Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past — not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in-mind when selecting plants, specially if they choose to “push” their hardiness zone by growing plants not rated for their zone. Although this edition of the USDA PHZM is drawn in the most detailed scale to-date, there might still be microclimates that are too small to show up on the map.
Southwest Florida is prmiarily in zone 10A — ideally, you would look to plant vegitation with this rating.
Many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. A bout of extremely cold weather early in the fall may injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. Such factors are not taken into account in the USDA PHZM.
Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.
- Light — to thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant’s temperature
- Soil moisture — plants have different requirements for soil moisture, and this might vary seasonally. Plants that might otherwise be hardy in your zone might be injured if soil moisture is too low in late autumn and they enter dormancy while suffering moisture stress
- Temperature — plants grow best within a range of optimum temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others
- Duration of exposure to cold — many plants that can survive a short period of exposure to cold may not tolerate longer periods of cold weather
- Humidity — high relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens