The key to patio gardening is simply high surrounding walls and an open sky.

In this 16th-century town of 300,000 people (the capital of Yucatan) hardly any of the old houses are detached, but run along in their pleasant congested way leaning on each other, with the interior rooms opening on a small courtyard.

Often as you walk down the narrow --see greenery through an open door and hear a little splash of fountain.

Suppose the court is 20 by 30 feet. There may be an arcade at one side with a stairway up to the second floor. Or more often there may be only the walls of the building itself.

The walls may be treated any number of ways. Usually they are stuccoed (the walls of Merida are limestone rubble, covered with a thin layer of stucco that may be moulded into pilasters and other fancies, or may be left plain) and given a coat of white or, more often, successive coats of earth colors like red and yellow iron oxides.

These are very soft colors. Sometimes mad emerald green or cobalt blue has been used at one point, then painted over later. Here and there only a trace of the former color shows through.

One reason this effect does not succeed in the United States is simply that the wrong materials are used here. Paint is not the medium. It is impossible to get, with paint, the soft transparent silky effect of old colored walls. And yet, if the various oxides are applied with a brush (look into concrete additives for the colors if you wish to try) the results will be just as they are in Merida or Seville or any Mediterranean town.

Because such a courtyard is constantly used for the traffic of the house, it is invariably paved. A soft earth-colored unglazed tile is best. Here in the north we would use brick, coated with a thin wash of buff or fawn or tawny-rose-colored cement.

The focal point of the court is commonly a raised bed of flowers, or a raised (maybe 20 inches above the pavement) octagonal or square or circular pool. Often this is simply stuccoed outside and colored either to match or contrast with the walls and paving. Or glazed tiles may be set into the Stucco. Just here let me warn against the use of too many tiles, not because they are gaudy and magnificent, but because they cancel each other out if used very abundantly.

In the best examples, there might be small (2 1/2-inch square) glazed blue or green and black and white tiles set in the unglazed pavement -- say, one every two feet or so. The entire outside wall of the raised pool might be faced with glazed tiles. It is inherently dumb to line the inside of a pool with tiles because they will not be seen. The inside of the pool (the pool is concrete or galvanized metal or lead) is left plain, however spectacular its outer tiles may be.

In a court this size there might be a large fan palm in one corner, a hibiscus or two, and a couple of the walls might support great clots of bougainvillea or rose-colored moonflowers or any other vine the gardener likes. In Washington, we would use grape or Carolina jasmine or Japanese clematis or Japanese honeysuckle or akebia or any other vine notable for grace. Or, for that matter, bougainvillea in a tub that is brought indoors for the winter.

Few shrubs or small trees are handsomer than crape myrtles in such a garden, especially since they flower in July and August when hardly any other first-rate plant is at its best.

It is a mistake, I have noticed, to stick colored tiles or brackets with pots of geraniums or other gew-gaws on the court walls. They look best plain, their only ornament being the clotted mass of a vine or two to contrast with their smooth surface.

A very small amount of ornamental iron goes a long way. One iron grille in an arched doorway can do wonders for such a courtyard, but this does not mean going to your friendly Swash and Mopbucket super duper store and inquiring what they have in the way of iron. No.

It is better not to have iron than to have the stuff commonly sold in the United States. Find somebody to forge the iron for you or order it from Spain or get it any other way you can think of. The trick is not to see how fancy you can find it, but to have the iron bars substantial and the design well-proportioned. The iron should be painted black. Period.

If there is a fountain, it is best for it to be a lead or bronze mask fixed to the wall with the water running out the mouth. I fully approve of lions, but anything that looks right would do. If the fountain is free-standing, then nothing is better than a simple pedestal like a fat balustrade with a very plain circular basin (it may be deep, it does not have to be saucer-shaped) for the water to run over.

Needless to say, there has to be a small pool or fountain basin beneath it. Fountains with sculpture are a mistake. The tremendous sculptured fountains of Versailles, for example, are even more vulgar than the rest of the place but at least they have several thousand feet of space around them so they do relatively little damage to the view.

In a small courtyard it is otherwise. Ladies holding swans or other baggage squirt as well as any other fountain, but never look right to me. No.A plain balustrade pedestal with a basin is the thing. Almost anything else looks contrived and cute, which is not what you want.

When in doubt, in a courtyard, put fewer things in it and make them larger. When in doubt, use fewer ornaments. When in doubt, use less ironwork. When in doubt, use the plainer pavement, however great the temptation may be to copy the marvelous one at St. Ildefonso.

As for plants, rely on box and yew, crape myrtle, grape vines, osmanthus, yuccas. Do not plant a batch of morning glories. Do not plant climbing roses. And let the plants, whatever they are, count as distinct bold masses against the plain walls. Sometimes in Merida they err by planting too many vines, thus losing the contrast of texture between masonry and foliage and winding up with a green wall.

A tub of the night jasmine (Cestrum nocturnum) or common basil or lavender verbenas, or yellow lantenas, might do well. One handsome light would probably do the trick at night. Fine effects can be had -- one of the few advances of the present century -- with concealed lighting.

There you have it. Plain soft-colored walls, glossy and noble foliage and not too much of it, good serviceable handsome paving, a bit of water, a few (again, very few) glazed tiles if you like, and a few spots of color from flowers, and you have it made.