THE IMPORTANT thing about tennis is that the backstop fences afford a great support for vines. And then, later, the area of the court will be found just tight for a rose garden, a terrace with herbs, a water-lily pool with arbor, or a vegetable patch.

The same thing is true of swimming pools, which can be converted with no trouble at all to a sanctuary for carp and aquatic plants, once the chlorine and splashing about are stopped.

But some, I know, will prefer tennis and swimming pools to even the most sensible alterations, and today we shall modestly confine ourselves to the wire fences commonly seen around both courts and pools.

Often I have heard people say, "But we can't plant anything there, the whole thing is paved in concrete."

Let me commend to everybody the jackhammer, the spike and sledge hammer and the mattock.

I once undid a whole stone wall of ashlar with a screwdriver and an ordinary hammer.

What idiots install, wise men can remove.

High tennis-court fences support climbing roses nicely, especially the ramblers that have long supple canes; things like the old 'Dorothy Perkins' so subject to mildew. But also the vigorous (I would not actually say rambunctious) ramblers like 'Gardenia' or 'Alberic Barbier' or 'Silver Moon.' Or 'New Dawn' or 'Blaze,' but a good bit of work is involved keeping the last five close against the fence since they like to bash out into the shape of a loaf of bread. That is good of them, but tennis players, as I understand it, dislike crawling beneath a low canopy of thorns in search of balls.

The sort of rose that does well, among the more supple kind of ramblers, is 'Violette,' for instance. This is a virtually thornless plant that ought to have most of its old canes removed every year, so the vigorous new canes (on which next spring's flowers will appear) can shoot up without obstacle.

The flowers are small, black-red, turning to purple and scentless.

Of course, gardeners either love the purple roses or not; and I notice the firm from which I acquired my plant no longer grows it, so slight was the bernand, and it may be procurable now. But it is just the sort of rose that's fine for tennis fences.

Another thornless rose, which is not vigorous enough for clothing high fences but is fine for lower ones, up to six feet or so, is 'zephyrine Drouhin,' which repeats its rose-pink (a good bit of blue in the pink), highly scented flowers off and on through the summer, though the only time it is showy is in its main spring flowering.

Once in Mississippi, on about the fiftieth of July, I visited friends who had a swimming pool out in the middle of the country without any trees around it, and a great distance from the amin farmhouse.

To thwart (and thereby maintain undrowned) tots, the pool was completely surrounded by a fence that must have been 20 feet high. They never were ones to under-build, on that farm.

They had planted cypress vine on the wire. The result was a sheer delicate feathery green cliff, spangled all over with bright red tiny flowers.

If this cypress vine (quamoclit) is planted from seed early in May, in a nicely dug up strip of earth at the base of the fence, it will make a feathery wall in the matter of a few weeks. Seed also can be started in paper cups, retrieved from drinking fountains, about the first of April and planted out in mid-May.

One of the most amazing of vines is the moonflower, Calonyction aculeatum. Like the cypress vine, it is tropical and easily raised from seeds, soaked a day before planting in pots of paper cups indoors and set out in the open garden in mid-May.

It is vigorous enough to do excellent work on a tennis fence, but should be within easy sight at night, when it flowers. The blooms are like morning glories; they open about 9 p.m., snowy white, and perhaps the size of saucers. It is highly perfumed, a nice sickly sweet.

Morning glories are obviouys, no doubt, and none the less beautiful for that. Everybody knows 'Heavenly Blue' and similar varieties of that type. Remember they all like full sun.

A grossly overlooked morning glory is Pharbitis learii, from South America. It used to be in every greenhouse, but is not common now, since greenhouses have shrunk in size and there is no great demand for vines to romp about the rafters.

This subtropical morning glory bears its flowers in clusters, deeper in color than 'Heavenly Blue' with a good bit of red masked by the seemingly pure and intense blue. Thus when the flowers fade, around noon, they trun magenta.

Possibly some gardeners, including me, are part bee. Bees are said to go quite to pieces over blue, and some of us certainly do. I never grew anything more gorgeously bule than this morning glory.

If young plants were set out in May, they would reach the top of the average fence by early July and bloom for some weeks. Then if the roots were covered with a bushel basket or two of mulch in November, the vine would almost certainly prove hardy in Washington, especially if planted on a wall facing south, and sunny.

Once I persuaded a skeptical newspaper fellow to plant this vine on a south wall of his bleak new house, in the same climatic zone as Washington. And for some reason he did so, though as a rule he is somewhat pig-headed.

It grew like mad, and proved soundly perennial, and even set seed (which P. learii is not supposed to do in gardens) and for a few months there, he thought I was God. I explained to him that I was not; I merely knew a few more facts then he did, which was a very easy thing to do, under the circumstances.

Now a gardener does not have to go quite as berserk as my old friend Jim, to see the glory of this excellent vine. I wish some of the garden centers, which are forever shipping in stuff anyway, would get a supply of Lear's morning glory in pots for May.

This is the merest surface we have scratched here. This may be the place to say most gardens would give more pleasure if there were a lot more vines in them. Anybody who can grow a zinnia or marigold can certainly succeed with any I have mentioned today. All of them, alas, require sun.