Intense sunlight can be as difficult for plants to live in as deep shade. And if your garden is set among buildings, the heat from walls may rebound into the airless site, producing a micro-climate hotter than the surroundings. However, nature has resolved the problem of hot, dry conditions by reducing the leaf size of some plants to cut down moisture loss, as is characteristic of many artemisias, eryngiums, lavender and rosemary. Some plants are covered with mats of fine hairs which deflect the heat from the leaf surface and prevents rapid evaporation; these include lamb's ears (Stachys byzantina), the low shrub Lotus hirsutus (syn Dorycnium hirsutum) and other species loosely described as "Mediterranean". Fleshy-leaved sedums, echeverias and crassulas carry their own reservoir of water.
Conserving water is crucial in hot or exposed situations and, because wind will also dry out soil, some screening is essential. Lightweight but firmly fixed boundaries of close-meshed trellis or woven bamboo will help, and a small irrigation system and a gravel mulch will do the rest. Drought is also the enemy of roof gardens because everything is likely to be grown in containers or the shallow soil of raised beds. So choose plants that tolerate dry conditions.
Because so many of the plants which thrive in hot, dry conditions are either silver-grey or strikingly formed, bold materials can be used with them. Rocks, pebbles and gravel create a natural appearance against which exotics like agaves, cordylines and cacti look good. With the softer effect of perennial plants like perovskias, sages and nepeta, timber decking may be more appropriate.
In hot climates, water is always welcome, cooling and refreshing - it may just be the restful patter of a tiny fountain or a small pool bubbling over pebbles. Moving water stirs and cools the air as well as animating the garden with flickering reflections.
Though we may revel in sunlight, there will be times of the day when shade is invaluable. If there is no tree canopy, shade can be achieved by fashioning an arbour. You may stretch a canvas awning over a seating area, while timber trellis fixed overhead can carry a vine, a golden hop or a wisteria. Whichever you choose, make sure it is sturdy enough to support climbers which thrive in heat. With a little care, summer jasmine, Solanum jasminoides and a grapevine will cope in temperate climates. Trachelospermum jasminoides, though somewhat tender, is a superbly fragrant climber and in totally frost-free areas you may be able to grow flame- coloured bougainvillaea or pretty blue Plumbago capensis.
At a lower level, grey is the natural foliage cover for dry heat. Classic associations with it are white or pastel flower colours. If you find this rather stark for a sunny climate, warm things up with hot-colour partnerships like Helianthemum 'Ben Afflick' and H 'Ben Nevis', with their orange and rust-coloured flowers. Add some of the smaller kniphofias like K galpinii, with spears of orange-yellow flowers, or the lemon-yellow K 'Little Maid'. Partner these with day lilies like Hemerocallis 'Stella de Oro' and try ornamental brown grasses such as the 30cm-high bronze variant of Carex comans. For dramatic emphasis include clumps of the "black" grass (Ophiopogon planiscapus 'Nigrescens') with Heuchera hispida, with its unusual bronze and orange flowers above evergreen leaf rosettes. Luscious dahlias, carnival alstroemeria, fiery crocosmia and other plants with strong flower colours look their dramatic best under a bright sky and will make your small space sing out.
If an oasis outside your window is more to your taste, greens will make a hot garden feel fresh and cool. Try a foundation planting of Euphorbia mellifera with irises, day lilies, green fennel and santolina. Myrtle and blue-flowered ceanothus could provide the backdrop, and the invaluable box will accommodate to any form or create the divisions needed in the garden. And the range of blues, from deep agapanthus blue to the powder-blue of scabious, are all effective - but then the sky can be cobalt too.
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