Alternative Approaches to the USDA Hardiness Zone Maps

Hardiness Zone Maps - Average Annual Minimum Temperature
In 2004 the National Arbor Day Foundation released an update to the 1990 USDA Map using 15 years of temperature data. The result is similar to the AHS Draft with many of the hardiness zones shifting northward.
The National Arbor Day Foundation map was revised further in 2006, showing a continuing northward movement of the climate zones even compared to the 2004 version. Click here for an excellent graphic representation of how the zones have changed from the 1990 USDA map.
In 1995 the Florida Climate Center issued a Hardiness Zone Map for Florida using the same methodology as the 1990 USDA Map. The Florida Climate Center Map was based on temperature readings from 1981 through 1995 and, like the AHS Map and the Arbor Day Foundation Map, showed a northward migration of the hardiness zones. See Palm Talk for a good discussion. Note, the Florida Climate Center Web site recently went through a major revision and the Hardiness Zone Map is no longer available.
Heat Zone Maps

The USDA Hardiness Zone Maps are based on the concept that, with respect to temperature, cold will have the greatest impact on plants. The American Horticultural Society (AHS) and other organizations have taken the approach that heat must also be taken into account in estimating whether a plant will thrive in a particular location. In 1997 Dr. H. Marc Cathey, issued the AHS Plant Heat-Zone Map. The Map is divided into 12 zones indicating the average number of days in a particular zone where temperatures in exceed 86 degrees (the point at which heat will cause experience damage to cellular proteins in plants).

More importantly, the AHS has combined the Heat and Hardiness Zone maps to create a four number rating system that indicates a plant's ability to withstand heat and cold. The first two numbers in the system indicate the optimum span of zones for cold hardiness and the second two numbers indicate the optimum span for heat tolerance. " For example, a tulip may be rated as 3-8, 8-1. This indicates that, while a tulip can grow in Zones 1 through 8 during the summer, it will only thrive (and bloom), if it receives the average minimum temperatures present in Zones 3 - 8. While this approach is a definite improvement over sole reliance on the 1990 USDA Map, it will require an overhaul of existing hardiness indicators for plants. This will require significant time and resources and it is not clear whether the horticultural community is willing to invest the effort.

See Plants That Can Take the Heat

The Florida Climate Center, following the AHS methodology, issued a Heat Zone Map for Florida based on temperature readings from 1981 through 1995.

Note, the Florida Climate Center Web site recently went through a major revision and the Heat Zone Map is no longer available. However, the map has been replaced with a much more detailed discussion of Florida's hot season with several very interesting graphics.

The Sunset Approach
Over the years Sunset Magazine has developed a more complex approach to hardiness maps that takes a number of factors into account - "A plant's performance is governed by the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, humidity. Sunset's climate zone maps take all these factors into account — unlike the familiar hardiness zone maps devised by the U.S. Department of Agriculture, which divide the U.S. and Canada into zones based strictly on winter lows." This approach works very successfully in the West where 24 zones have been defined to the point where it is referred to far more frequently than the USDA hardiness Zones. However, in the East the Sunset National Garden Book Map is less useful.
Chilling Hours
Another element that needs to be considered in developing hardiness maps - Deciduous fruit trees, bulbs and other plants that go dormant during the Winter need a minimum number of chilling hours (hours where the temperature is between 32 and 45 degrees Fahrenheit).

If hardiness zone maps can be defined as the measurement of temperature changes in a particular location as an indicator of whether plants will thrive, phenology is the measurement of the plant lifecycle as an indicator of changing climate. As defined by the USA National Phenology Network, "Phenology ... is the study of periodic plant and animal life cycle events that are influenced by environmental changes, especially seasonal variations in temperature and precipitation driven by weather and climate. Wide ranges of phenomena are included, from first openings of leaf and flower buds, to insect hatchings and return of birds. Each one gives a ready measure of the environment as viewed by the associated organism. Thus, timings of phenological events are ideal indicators of the impact of local and global changes in weather and climate on the Earth's biosphere."