Limitations of the Hardiness Zones Map for Landscapes
However, there are drawbacks to the hardiness zones with used without consulting supplementary information such as heat and rainfall. One problem of the hardiness zones is that they do not take into consideration the reliability of snow cover. Snow acts as insulator in extreme cold, protecting the root system of hibernating plants. If the snow cover is reliable then the roots are subjected to lesser temperature extremes than indicated on the hardiness zone map.
For plant survival the hardiness zone considers soil moisture, humidity, number of annual frost days, and the risk of a rare disastrous cold snap. Often this information would be more useful than just the average conditions.
Another factor to consider in plant survivability is the day length is insufficient or if they require a particular duration of low temperature (vernalization). One can go to the library and research gardening books such as Sunset Books that redefine in finer detail than the USDA zones. Sunset Books reflect the hardiness zone as 45 distinct zones which incorporate temperature ranges by season, precipitation, wind patterns, elevation, and length and structure of the growing season.
Hardiness zones a useful reference, if used in conjunction with other data such as heat zones, humidity, and rainfall distribution. Below is the American Horticultural Society (AHS) heat zone chart that can be used with the USDA hardiness zone map to make an informed decision of plant survivability in your area.
The table shows the number of days temperatures exceed 86 F by zone. ZONE FROM-TO 1 210