If you were to ask ten water gardeners what they considered to be the worst problem associated with, you would probably receive but one single answer from all ten. A that leaks constitutes the worst problem.
No water, obviously, means no pond. Fortu-nately, leaks are rarely so severe as to leave a pond empty. But any leakage that is discernible creates problems. The constant topping up of the water level with the garden hose is not only an added chore, but also adds unnecessarily to the chemical content of the water upon which algae thrive. Tap water will also tend tothe water in the pond which in turn will reduce the vigour of the water-lilies and they may not bloom so freely. Evaporation during the hot days of summer is bound to lower the water level to some extent, and topping up cannot be avoided entirely. Leakage may increase the amount of topping up to an unsatisfactory degree. And finally, a leaking pond often means the inconvenience of having an area of lawn or pathway which is constantly soggy.
Concretecan develop leaks for a number of reasons, all but one of which can be avoided during the course of construction.
Only fresh and clean materials should be used. If there are any lumps in the cement these should quickly turn to powder when lightly rubbed between the fingers. If they are in any way hard, then the bag of cement should be rejected; there should be no lumps of cement in the mix. In the pond in which the cement, sand and aggregate have not been well mixed, soft, porous areas are bound to develop quickly. These areas will be immediately obvious by their pitted appearance and by the fact that they become powdery when given a sharp tap. This problem should never arise. It cannot be over-stressed: mix the ingredients dry, break down any lumps of sand or cement that form, and mix the lot thoroughly. And when placing the concrete in the site, do not throw it in but shovel it in with some care, otherwise the in-gredients may tend to separate. Some writers recommend giving the pond a facing of cement and sand in the ratio 1:3. The only reason for doing this is to provide the pond with a smoother finish. But the interior surfaces become virtually invisible once the pond has been filled with water and planted, so I do not see much point in finishing the pond in this way. It should certainly not be carried out with the intention of making the pond waterproof. The original concreting should do that. It is perhaps worth mentioning, since I know a case in point, that if you come across a massive boulder while excavating the site, it must either be removed or concreted right over (to the required thickness). It is no use concreting around such an obstacle because concrete and rock will not form a watertight bond.
Assuming that you have carried out the con-creting properly, the walls are of uniform thickness, the reinforcing wire is thoroughly embedded in the base and walls, and the foun-dation is perfectly stable, then the only other reason as to why your pond might crack lies in the formation of ice in winter. If you live in a climate in which several centimetres of ice is the norm, then I would be inclined to make the walls of the pond somewhat thicker than 15 cm. This is particularly important if the walls of the pond are perpendicular. Even a slight slant outwards offers a considerable safeguard against cracking. And the pressure of the ice can be reduced furthei ^» keeping a rubber tyre or a few lengths of s .n wood in the pond during severe weather.
If your pond does develop a crack there are few courses of action you can take. If it is no more than a hairline crack, clean the area surrounding it with a wire brush and apply a coat of bitumen paint or of rapid-hardening patent cement. The application, in either case, should not be confined to the crack but spread a fair distance around it as well. In many cases a waterproof join will result but there is no certainty about it. In the case of large or widespread cracks the simplest remedy is to install a liner over the concrete, having first removed any sharp projections. Otherwise there is nothing for it but to re-concrete the entire site. There is no need to remove the old concrete, except, of course, the reconstituted pond will be smaller in area by the additional thickness of the new walls and base.
Liner ponds will not crack, but they are probably more vulnerable to children who enjoy ‘messing about with water’. And what child does not enjoy playing beside, and given the chance, in water? A broken glass, a garden fork, not to mention the innumerable knick knacks with which children play, are all a potential means of puncturing the skin of Butyl rubber or PVC. To be sure, these materials can take a certain amount of prodding and misuse, nevertheless the possibility of a puncture is always present. Puncturing may also occur if any sharp stones or the like are not removed from the foundation before the liner is placed in. Land movement also offers a hazard: sharp stones may become exposed which had been concealed before. A subsidence of soil in the base or even the sides may leave a hollow beneath the liner and should someone step into the hollow then the liner may be forced to stretch beyond its elastic capability and a tear is inevitable. And the walls of the liner should not be placed under too much tension. Make certain the liner is lying smoothly over the entire base and fitting snugly into the corners. When making a raised pond this point should be particularly noted, as the temptation is to raise up the liner a little so that the sides are taut. Repairs to Butyl rubber and PVC are permanent, but do not underestimate the trouble involved in finding and repairing a leak. It requires emptying, cleaning and drying out the pond, and it may involve examining the liner centimetre by centimetre until the leak is located. Perhaps the old saying: ‘prevention is better than cure’ was concocted by an old pond builder!
In hot weather, the occasional topping up of the pond should not greatly increase the algae growth. But there is one situation where, regardless of the topping up involved or the number of oxygenators present, algae seem to thrive at a high level. The reason for this — all else being equal — is the ratio between the surface area of the pond and its volume. If the pond is very shallow in relation to its surface area then there is proportionately little water to absorb the light and heat and algae may thrive. The worst shape for a pond is a saucer shape: broad, gently curving sides and little depth. Ponds that are very shallow are unsatisfactory for other reasons too. The shallower the pond the more rapidly will the water change temperature. Neither water-lilies nor fish care for that. Moreover, in winter, fish like to hibernate at a reasonable depth, out of the way of ice and prey. Floating pond heaters are now available which will keep an area of water free of ice during cold weather. The idea is that toxic gases can thus escape from the water and oxygen enter it. However, fish use very little oxygen while
hibernating, and provided your pond is free of deadand is of a reasonable depth, there is no need of such mollycoddling. Incidentally, never crack ice: the vibration can seriously affect your fish’s balancing mechanism. It is safer to melt it with water from a hot kettle if you must. Snow over ice may be more of a problem than the ice itself. If light is completely excluded from the pond over a long period, it is possible that the oxygenating plants will die off. Brushing the snow off a section of the pond should remove this risk.