Check out these cool plants for hot walls

Columns - Regional Plant Specialist

June 22, 2011

Heat is a way of life in the gardens of the Southwestern deserts. The intense solar radiation of the low deserts causes more than just personal discomfort. The heat load on an unprotected south or western wall can increase the heat load on the building by up to 30 percent. All that heat significantly affects how hard an air conditioner must work to cool down the interior.

Plants growing in front of a south- or west- facing wall that are not well suited to the increased solar radiation and, therefore, suffer heat stress, sunburn, and short lives. This is a particular problem for vegetable gardens where the increased heat can mean the difference between plants lasting through the summer, and giving it up after a few weeks of growth.

The heat of the sun’s rays is collected by walls, particularly those made of stone, and bounced back onto anything (plant, wall, building, person) nearby. In addition, pavement or pool decking collects and re-radiates heat and, in small yards, this combination can make the entire area uncomfortably hot.

So what to do? It is going to be hot in the summer, but there are a number of ways to cool down and mitigate the heat buildup on walls. The solutions revolve around choosing plants that thrive in such a situation and planting them generously.

Evergreen plants, those that do not lose all their leaves at one time seasonally, are the first line of defense for a hot wall. The thick array of leaves on such plants not only blocks the radiant heat of the sun’s rays, but absorbs them for its own photosynthetic needs.

If there is ample room, use desert trees like palo verdes (Parkinsonia florida, P. microphylla and P. praecox), mesquites (Prosopis sp.) and ironwood (Olneya tesota) and allow their limbs to sweep the ground. These plants require a space of up to 20 feet from the wall for best results, but are excellent protection against the western sun.

All varieties of citrus have immense heat tolerance and thrive when grown against a western wall or window. Citrus is evergreen, and naturally grows in a dense, more or less spherical shape. If planted at a distance from the wall, give it 12 to 15 feet clearance. Citrus can also be positioned directly adjacent to the wall and kept either formally espaliered or pruned to grow to a one-sided plant.

Other large evergreen plants with ample heat tolerance to protect a hot wall include oleander (Nerium oleander), hopbush (Dodonaea viscosa), rosewood (Vauquelinia californica and V. corymbosa) and Texas ebony (Ebanopsis ebano).

Another strategy to manage the heat of a hot wall is to use deciduous trees or shrubs. Deciduous plants are particularly useful for unprotected western or southern windows, or the sides or roofs of patios or ramadas. The benign rays of the winter sun help warm a cold house or seating area, while the dense growth of summer protects them.

Large evergreen choices, those that grow to well over 12 feet tall, include tree forms of yellow bells (Tecoma stans), pomegranate (Punica granatum) or desert willow (Chilopsis linearis). Smaller shrubs, growing up to 10 feet tall, are shorter yellowbells (Tecoma stans), red bird of paradise (Caesalpinia pulcherrima) and red fairy duster (Calliandra california).

Desert willow (Chilopsis linearis)

Heat builds up on a wall from not just the direct absorption of the sun’s rays, but also from the reflection of the sun’s rays as they bounce up from the ground on to the wall. This is a particular problem when the ground is covered only with gravel or pool decking. To calm down the bouncing rays, a third strategy—planting in stages against the wall—comes to the rescue.

Begin by placing the tallest plants nearest the wall, then place shorter, usually evergreen, plants in the next ‘row’ and end by having something short, even a groundcover, as the final ‘row.’ It is not necessary, and rarely desirable, to have the planting lined up like soldiers against the wall, but by choosing to cascade the sizes of plants from the wall out as far as practical, this style of planting greatly reduces the effect of bounced back heat.

Mid-size varieties
Any combination of plants will do for such a planting scheme and most of the taller ones mentioned earlier are good choices. Other mid-size choices include jojoba (Simmondsia chinensis), Texas rangers (Leucophyllum sp.), emu bush (Eremophila maculata, E. laanii) and little leaf cordia (Cordia parvifolia).

Shorter plants to finish off such a planting might include black dalea (Dalea frutescens), verbena (Verbena pulchella, V. rigida), or desert petunia (Ruellia brittoniana). Large desert perennials like desert spoon (Dasylirion wheeleri) and ornamental grasses such as deer grass (Muhlenbergia rigens) and other members of its genus are also excellent choices for layering to minimize heat build up.

Vines have been used in all climates as seasonal shade producers and such plants work just as well in the desert. Placing a vine against a hot wall offers the same protection from the sun and the heat as the shrubs identified previously. Again, evergreens are the densest and desert choices including lilac vine (Hardenbergia violacea) and yellow butterfly vine (Mascagnia macroptera) and bougainvillea (Bougainvillea spp.). Deciduous vines like Queen’s wreath (Antigonon leptopus), podranea (Podranea ricasoliana) or yuca vine (Merremia aurea) provide the same seasonal protection as deciduous shrubs. All of these vines need a support or a trellis to stay where you need them. Growing vines is a charming way to increase color and calm down the heat on the walls or roofs of patios, ramadas or gazebos.

A wall that is protected by heat tolerant plants creates a zone in front of it in which plants with much less tolerance for reflected heat can be grown more successfully. This is especially useful for vegetable, herb, or flower gardens that are adjacent to a hot wall.

The great heat of summer is one of a desert gardener’s great challenges. By using some of these strategies, gardeners can calm down the heat load on their homes or walls, increase the success of less heat tolerant plantings and make their garden a more congenial place to sit and relax, even when the temperatures soar.

Mary Irish is a freelance writer, book author and speaker in Scottsdale, Ariz. She’s the former director of horticulture at the Desert Botanical Garden in Phoenix.