Waterside and Marsh Plants

Planting the pool surround is a separate exercise from the stocking of the pool itself, and will depend very much on the moisture content of the soil. The banks of streams and natural ponds, where a constant supply of moisture is assured, provide ideal conditions for such traditional waterside plants as astilbe, trollius, rodgersia, Iris sibirica. Iris kaernp-feri and primula.

But the situation round an artificial pool, where the soil is completely separated from the pool water by a barrier of concrete or plastic, is not likely to be naturally suitable for these moisture lovers. If, in spite of this disadvantage, you are still keen to enjoy the beauty of the so-called ‘bog plants’, is there any way in which suitable conditions can be created for them in conjunction with a pool of artificial construction? There is, but before looking at ways and means let us look closer at the plants in this group to see exactly what it is they need.

One thing they certainly need is a different name from ‘bog plants’. For me, with memories of the Dorset heathlands, a bog is a spongy morass of sphagnum and coarse grasses and very acid water, and bog plants are sundews and butterworts and bog myrtle. In a genuine bog of this sort the plants we want to grow would simply curl up and die so, at the risk of being thought pedantic, I suggest we stop using the term bog plants and call them marsh plants instead.

I have another Dorset memory, of a stretch of lush water meadows. The soil is not sour and acid, but a good rich loam. A stream runs clear and shallow through beds of starwort and water crowfoot to lose itself in a thicket of willows and alders where sweet flag and reed mace grow in muddy shallows between primrose-covered islands of firmer ground. On the wettest parts of the open meadows colonies of yellow flag iris flourish while clumps of shining marsh marigolds grow equally well in the sunny meadows and in the willows’ shade, in waterlogged hollows and on the firmer mounds between them. Later in the summer, meadowsweet and loosestrife will raise their plumes along the stream banks, in soil that remains above water level but always has an ample reservoir of moisture beneath it.

Here are exactly the plants we want to grow – iris and caltha, acorus and primula, lythrum and spiraea. Note that they are not sitting in a soggy pudding of sour soil and stagnant water: there is sweet water moving through the soil all the time. Note that they all need water but in varying degrees. Their distribution is determined primarily by the level of water relative to soil – and root – level, and they fall broadly into three categories. There are the swamp dwellers that flourish where the soil is covered permanently by a few inches of water. There are those that live on or a little above the waterline, with their roots in waterlogged soil. And there are those that must have their crowns well above the level of saturated soil, but are able to send down roots to find the abundant moisture they need to make their summer growth.

The successful marsh garden must imitate these different levels, combining wet hollows with higher mounds and suiting the degree of moisture to the needs of different plants. Some, like the calthas, will adapt themselves cheerfully to life at any level of the marsh, but to most varieties their relation to water level is crucial and a few inches can make the difference between success and failure.

One of the most effective ways of creating a marsh garden is to incorporate it within the pool as an integral part of the pool design. As we have seen, it is customary to include shelves about 9 in. deep for marginal plants. If, on one side of the pool, this shelf was made several feet wide instead of the usual 8 to 12 in., then an area suitable for marsh plants would be created. The soil in this area, built up to provide the desirable variety of levels and to be at least 6 in. above water level in places, must be prevented from seeping into the pool by a retaining wall of bricks or stone low enough for the pool water to extend over into the marsh.

If the main pool already exists it will not be possible to tack on an extension. It is still possible, however, to make a marshy area close enough to the pool to appear part of it, although in fact there is no connection. This is clone by excavating the soil from the chosen area to a depth of 12 in. and laying a sheet of polythene. It can have a few holes and it need not be turned up at the edges, since the intention is to slow down water movement through the soil but not to prevent it altogether. Then soil is returned, liberally mixed with damp peat, and firmed to a depth of 6 in. over the polythene. On top of this goes a length of £-in. polythene tubing pierced with holes at 12-in. Intervals. Laying it straight across the middle or the diagonal will usually do, but for larger areas bends can be inserted to take it round in a U shape. One end of the tubing is stopped and the other left above the surface to be connected, via a length of garden hose, to the water supply in periods of drought. The rest of the soil/peat mixture is then returned over the tubing.

