Making gardens in the sky. For urban dwellers, greenery brings charm to the concrete jungle

MY husband and I have sat under apple trees, picked herbs, and babied snapdragons on our Manhattan roof 14 stories above Park Avenue for more than a dozen years. We know firsthand that the best investment an urban dweller can make is a garden, be it a few feet on a condo balcony or the whole roof of a penthouse. Leaf patterns and bright blooms charm concrete as nothing else can -- the harsher the environment, the greater the garden's gift.

The most rewarding gardens are those designed to be viewed from indoors as well as enjoyed outdoors. What the garden suggests is almost as important as what it does. A small table and two ice- cream parlor chairs beside a young willow imply a world of outdoor living, even if it's mostly stage set; a Chinese ceramic garden stool, chair, or chaise longue beside a pot of geraniums with ivy at its feet makes promises of rest and comfort. Floodlit at night, the smallest rooftop garden is magical. Designing the garden

A garden is any grouping of plants that unfolds the seasonal play of earth and sky with skill and taste. Space can always be found somewhere. If there's no horizontal space, plan a garden in moss-lined mesh baskets or big clay saucers hanging outside a sunny window. If there's just enough space to stand or sit and a scrap of sun, design the garden in groups of pots and hanging baskets.

Small apartment balconies usually accommodate one 18-inch deep planting box 12 by 36 inches, along with several 12- or 16-inch pots, a strawberry jar for herbs, a few hanging baskets, and even a modest trellis to support a tomato plant and a climbing rose. Larger spaces can handle everything from corn to waterlilies.

Since the largest plant will be the dominant one in the overall plan, choose it first. Then select secondary plantings to complement and contrast with its mature form -- columnar, weeping, horizontal, and so on. Contrast adds visual tension to a design; repetition enhances basic shapes.

To winter successfully in a cool climate, a tree will require a planter 3 feet across and 2 feet deep. A shrub succeeds in a tub 2 feet across and 20 inches deep. Use chalk to outline the space your largest plant will take, then you'll know how much space you have left for other elements.

Before leaping to implement a plan, submit it to the building supervisers for an OK. If it's going to be an ambitious rooftop project, check it out with the fire department and the city as well. Be aware that planters must be three inches off the roof or terrace to avoid possible damage to ceilings below. And make sure water for the garden will be available and handy.

Roof surfaces that are neither tiled nor decked (tar paper, for instance) are fragile, especially when hot. Get permission to deck or tile the garden before proceeding to design. In the Northeast, wood decks require annual rejuvenation. White asphalt tile is the currently favored low-cost tiling material used in luxury penthouses in Manhattan. Baked red ceramic tiles are gorgeous and more expensive than Fort Knox. Light is the key

Light is the first requirement of all green plants. Study the number of sunlight hours your garden space offers, and chalk outlines on the floor and wall areas where there is full sun (at least 6 hours), half sun (3 hours), and low sun (less than 3 hours).

In her book ``The Complete Urban Gardener'' (Harper & Row, 1985), Joan Puma gives good basic lists of trees, shrubs, and flowers suited to rooftop gardening -- and she groups them according to light preferences and regional hardiness. This is information that a garden design must be built on. Some species of tree, shrub, and flower will thrive in whatever light is available, but for most to succeed the right plant must be in the right place. Some of the showiest blooms thrive in low light -- the brillia nt impatiens, for instance, and begonias.

To upgrade the light available, paint walls and floors white, or a light color (if permitted). Facing dark corners with mirror can double the footcandles of light plants receive, and the vision of a garden you receive. Flood-lighting can lengthen the light hours available in dark corners and solve the shadows cast by overhead balconies. Water is crucial

Moisture is the other crucial factor in the success of an urban garden. High winds and hot sun suck moisture from leaf and soil surfaces. That's why large containers are preferable, and soil mixtures should include at least half moisture-retentive organics such as peat moss or vermiculite (makes for lighter containers, too, and that's good). To compensate for lack of nutritive value in such soil mixtures, feed often -- before planting in the early spring, monthly thereafter until cold weather. MD BRHouseplant foods are suitable. Use balances such as 7-6-9 for rapid foliage growth, 5-10-5 for young plants where lots of rapid root growth is wanted.

Water containers daily in hot weather, and keep the soil moist to the touch at all times. Not soaking, moist. A tree in a 36-inch diameter tub needs at least one big bucketful of water a day. If possible, include in the garden budget the cost of an automatic watering system with an electric timer. The best you can afford. It will cost less than a plant sitter when you go on vacation, and less than a lost garden. The systems are made of plastic tubing with leads for individual pots and plants and they ar e easy to install.

The more soil a plant has, the less often its growth is checked by drought and heaved by spring's frost and thaw cycles. This means the bigger the container, the better the plants like it. However, containers must first be scaled to the size of the plants they will hold and to the garden they will grace. One planter 18 inches deep and 36 by 12 inches in length and width will hold a rich variety of flowers and perennials, more than you will get from six 6-inch pots, or three 12-inch pots taking up equiva lent space. Pots are best used to give flexibility and variety to displays. Geraniums, impatiens, mums in late summer and fall, and pots of foliage plants, such as ferns and silver mound (artemis Schmidtiana), can be moved to bring color to dreary corners, to enhance the plants' light reception, or to hide blooms gone by.

Choose quality containers and empty and store them upside down for winter. Or look for containers that can be dumped when they fall apart. Spray-painted buckets that held ice or chips, bushel baskets, burlap bags, anything roomy that can give shape to a heavy-duty plastic bag can be a planter. Make several drainage holes in the bottom before you add the soil. Plants growing in water-logged containers drown from lack of air.