottonia inflata, the flowers are always white. The Water Violet is not always an easy plant to transfer. Featherfoil is another name for the plant, not to be confused with Parrot’s Feather, which is the common name ly regard this lovely plant as an ideal oxygenator since it likes nothing better than to rise out of the water and, given the opportunity, hang gracefully over the edge of a bank or side of an aquarium. It has been used on occasions in hanging baskets filled with water, so that the finely cut leaves and the gorgeous light green colour of the plant appears right before one’s eyes. The lovely feathery foliage, as well as the colour, certainly justifies such treatment. And Parrot’s Feather is well worth including in the pond for its ornamental value. It does need to be wintered indoors as it is not fully hardy. Spiked Water Milfoil, Myriophyllum spicatum, is quite hardy, has feathery foliage too and is common in many waterways in Europe. In summer it sends up very small but nonetheless conspicuous red and yellow flowers, which is an added bonus in a plant which is an excellent purifier. More widespread is Myriophyllum verticillatum which is found in North America and Australia as well as Europe. The plant is of similar appearance to the spiked form but the flowers are insignificant.

For a mass of white buttercup-shaped flowers floating or just raised above the surface, grow the Water Crowfoot or Ranunculus aquatilis. It is a common and very easy plant to find and it is a good example of how adaptable nature can be. The plant has two distinct kinds of leaves. Those under water are finely divided, which enables the functions of collecting carbon dioxide and giving off oxygen to be carried out efficiently. The leaves on the surface are not more than deeply lobed and are, of course, quite flat so that the leaves can float and maintain their equilibrium. A carpet of Water Crowfoot running the length of the bank is an attractive feature in spring when the flowers appear. If you want to use it as an oxygenator, plant it in reasonably deep water otherwise the underwater leaves may not develop. Two other plants are worth mentioning, for if you do not find them in the wild, you are sure to find them in an aquarist’s shop. One is Tape Grass, Vallisneria sprialis, which bears long, twisting and strap-like leaves emerging from the base. It forms runners and so useful colonies of these plants develop in a tray or in soil laid on the bottom of the pool. The other, Poiamogeton crispus, Curled Pondweed, has broad, buckled and crimped leaves of a bronze hue, which makes the plant distinctive in any pond.

So-called floating plants, those which for at least part of the year live on the surface of the pond, are not effective as oxygenators. But they do play a part in keeping the water clear in so far as they cut down the amount of sunlight the algae receive and by using some of the nutrients in the pond. They are primarily grown for their ornamental value. The Common Duckweed, Lemna minor, is a very familiar sight: those little bright green discs which cover many a wild pond and ditch. Fish may enjoy it, but it is rather prolific and if you introduce it into your pond, you may involve yourself in tiresome thinning-out op-erations. The same is true for all the Lemnas with one notable exception. That is Lemna trisuka, the Ivy-Leaved Duckweed. It consists of oval or elliptical fronds of a greeny transparency, and the fronds are linked together by thin strands. If you have experienced trouble in keeping your pond clear, get hold of this duckweed, it is excellent for purifying water. If you have seen the Fairy Moss, Azolla caroliniana, you are bound to have been struck by its delicate tones of green in summer and vivid red in autumn. It is, without doubt, a beautiful plant; but be warned, it is not simply prolific but can take over a pond in the manner of a wall-to-wall carpet. So thorough is the Fairy Moss in covering the surface of a pool that it has been used in Panama and elsewhere to prevent mosquitoes laying their eggs in the water.

Rather like a miniature water-lily is Frogbit, Hydrocharis morsus-ranae, which has light green, round to kidney-shaped leaves not much larger than a thumb nail, and little white flowers. In winter the plant dies off but not before producing terminal buds which become separated from the parent plant to lie on the bottom of the pool until the following spring. Of similar habit is the Water Soldier, Stratiotes aloides, which only makes a brief appearance on the surface and looks like the top of a large pineapple. For most of the time the Water Soldier is submerged. Like the Frogbit, it produces side shoots or bulblets which lie dormant until the spring and the mature plants only appear to produce their white flowers. The plants then sink back again. Stratiotes aloides is calcareous, that is to say it prefers limestone waters. And this may explain why it does not always transfer readily. On the other hand, it can be prolific; but the plants are so large (30 cm. Across) that they can easily be removed by hand as necessary.

If you are prepared to go to the trouble of wintering plants indoors, then you can try growing the most beautiful of all floating plants, the Water Hyacinth, Eichlwrnia crassipes (speciosa). The entire plant is a shiny light green. It would be worth growing for that alone, but the flowers are quite superb, a light lilac blue with a spot of yellow, borne on spikes held high above the plant. The bushy, dangling roots, incidentally, make an excellent spawning ground for goldfish. Another plant worth growing for its foliage is the Water Lettuce, Pistia stralioles, which indeed looks rather like a flat lettuce, one that has been gently sat upon. The leaves are a pale shade of green. This is not an easy plant to grow. It requires a temperature in excess of 20°C, and soft water; and it does better in shallow water with its roots within reach of soil. Finally, if you are lucky enough to be able to find them or obtain a few seeds of the Water Chestnut, Trapa nutans, a European native naturalized to a limited extent in North America, you can enjoy the fruits raw or roasted. They are sweeter before the shells have hardened. Apart from being edible, the plant is worth growing for its beautiful bronze and green foliage. It is an annual, the fruits rarely ripen in Britain and the plant appears to be growing scarcer on the European continent.

If you happen to live within easy reach of a horticultural establishment which specializes in aquatic plants, do not let your enthusiasm run away with you. Can I make a plea for a stretch of plain water? In every pond, whatever the size, the reflection of waterside plants is one of the finest qualities that water can provide. Too many floating plants and oxygenators breaking the surface will spoil or destroy that quality. And besides, there should not be so many floating plants on the surface as to detract from the greatest of all aquatic plants: the water-lily.

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