Cold Air and Chill Requirements​

​At a local scale, cold air tends to sink while warm air rises. When night approaches, especially in fall, winter and spring, cold air from higher elevations of a landscape can flow slowly downslope and collect in low areas at the bottom. Valleys carry “rivers” of cold air. Is your site located at or near the bottom of a drainage, canyon or wash? Do you occasionally or frequently have frosts or heavy freezes?​

​Accumulations of cold air can be a problem for trees that are cold sensitive. Since this cold air may cause late spring freezes, do not place cold-sensitive trees in low-lying areas. If your site is in a low-lying area, place trees in warmer locations such as the south side of buildings where heat absorbed by walls on sunny winter days can re-radiate to nearby plants at night. Other built and natural features can moderate cold temperatures, such as rainwater harvesting tanks, pavement, walls, thick vegetation and ponds. ​

​Cold air flows down slopes to pool at the bottom. If your site has an elevation gradient, to increase fruit production place cold-sensitive trees higher on hillsides—if soil is deep enough to support them. A vertical distance of a few feet to a hundred feet above valley floors can make a difference in cold conditions and frost events. Placing cold-sensitive trees on south-facing slopes will provide them with warmer winter temperatures than nearby north-facing slopes would provide. In contrast, desert locations where cold air accumulates can benefit trees that need a certain number of chill hours to bloom and fruit. Planting fruit trees on north-facing slopes in desert areas may lengthen chill hours, delay spring bud break and help avoid damage from late spring frosts. ​

​Climate change could result in decreasing chill hours in Arizona regions in the future. However, having fewer chill hours does not necessarily mean having fewer freezing nights and less frost risk, so continue to protect vulnerable trees from cold. ​