This method allows the area to be given a thorough soaking at the turn of a tap whenever it is needed -best indicated by signs of flagging plant growth – and without the surface-panning disadvantage of overhead watering. There is the nuisance of having to connect to a tap but, compared to the ‘extended shelf arrangement, this method has the advantage that water is not static but periodically moving through the soil and thus avoiding sourness.

The sui’face of the marsh garden needs undulations to give it character, as well as to provide niches for different plant needs, and some well-placed pieces of weathered stone help greatly in this – as well as providing the firm footing that will be needed here and there to facilitate planting and weeding. Remember-ing that the permanently moist soil constitutes an open invitation to weeds, constant vigilance is essential to prevent the marsh garden being swamped by undesirables, most often in the shape of coarse invasive grasses. The outcrops of stone may well run back to the beginnings of a rock garden beyond the edge of the marsh garden so that the one feature develops naturally into the next and creates a pleasingly natural overall design.

If you are dismayed at what is involved in changing the conditions beside your artificial pool to suit marsh plants, then you’ll have to forget about marsh plants and choose others that will flourish in the soil as it is. This may seem like taking the easy way out, but I can offer at least one reasonably logical idea with which you can justify such a decision.

The conventional waterside plants, like the lilies. And marginals in the pool, are perennial. When they are in growth they are all in growth together, and the chances are that one will spoil the view of the other. When the lilies and marginals die back, so do the marsh and waterside plants and throughout the winter all is bare and desolate. It’s all or nothing.

Might it not be better if the summer beauty of the marginals had a subdued but pleasing background that set them off instead of competing with them? And when the marginals died might it not be better for the pool to be surrounded by living foliage and flowers than by dead stems? It can be done, and done very easily with plants that will be happy in ordinary soil around the pool; and the plants that will do it are alpines, heathers and a conifer or two.

When the hole is dug to make the pool the easiest way of disposing of the soil is to mound it up at one end to make a low rock garden and provide a convenient site for a watercourse. Why not develop this theme with a sloping bed curving along the back and side of the pool, with a few large rocks all but buried in it as scattered outcrops? As the pool was deliberately put in a sunny spot, and the soil will be well drained and on the dry side, conditions will be perfect for many alpines. The most useful will be those that make a good splash of colour in the early part of the year before the marginals grow tall enough to hide them. There is nothing that will do this job so well as the bright lemon Alyssum saxatile citrinum in the company of aubrietas in shades of red, pink, blue and purple. Add the bright yellow of Achillea tomentosa, the soft blue of Phlox stolonifera Blue Ridge, the bright crimson of Phlox Temiscaming, and a golden carpet of Potentilla chrysocraspedia and there will be ample colour to brighten the surround until the pond plants take over.

The policy should always be bold groups of a few types rather than a spotty one-of-everything mixture. The same is true of the heathers that provide colour during the winter months. There are many varieties: I shall mention only four. Three of each of these will be more effective than a mixture of twelve different kinds, and six each of two varieties will be better still.

One of them must be Erica carnea Springwood whose flowers, produced from January to April, are pure white. The perfect partner for it is Erica carnea Pink Spangles which has the same low, spreading habit. Erica carnea King George is rich pink and starts to flower in December. Erica darleyensis Furzey makes wide cushions 15 in. high covered with pink flowers from December to April. When they are out of flower these tidy evergreens are no less pleasing to the eye. All grow as happily in limy as in acid soil.

It only remains to add the all-year-round form and colour of one or two dwarf conifers. Dark rich green colour and superb texture of crisply whorled foliage make Chamaecyparis obtusa nana a first choice. Of the spreading near-prostrate conifers I particularly like Juniperus horizontalis Bar Harbor or J. communis hornibrookii. For a touch of gold to catch the winter sunshine, and an interesting chunky upright shape to contrast with the spreading forms, Thuja orientalis aurea nana is highly recommended.

Now the picture is complete, the canvas of the pool framed in a varied, changing but always attractive setting. Only one thing remains to be said on the subject of planting around the pool. If you are thinking of planting a weeping willow-don’t. A 50-ft. Tree crouched over a modest-sized pool doesn’t merely look silly, it is a positive danger. The pool will never be free of decaying leaves and since they contain aspirin the toxic effect will be even worse than usual. Leave the willow to its proper setting, beside the lakes of stately homes and public parks.

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