Blasted by the sun and battered by the wind, roof gardens, or roof terraces are prime examples of exposed sites. They are not the best places to grow garden plants, but if you live in a city apartment, a roof garden can make a useful, extra room, bringing greenery closer to the urban home and providing a pleasant and relaxing place to sit, with the sky as a ceiling.
The large size and sturdy structure of the roof terrace at the top of an apartment block makes it possible to grow a selection of heavy specimen trees in pots.
The owners should have checked with an architect how much weight the roof could take, as large containers can be very heavy, especially when they are wet. Usually, it is best to place heavy containers near load-bearing walls, rather than arrange them in the middle of the terrace where the roof is unlikely to be supported.
Provided they are watered and fed well, trees and large shrubs grow well in good-sized containers, even in exposed positions. Generally, pots measuring 1m (36in) across will take a 3.6m– (12ft-) high tree, although in exposed sites it is best to choose species that are not higher than 3m (10ft). Unless they are well anchored, they may blow over in high winds.
When it comes to selecting trees, it is best not to consider fast-growing species, such as eucalyptus and ailanthus – they will soon grow too big for their tubs. Conifers do well if they are watered frequently, although just a short spell without sufficient moisture kills them. For exposed spots, however, pines and junipers are the most suitable conifers – high winds will brown the foliage of other species.
In an exposed site, trees in pots need frequent watering because of the drying effects of the wind and sun. Even in winter, you should water them, except when the temperature is below freezing. Spray the leaves of large, newly planted specimens frequently, but avoid doing so in hot sunlight. After their first year, underwater rather than overwater them.
Bamboo in large containers
