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.