Plants release water through pores in their leaves by a process called transpiration. As warm air passes over leaf surfaces, heat is absorbed by the water, which then evaporates, and lowers the temperature of the air. Most Tucsonans are familiar with this process, called evaporative cooling. It's the same process swamp coolers utilize to cool the inside of our homes. 

The result of evaporative cooling by plants outdoors is the lowering of air temperature immediately surrounding vegetation, by as much as 9 ° F. The greater the leaf-surface area in the landscape, the greater the cooling effects.

Paved and graveled surfaces around the home contribute substantially to summer heat loads. These surfaces absorb the sun's heat or reflect it back into the immediate environment. This translates into increased discomfort experienced by people during the day. 

Paved and graveled surfaces also store heat during the day, keeping temperatures high around the home well into the evening and night-time. Temperatures over ground covers can be 15 to 25 ° F lower than over asphalt and concrete.

When selecting ground cover plants for the home landscape, choose ones which stay relatively low, but spread out significantly. Some of the best include; verbenas, trailing lantana, ice plants, Mexican primrose, trailing indigo bush, myoporum, prostrate acacia, and gold dalea. Spacing between plants depends on the ultimate spread of the variety. Ground covers grow fast, so don't crowd them! Planting too close will result in a tangled mess of growth that requires frequent pruning. Crowding can also increase the likelihood of insect and disease problems.

Generally, ground covers are established in two years, although some species require slightly more or less time. During this period of establishment, water on a regular basis. This could be as often as once a day, immediately after planting. The intervals between waterings can be gradually increased. When established, most species of ground covers will do nicely with one or two watering weekly during hot, dry weather.

The best time to plant ground covers is from mid-September to mid-October. Temperatures have cooled sufficiently to lessen stress on new plants, but soil temperatures are still warm, allowing for good root development. 

Ground covers should be planted in groupings of several plants of the same variety. It is beneficial in such bed plantings to improve the soil with the addition of organic matter prior to planting. A layer of peat moss, compost, or well-rotted manure should be spread over the soil 3 to 4 inches thick. Along with the organic matter, apply a small amount of fertilizer such as ammonium phosphate (16-20-0), bonemeal, or other material high in phosphorous. The phosphorous helps stimulate root growth. Mix the organic matter and fertilizer into the top 12 inches of soil. 

After planting, apply a surface mulch over the soil around and between plants. Mulching insulates the soil from heat, holds in moisture, and keeps out weeds. The best mulch for this purpose is forest mulch or ground wood mulch. Gravel mulches are also commonly used, but remember, they heat up much more than do wood mulches. As they decompose, wood mulches also have the advantage of adding beneficial organic matter back into the soil.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona,  520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on August 23, 1998
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