Whether on one-story structures or on skyscrapers, rooftop gardens are havens with a charm of their own. For the owners, they provide private worlds in which to grow plants and escape the bustle of city life. All this, of course, is made possible with soil brought in and carried to the top of the building for the pots and boxes that comprise the rooftop garden. If you have ever seen a penthouse garden, you know what a feeling of space it gives, especially if the building is high. It is like being on a mountain top, with a panoramic view that on clear days seems limitless.
Delightful as these skyline gardens are, they do present problems. The wind, for example, snaps trees and tears up plantings. Arrangements must be made to provide shelter in the form of fences or other barriers. These also give needed privacy. Winds constantly dry out the soil so that in summer when the sun is hot, plants often need watering two or three times a day. Pergolas, lattice fences, wood panels, and laths can be erected to provide shade but still allow air and sun to enter.
Winter cold is another problem. In cold regions, where soil freezes solidly, evergreen plants are often windburned through loss of moisture that is hard to replace when the soil is frozen. The sun, too, draws off moisture and causes sunscald. Rooftop gardens only a few stories up are less affected by wind and are often easier to care for than plantings on the ground. They are usually protected by buildings on one or more sides and get sun for only a part of the day.
Roof Must be Strong
At the start make certain the roof is strong enough to support the weight of containers filled with soil. Modern buildings usually are, but you will be wise to have your structure checked by a building inspector. Then make sure that water can be drained away through pipes. Most important, build a wall around the edge of the roof high enough to serve as a guard. This can be constructed with some harmonizing materials such as concrete, brick, and wood.
The next step is to make a plan. On the whole, simple, formal designs are best in the limited area of a roof. Allow for some large boxes for trees and shrubs and for planters or raised beds, which will give the feeling of flower borders. Erect fences and lay out several enclosed areas for dining, sunbathing, and reading.
Only those who have lugged soil in baskets and boxes onto elevators or up flights of stairs know what this involves, yet without it there can be no garden. If you go to all this effort, obtain good soil, since the labor and cost for good and bad soil are about the same. As for containers, be certain they are large enough to hold sufficient earth. Shrubs, vines, and roses need a depth of eighteen inches; trees need considerably more. For perennials, annuals, and bulbs, a depth of ten to twelve inches is satisfactory. If boxes are equipped with wheels, it will be easy to move them around and water will pass through the holes without interference.
Since rooftop space is limited, try to have boxes fit specific areas. Here is your opportunity to introduce interesting shapes suited to the overall design. If you set up boxes in step fashion, you can grow more plants in a limited area. Allow some space for vines and espaliered plants to cover walls, fences, and other vertical surfaces.
Instead of adding soil to all the containers, fill a few of the largest with moist peat or sphagnum moss. Flowering potted plants can be plunged directly into these and be replaced when they are past their prime. This may be expensive, but it always seems worthwhile, and you do not have to replace a large amount of worn-out soil after a period of years.
Containers for Rooftops
All kinds of containers are suited to the roof garden. Glazed pots stay moist longer than clay, as do wooden tubs and boxes, which keep soil cool. If containers are not heavy enough to stand up in wind, they will need securing. Light plastic pots have to be reinforced by being closely grouped or placed in tubs, jardinieres, or planters.
With so much wall space, think what you can do with wall brackets. Try grouping pots of the same size, including some trailing English ivy or the weeping variety of lantana. Hanging baskets can be attached to walls if these are not exposed to strong wind. When suspending plants on walls, avoid positions from which they might fall on someone. Secure the pots with strong hooks and wire and keep them low enough to make watering easy.
Trees for Height
Every rooftop garden requires a tree or so for height. They also add interest of foliage and blossoms. Willows, which are fast growing and resilient, have been used successfully. When they get too large for their containers, they are easily replaced. Oriental flowering cherries, crab-apples, apple, pear, ailanthus, silk tree, linden, birch and upright maples or lindens for slender height are all good trees for rooftop gardens in the North.
Plan for some evergreen trees and shrubs for year-round color and mass. Scotch, mugo, and Japanese black pines, hollies, Japanese yews, pieris, mountain laurel, camellias, azaleas, and rhododendrons will flourish with some protection. Among the deciduous shrubs, privet, forsythia, spirea, firethorn, mock-oranges, lilacs, and viburnums have proved their worth.
