x Abu Dhabi, UAETuesday 23 January 2018

Choosing plants for rooftop gardens

On rooftop gardens, plants and shade sources must be chosen carefully.

Recently I asked Geoff Sanderson, the landscape architect and principal of the Dubai-based design group GCLA Realm, for any advice that he might have for the UAE's amateur gardeners.

"Shade, shade and more shade," was his first response - said partly in jest - but they are words that immediately came back to me as I started to think about a starting point for a column about roof gardens.

Although it might seem obvious that shade should be necessary in any garden in the UAE, it should be a particular consideration for roofs. Like roads, roofs frequently reach temperatures that are even higher than that of the surrounding air and spend most of each day soaking up the heat until much of it is released again in the evening, just when you want to be able to enjoy your cherished roof space. (Did you never notice that the city actually gets hotter in the evening before it finally cools down?)

Unfortunately, shade alone will not provide the kind of thermal comfort that you and your plants are looking for. Recent studies have shown that, far from reducing temperatures, artificial shade alone can actually increase ambient rooftop temperatures.

Equally, roof gardens that depend solely on plants to create shade and reduce temperatures also suffer from the fact that the plants' exposure to the extremes of heat and wind simply means that even more water is required to keep them alive.

Luckily, a combination of plants and artificial shade has been shown to create a cooling effect that is actually greater than the sum of its parts - and it's something that all roof gardeners should aim for. A shade made from light-coloured, reflective materials will be more effective at helping to reduce rooftop temperatures than one that is absorbent and dark.

When it comes to plant selection for roof gardens, there are many myths surrounding the suitability of some species over others. I have heard it argued that a roof garden is the perfect place to grow the widest possible selection of plants because where else would they be subject to such careful and regular maintenance, such good quality compost, fertiliser and irrigation and so little intrusion from such destructive pests as the great general public?

Unfortunately, for every hanging garden I've seen in more tropical and temperate climes (complete with full-sized trees, lawns and water features), I've also lost count of the number of local rooftop gardens I've visited where the owner has bemoaned the scorched and struggling state of their Camellia, bamboo and other tropical species, which may survive here given optimal conditions and care, but which are ultimately unsuitable for the harsh conditions on our rooftops.

If you're a very experienced gardener, or one for whom money, water and time are not issues, the options are indeed wide. However, if you have less experience, time and cash it is probably wiser to choose plants that are best able to withstand high temperatures.

Surprisingly, perhaps, maritime plants often thrive on roof gardens because they are naturally adapted to cope with harsh, salt-laden winds. Equally good are species that have developed glaucous, silver or hairy foliage that reflects the sun's rays. The silver buttonwood from the white mangrove family (Conocarpus erectus var. sericeus) is a maritime species that also has grey foliage. More commonly seen as green buttonwood in the UAE (Conocarpus erectus), this normally grows into a large tree but can also be trained into an excellent hedge and useful screen that can help to provide privacy on overlooked rooftops. It is particularly attractive in this glaucous, grey-leaved variety.

Other glaucous or grey-leaved species that spring to mind include the Mediterranean saltbush (Atriplex halimus), which is very good for creating low, closely clipped topiary hedging, the mighty Bismarkia nobilis from Madagascar (for which you would need a very large rooftop indeed) and the ever-popular olive (Olea europaea).

Although they don't have grey foliage, fan palms (Chamaerops humilis), frangipani (Plumeria obtusa) and sea grape (Coccoloba uvifera) also make excellent rooftop specimens and, most importantly, also thrive in pots - a key consideration with most domestic roof terraces.

Although it's also tempting to consider cacti and other desert succulents from the perspective of hardiness, I've found that they are frequently perceived as pests because gardeners, guests, pets and children are forced to navigate their way around their needles and spines.