Planting zones, also called hardiness zones, divide the U.S., Mexico and Canada into 11 areas. Each of the gardening zones is based on a 10 degree Fahrenheit difference in the average annual MINIMUM temperature.
Planting Zones 1 and 2a, are the coldest and represent Canada. The U.S. falls within zones 2 through 10. Hawaii and Mexico are represented by zone 11, the hottest score.
Suitable hardiness means a plant can be expected to grow in the zone’s temperature extremes, as determined by the LOWEST average annual temperature.
As an example: If a plant is recommended for a range of zones 5 – 9, the plant is suitable for growing in zones 5,6,7,8 and 9. The Planting Zone chart should be used only as a guideline to help select the right plant for your location. The Planting Zone map is updated every 10 years and the new map will be published this year. It is likely that the zones will be moved downward to reflect the cooler weather that we have experienced the last several years.
South Florida Hardiness Zones:
9a: 20 to 25 degrees
9b: 25 to 30 degrees
10a: 30 to 35 degrees
10b: 35 to 40 degrees
Ft. Myers in general is considered zone 10a. In reality, areas east of I-75 often experience frost conditions and reflect zone 9b. Our nursery, Alva, Buckingham, Lehigh, and much of LaBelle are examples of that zone. Zone 9a is farther east and north and can often experience hard frosts for several hours.
There are other factors that can influence whether a frost occurs or its severity. These include the proximity to large bodies of water, low-lying areas, and how sheltered plants are. As an example, plants under the canopy of a tree; the close proximity to a building, and plantings on the east and south side of a structure may be spared the worst effects of a frost.
*(Much of this information is from the USDA Planting Zones website.)