Vines for Patterns
Vines cover bare walls and provide flower and foliage patterns. Wisteria is one of the best; but Japanese honeysuckle, bittersweet, the fast-growing Chinese fleece vine, Boston ivy, and Virginia creeper or woodbine are all excellent. English ivy, as a climber or ground cover, will hold its own on rooftops, though it must be kept out of strong winter sun in the North. Scarlet runner beans, morning glories, cypress vine, and moon flowers are annual kinds to try.
Roses for Fragrance
Every rooftop garden, even the smallest, should have some roses for color and fragrance, as well as their ability to take wind. Train some climbers over the walls and concentrate on such floribundas as Betty Prior, Pinocchio, Carrousel, Floradora, Spartan, Vogue, and Fashion. Miniature roses are ideal for small containers or for edging larger planters. You'll like the pink Sweet Fairy, the deep crimson Tom Thumb, the yellow Bit O'Sunshine, and Pink Joy. Rouletti, a rose-pink that is one of the hardiest, grows six inches tall.
Some perennials are essential, so make room for day-lilies, astilbes, iris, veronicas, shasta and painted daisies, balloon flowers, hostas and chrysanthemums. If climate allows, plan for spring bulb displays of crocus, hyacinths, daffodils, and tulips. During the summer all kinds of tender bulbs can be grown-dwarf dahlias, tigridias, gladiolus, montbretias, Peruvian daffodils, calla lilies, and fancy-leaved caldiums. Hardy lilies can be bought as pot plants in early spring for setting out in suitable containers.
Annuals will provide riotous color, so allow for some of these-marigolds, zinnias, petunias, nicotiana, nierembergia, Madagascar periwinkle. (Vinca rosea), cleome, snapdragons, annual phlox, verbena, dimorphotheca, ageratum, and heat-loving portulaca. Coleus will thrive in shade, and heliotrope will give fragrance. Fuchsias and geraniums offer vivid splashes of color; and in the constant wind and intense sunshine of the rooftop, succulents and sedums are without peer.
The concentrated area of the roof garden offers opportunity to display attractive containers. There you can give prominence to a handsome decorated jar, a choice piece of glazed pottery, or a hand-carved wooden tub. On the wall, you might hang a bird cage filled with foliage plants or a hand-painted pot with grape or kangaroo ivy. Beside a doorway, place a glazed strawberry jar planted with sedums and succulents or an ornamental well-head with trailing grape ivy will be attractive.
A Boston Rooftop Garden
A delightful example of a roof garden is that of Mr. and Mrs. Joel E. Harrell of Commonwealth Avenue, Boston. Located on the seventh floor of their modern apartment, the thirty-one- by twenty-five-foot tiled terrace commands views of the Commonwealth Avenue Mall, the Boston Public Garden and the city's picturesque rooftops and chimneys. Sheltered by the apartment on two sides, there is a four-foot brick wall along the edge of the other sides to cut the force of the wind and give privacy to the sitting area.
Lined along the wall are natural redwood tubs and planters, with Japanese yews, mountain laurel, pieris, junipers, Hinoki cypress, azaleas, and trailing Baltic ivy. There are eleven planters and six large octagonal tubs. Though this is primarily an architectural, easy-to-maintain green garden, in the summer, geraniums, ruffled white petunias, and blue lobelias are planted in front of the evergreens. On the axis of the glass living-room doors stands a charming figure of a little girl holding a large bowl above her head. The bowl is kept filled with water for the birds and the figure stands in a small pool with pots of English ivy instead of water. Two lead fan-tailed pigeons complete the composition. Sometimes potted flowering plants, like white chrysanthemums in the fall, brighten the picture.
According to Mrs. Harrell, all the soil was brought up by elevator in wheelbarrows and boxes. After containers were filled, humus was added, and each year a few inches of old soil is removed from the top and replaced with fresh mixture. The evergreens are fed twice a year-a heavy application in the early spring and a lighter one in the fall. Plants are watered daily during the growing season. Shade is provided by a large fireproof awning of dark green.
Because of its good design, this three-year-old rooftop terrace is equally lovely in winter. In fact, the Harrells like it best just after a light snowstorm. At Christmas, the statue is moved to one side to make room for a lighted and tubbed Christmas tree which adds lively color to this rooftop haven.