Climate Zones
Two of the most widely used climate zone maps are the Sunset zones, also known as the Western Plant Climate Zones, and the USDA Hardiness zones. The Sunset zones, popularized and researched by Sunset Magazine, are derived from climate zones developed by the University of California Cooperative Extension. Sunset zones are based on a combination of extremes and averages of minimum, maximum, and mean temperature, rainfall, humidity, and the length of the growing season. As such, this system proves the most useful when trying to judge plant hardiness and adaptability to a certain region. The original zones mapped out the Western United States, and range on a scale from 1-24. The zones have since expanded to include the whole United States, as well as Alaska and Hawaii, comprising a scale from 1-45, plus zones A1-A3 in Alaska, and zones H1-H2 in Hawaii. For maps of the zones, please refer to the excellent Sunset Garden Books. Generally, zones 23-24 comprise the mildest parts of Southern California where most subtropicals and near tropicals can be grown. Zone 25 includes South Florida and the Keys, and is generally frost free. Zones 16-22 include milder areas of Southern California and the coastal regions of Northern California.
USDA zones are more widely used. These zones are based on the average minimum temperature in a given region. In some areas however, the minimum temperature does not tell the whole story. Conditions such as prolonged cool winters (40-55F) and/or summers can be just as lethal to tropical plants as frost, hence the USDA zones may not be as useful, depending on your location. Below is a general chart of USDA zones: (Note that the Sunset equivalent zones are rough approximations and do not correlate completely with the USDA zones.)
| USDA Zone | Temp. (F) | Temp. (C) | Sunset Equiv. |
| 1 |
below -50F |
|
A1 |
| 2a |
-50 to -45F |
A1 | |
| 2b |
-45 to -40F |
A1 | |
| 3a |
-40 to -35F |
A1 | |
| 3b |
-35 to -30F |
A1 | |
| 4a |
-30 to -25F |
A1 | |
| 4b |
-25 to -20F |
A1 | |
| 5a |
-20 to -15F |
1A, 1B, A2 | |
| 5b |
-15 to -10F |
1-2 | |
| 6a |
-10 to -5F |
1-3 | |
| 6b |
-5 to 0F |
1-3 | |
| 7a |
0 to 5F |
6 | |
| 7b |
5 to 10F |
-14.9 to -12.3C | 6-7 |
| 8a | 10 to 15F | -12.2 to -9.5C | 4, 6-9 |
| 8b | 15 to 20F | -9.4 to -6.7C | 4-5, 7-9 |
| 9a | 20 to 25F | -6.6 to -3.9C | 16 |
| 9b | 25 to 30F | -3.8 to -1.2C | 16-22 |
| 10a | 30 to 35F | -1.1 to 1.6C | 17-23, 26 |
| 10b | 35 to 40F | 1.7 to 4.4C | 23-24, 26 |
| 11 | above 40F | above 4.5C | 25, H1, H2 |