Planting Heat Tolerance Index

Development

• Like the USDA Plant Hardiness Zone Map, the AHS Plant Heat Zone Map was developed from nationwide National Weather Service data reported during a sustained time period. The AHS map is based on weather data from 1974 through 1995. Except in Alaska and Hawaii, only weather stations that maintained daily reports for 12 or more years were included; an exception was made for missing data in Alaska and Hawaii, where stations with seven or more years of reports were included. Weather stations on or close to mountaintops in sparsely occupied areas were excluded. From a pool of 7,831 stations, 4,745 were used for the development of the AHS map.

Scale and Range

• The AHS map resembles the USDA map in its use of color codes to represent temperature gradations. On the 2012 USDA map, 26 color-coded zones express a span of average annual extreme minimum temperatures from minus 55 to minus 60 degrees Fahrenheit in USDA plant hardiness zone 1a to a 65 to 70 F in zone 13b. AHS map's 12 zones express "heat days;" each zone reflects its average number of days annually that exceed 86 F, the temperature at which plant tissues can sustain heat-damage. AHS index steps increase in breadth from zones 1 through 12. The heat range in zone 1 is zero to one day above 86 F; in zone 6 it is 45 to 60 days; in zone 10 it is 150 to 180 days; and in zone 12 it is 210 or more days.

Usage

• The AHS heat-days information allows gardeners to refine their choices of plants that grow well in their climate. Cold-hardiness information may not be adequate for that task. When using only the USDA map, such disparate places as Portland, Oregon, and Dallas, Texas, should be hospitable to the same plants. Adding AHS information can help determine a plant's full temperature-tolerance range. Portland has a heat-zone score of 1 while Dallas' score is 8. In order to prevent confusion, USDA zones for a plant are expressed from coldest to warmest, as in "hardy in zones 3 through 7," while AHS' information is expressed from hot to cold, as in "heat-tolerant in zones 12 through 6." A combined rating for a single plant might read: 3 through 7, 6 through 1.

Considerations

• The process of using the USDA and AHS information requires consulting two references but can prevent you from losing plants to summer heat even though they survived winter. Delphiniums (Delphinium spp.), hardy in USDA zones 3 through 7, would do fine in Nebraska's zones 4 through 5 winter, but their AHS score of 6 through 1 suggests they will flourish in Omaha, Nebraska's heat zone 4 but may struggle in Grand Island, Nebraska's heat zone 6 that borders on 7. If you want to assess the suitability of a single plant, consulting a reference such as PlantMaps.com can speed the task of finding the cold and heat ranges for your location.

  Although as of mid-2013 AHS heat scores for more than 30,000 plants were readily available in only print form, the society has zone information available on its website, ahs.org.