December and January are the coolest months of the year in the Salt River Valley. The coolest over night lows normally occur from 30 minutes before sunrise to 30 minutes after sunrise. It is the duration of the cold, below 28 degrees Fahrenheit, that does the most damage. On December 1st, the Phoenix sunrise and sunset is 7:14AM and 5:20PM respectively. Weather conditions that favor the coolest temperatures include clear skies, calm conditions (no breezes), lower dew point temperatures (in the 20's and lower), and air flow out of the north-northwest. If a light breeze develops over your area during the night, the air temperatures will rise because of the mixing of the air. However, as day break approaches, the breeze will drop off to calm, or nearly calm, allowing the temperature to drop down to the previous low, or worse, even cooler than recorded earlier in the evening. The national weather service puts out a new forecast daily at 4 am & 4 pm, with updates as needed. Check them out.
For frost protection in the winter months, cover your young citrus trees with burlap or old sheets. Also, to help minimize frost damage, fill the well with water the night before the frost. However, a newly planted citrus tree does not have the established root system of a mature tree to absorb water fast enough to offer sufficient protection. For that reason, we recommend covering your citrus tree to help keep frost from settling on their defenseless branches and leaves.