The crucial difference between Arizona’s intermediate desert (Zone 12) and the low desert (Zone 13) is winter cold. But though the intermediate desert averages only 5 more freezing nights than the low desert (20 in Tucson compared with 15 in Phoenix and El Centro), it has harder frosts spread over a longer cold season. Zone 12 averages about 8 months between freezes, 9 months between killing frosts of 28°F (–2°C) or lower. Zone 13, on the other hand, averages more than 11 months between killing frosts, when it gets them at all. Extreme low temperatures of 6°F (–14°C) have been recorded in Zone 12.
The mean maximums in July and August are 5 or 6°F cooler than the highs of Zone 13. Many subtropicals that do well in Zone 13 aren’t reliably hardy here, but succeed with protection against the extreme winters. Although winter temperatures are lower than in Zone 13, the total hours of cold are not enough to provide sufficient winter chilling for some deciduous fruits. From March to May, strong winds (to 40 miles per hour) can damage young tender growth. Windbreaks help. Here, as in Zone 13 and the eastern parts of Zone 10, summer rains are to be expected and can be more dependable than winter rains.And as in Zone 13, the best season for cool-season crops (salad greens, root vegetables, cabbage family members) starts in September or October.