USDA Plant Hardiness Zones are based on the temperature conditions of the place. They range from zones 1a to 13b. Most of the United States comprises zone 3a, which has a low temperature of around -40 and -35 degrees (F), and zone 8a, with 10 to 15 degrees. Outdoor gardening is on hold as by December, most annual plants have finished their cycles and been taken up, and many perennials are dormant.