This may seem a surprisingly early stage to discuss a problem that can only crop up after a pool is made and stocked. The problem is green water, a condition that all pools go through briefly but that plagues some so persistently that their owners are reduced to nail-biting despair. The reason for raising the spectre at this point is that the design of the pool has a pro-found effect on its liability to suffer from the green water nuisance. If we play our cards right at the design stage we can go a long way towards ensuring a well-balanced pool with clear water.
There are other factors that either encourage or inhibit greenness in the water and it will be necessary to refer to the phenomenon more than once in succeeding chapters. But since it is undoubtedly at the design stage that the battle for clear water can largely be won or lost, this is the time to summarise the nature and cause of green water, the better to understand the steps recommended to defeat it.
Wherever there is light and water you will find algae (pronounced Algy). Algae are simple types of plant life ranging in form from microscopic specks to seaweeds. The most common of those that affect the garden pool are the free-swimming algae which, though individually minute, produce a total effect that makes the pool look as if it is filled with green distemper or thick pea soup. Then there are the fila-mentous forms that grow in strands. The sorts that make short furry growths on lilyand the sides of the pool are of no consequence, but the one known as blanketweed, which makes tangled masses of long silky threads, can be a choking nuisance if it gets out of hand. Another, which forms fioating brown scummy blobs, occurs less frequently. Except for particularly heavy infestations of blanketweed, algae are an unsightly nuisance rather than a menace. But it’s not much consolation to know that fish are perfectly happy swimming around in the pea soup if it is so thick that you can’t see whether there are any fish in the pool or not.
Algae are the product of tiny spores that are drifting about everywhere and the quaint idea that blanketweed can only be introduced on plants is totally mistaken. Algae thrive under the influence of sunlight and warmth combined with high carbon dioxide and dissolved mineral salt contents in the water. Inevitably, therefore, they will appear in newly stocked pools in which the high mineral salt content of tap water is increased by the minerals dissolved out of the planting soil. In addition there is no obstacle to the maximum penetration of the water by sunlight because the new plants have not yet developed the growth that later on will cut off much of the light at the surface and also use up a lot of mineral salts.
So at the beginning everything is in favour of the algae and they duly appear and fill the pool with soup. This is natural, inevitable and harmless. If you ignore it and provided the pool is well designed and properly stocked with plants, it will go away. Unfortunately many pool owners panic at this point and change the water. Within a day or two it is as green as ever because all they have achieved is the introduction of a fresh supply of mineral salts with the new water and they are back to square one. If only they would be strong minded and not change the water they would find one morning-and it can hap-pen just like that, overnight – that the water was as clear as gin. And after that, except perhaps for a brief seasonal outbreak in the spring, the pool would remain clear and untroubled by algae for years.
This happy state of affairs is usually referred to as a balanced pool. In the biological sense I suppose that there is no such thing as ‘pool balance’ if it is taken to imply that the ecological processes in the pool arrive at a final condition after which there is no change. Far from it. Within the enclosed watery world of the pool, life flourishes in myriad forms, con-suming and producing, competing but interdependent, preying and being preyed upon, waxing and waning. But since most of them are invisible the onlooker is completely unaware of their cycles of increase or decline. Not so with the algae. When they are in the ascendant they are all too obvious. Pool balance, in the water gardener’s vocabulary, is a convenient term to describe the situation when a pool that was green becomes clear. The minute algae that swarmed in the light, warm surface water have vanished. Having consumed what mineral salts were left to them by the competition of oxygenating plants, and being deprived of light by the developing surface foliage ofand floating plants, they have dwindled away and died and sunk to the bottom.
In this way mineral salts are cleared from the upper water layers and deposited on the bottom. There may be an occasional ‘bloom’ of green water after this. If the accumulation on the floor of the pool is thoroughly stirred up – as in the course of pool cleaning or replanting – mineral salts will be redistributed in the upper water to nourish a fresh generation of algae. And in the spring, when warmth and light are increasing, algae may flourish briefly until overtaken by the new season’s growth of oxygenators and lilies. Apart from sporadic outbursts of this sort, usually quite brief, the pool when once balanced should remain clear.
And yet there are some pools that will never clear. They have their proper complement of plants and they have no constant inflow of mains or stream water tothe algae continually with fresh supplies of mineral salts. What is the difference between the pool that clears and the pool that stays green? The answer probably lies in the shape. Not the surface shape, but the sectional shape (or profile) that would be revealed if you sliced the pool in half and looked at it edge on. The difference this can make is illustrated by the examples below.
Here are two pools. They have the same surface shape: for the sake of argument say they are circular and 10 ft. across. That gives them both a surface area of about 78 sq. ft. One is 18 in. deep all the way across while the other is saucer shaped and 18 in. deep only at the centre. The difference in the contained volume of water is surprising. The saucer shape holds 380 gallons, the other 740 gallons, nearly twice as much. They both have the same surface area to admit sunlight, but one has twice as many gallons of water to absorb its effect. The smaller amount of water in the saucer will clearly be much more affected by light penetration and heat, making conditions far more favourable to the growth and persistence of algae.
Another effect of the low volume/surface area ratio is a quick reaction to temperature changes, as between night and day, producing much more fluctuation than is good for fish. The saucer shape is clearly a bad shape; the ideal pool section will be much nearer the cylinder, having the cushioning and stabilising effect of the maximum number of gallons for every square foot of surface area.
A variety of practical considerations, including the shaping of a firm pool profile when excavating in loose soils, make absolutely vertical sides undesirable. Sloping them at about 20 degrees from the vertical satisfies all needs without any serious loss of volume.
