Evaporation of water from the soil surface eventually dries it and more radiation is used to raise the soil temperature, increasing the daytime maximum and imposing greater stress upon the plants. Maximum temperature on the bare, sunlit soil surface could reach a lethal level for fine roots and cambium of tender shoots. Nighttime minimum soil temperatures tend to be high because of the high air temperatures from the heat retention of structures. Plant metabolism rates potentially remain high. Few actual soil temperature data are available for urban soils, so inferences must be made from air or surface temperature measurements and heat budget evaluation at the mesoscale level,or from observation of plant response.
Soil temperature is important since it controls the growth environment of roots and soil organisms, and inorganic chemical processes. Warmer temperatures increase rates of chemical and biological processes. Root growth is extended from the fall into early winter. This may prevent hardening off of the plants in northern latitudes.
Mulching or other protection of the soil surface will do much to lower daytime maximum emperatures and prevent the drying out of the soil,benefiting fine root growth and development.