The design of the pool must take account of the depth of water required by various types of water plants. These will be referred to in more detail later: at this point I will ask you to take my word for it that there is no need to make an elaborate staircase of shelves at a dozen different levels. Two planting levels only will be entirely adequate. One will be the floor of the pool, and the depth of this is determined by two considerations: it must be deep enough to ensure an adequate gallonage of water for every square foot of surface, and it must b« a comfortable depth for the type ofappropriate to the pool size. Experience supports the following conclusions:
Any pool less than 15 in. deep is asking for trouble. 18 in. is deep enough for pools up to 100 sq. ft. in area. Beyond 100 sq. ft. a depth of 2 ft. is beneficial. At 300 sq. ft. the depth might well be 30 in. At 1000 sq. ft. or more 3 ft. is permissible. No pool, however large, need be deeper than 3 ft.
These figures are, of course, intended as a general guide rather than a precise specification. It is certainly not to be taken that an 18-in. Depth for a pool of 120-sq. ft. surface area would result in serious problems, or that a depth of 2 ft. would be out of the question for a pool of 80 sq. ft. I have indicated, for very arbitrary divisions of pool sizes, the depths that experience has proved to be enough.
At this point it may be as well to knock two widely prevalent notions firmly on the head. The idea that a pool must have an area 3 or 4 ft. deep either to protect fish in winter or to grow the more vigorous water lilies is entirely without merit.
A second planting level must be provided to accommodate the marginal plants such as water iris, reed maces, kingcups and arrowheads that flourish only in shallow water. They will be planted in containers 6 in. deep and most of them like 2 or 3 in. of water over their. They are well catered for by a shelf 9 in. below water level and 9 or 10 in. wide. You will certainly not want a complete hedge of marginals all round the pool so a continuous shelf is unnecessary. However the shelves are disposed to suit the individual pool design, a total shelf length equivalent to about a third o’f the pool’s perimeter will usually be adequate to accommodate the marginals in with plenty of space between them.
The pool now has two planting levels. One is the bottom, which for the great majority of pools will be 18 or 24 in. deep; the second is the shelf, which will invariably be 9 in. below the surface.
The surface shape of the pool will depend to some extent on where you put it. Wherever it is, it must be in the sun. Most aquatic plants, and water lilies above all, flower splendidly in sunshine and sulk without it. Sunshine for half the day will do, but more is better if you can provide it. When you cast about for a site avoid shady corners and also get away as far as you can from trees and shrubberies – not only because of shade but to avoid the pool being fouled in the autumn by an accumulation of decaying.
Bear in mind that the pool will need filling to start with and occasional topping up later, and make sure it is within the range of your garden hose. Or, to put it another way, make sure you have a hose long enough to reach the chosen pool site. Remember, too, that pumps, lights and pool heaters need electricity, and consider the length and route of the cable-run from the pool to the handiest undercover socket outlet. A run of more than 100 ft. or so could result in voltage drop.
The spot finally chosen will dictate the general lines of the pool. On aformal shapes are called for and a circular, square or rectangular pool will be most suitable. Placed at the foot of a rock garden, as if water had collected there naturally, a pool must have easy ‘natural’ curves. Even if it is in an area of open lawn its shape must still repeat the general pattern of the garden, complementing straight paths and right angles with a formal geometrical shape, or matching curving paths and borders with flowing, informal lines. Within these general limits the choice of shape is yours.
If you decide to use a glassfibre pool your choice of shape will be limited. There are circular and rectangular models available which are ideal for. The informal designs suffer, in general, from too much irregularity ofline: they have lots of wiggles but not much water area. I can think of only one that has enough stretch of water to accommodate a modest fountain without losing most of the spray beyond the edges.
If you use a plastic liner you will be able to make the pool shape whatever you fancy. But don’t let that fancy tempt you into an excess of narrow necks and promontories with edges zig-zagging like a demented snake. Avoid dumb-bells and crosses and L shapes and serpentine canals. If there is one thing I have learned from many years of making pools and looking at pools that other people have made it is this. Keep the shape open, keep it simple. If you have decided to devote a certain area of your garden to water, then fill that area with as much water as you can. Water is what you want, the broadest stretch of water you can squeeze in, to bear the spreading blooms of water lilies, to receive the falling spray of fountains, to catch the light and reflect the passing clouds. So keep it simple and keep it open.
How large should a pool be? At least 50 sq. ft. I would say, though there are certainly many successful pools that are smaller. Fifty square feet, after all, is not so much when you think in terms of a rectangle 10 by 5 ft. or a 7-ft. Square, or a circle barely 8 ft. across. The answer is to make it as large as you can manage. Limits will be set by the size of your garden or how big a bite out of your lawn you can spare. You will not want to be extravagant of space or of cash, but within whatever limits there are do make the pool as large as possible. The smaller a patch of water, the more it suffers from temperature fluctuations and the more difficult it is to get it settled and balanced.
To help you resolve the question of what shape and how large I suggest that you illustrate your tentative answer with a diagram. Draw it on the ground at the chosen spot, using a length of hose or a clothes line to represent the intended pool. Then stand back and consider: is it easily visible from the important points of view (of which the most important, I am given to understand, is the kitchen window)? Is it well served by paths ?(Visitors will always wanttomakeabee-line for the pool). Is the shape satisfactory? (If not, make adjustments). Where will it be best tothe marginal shelves so as not to obscure the view of the water with reeds and rushes? Where to the waterfall, if one is intended, and the backing rock garden if none exists? Above all, is the pool large enough?
Testing the possibilities in this way will often reveal scope for improvement and enable you to arrive at the final ideal plan without so much as making a scar on the turf. In particular it may avoid later regrets, when it is too late to do anything about it, that you didn’t make it the few feet larger that would have made so much difference.
You now know where the pool is going, its depth and profile, its dimensions and its shape. The next thing to consider is which of the pool-making materials and techniques available will most effectively, economically and easily translate your plans into reality.