Living in USDA zone 8a, you know you can grow perennial crops that overwinter in temperatures as cold as 10 degrees F. Likewise, gardeners in Zone 3b are limited to those surviving in temps as low as -35 degrees F. These designations taken from the USDA Plant Hardiness Zone Map, which tells you which plants are most likely to thrive based on your average annual minimum winter temperature. This concept has been around since 1960, and it’s pretty widely accepted among gardeners in this country. The flip side to being prepared to overwinter plants in cold weather is planning a garden that tolerates hot weather, and for that, the American Horticultural Society’s Plant Heat Zone Map has you covered.
The late H. Marc Cathey, PhD, AHS president emeritus, assigned heat zone codes to thousands of plants for use in nursery catalogs, books and in the AHS magazine, The American Gardener, starting in 1997. (Since Cathey’s passing, AHS staff has taken over coding.) The plant’s heat zone designations are most commonly seen for ornamental plants, though you can find them for some vegetables and herbs, too.
The idea behind the heat zones is that both cold and heat are determinants in our plants’ survival. As the AHS points out, this is particularly true during seasons of drought. The AHS developed its Plant Heat Zone Map to educate gardeners about the “heat days”—days with temperatures hotter than 86 degrees F—in their areas. “That is the point at which plants begin suffering physiological damage from heat,” AHS explains. The zones range from zone 1 in Alaska, with less than one heat day per year, to zone 12 in Puerto Rico, with more than 210 heat days per year.
“Vegetable gardeners haven’t traditionally relied on the conventional climate zones (including the USDA hardiness zones) in planning what they plant. In part, this is because most vegetables are annuals, completing their lifecycle in one year or less,” explains David Ellis, AHS director of communications and editor of The American Gardener. “The climate zone maps are geared more toward perennial plants, which are usually a bigger investment in terms of money and time than are annual plants. So there’s a bigger incentive for gardeners to select perennial plants that are acclimated to their region and will thrive for many years in their gardens.”
Still, the USDA plant hardiness zones have long served as a guideline for planting times and vegetable varieties, as can AHS plant heat zones—particularly for farmers and gardeners in warmer climates.
“The assigned codes for both map systems include useful information that will help gardeners select the kind of vegetables and herbs to grow in their region and also what time of year they should be planting different vegetable varieties,” Ellis continues.
Using Heat and Hardiness Zones
The USDA and AHS maps can offer a guideline to your planting schedule. The hardiness zone designation is found on most every seed packet or catalog seed description. The heat zone is not as easy to find, though AHS members can readily access heat zone information, and Ellis says the organization would like to make that archive accessible to nonmembers in the future. Even without knowing exactly what heat zone your chosen broccoli variety is rated for, you can use the temperature information for your heat zone to make planting decisions.
“In regions with higher heat zones (say 8 through 12), cool-weather crops, such as lettuces or kale, must be grown at the coolest time of the year, usually in late fall to mid winter,” Ellis says. “In regions with lower heat zones, these cool-weather crops are generally sown in very early spring or in late summer for a fall crop.”
“For instance, corn (Zea mays) is coded USDA hardiness zones 1 through 11, AHS Heat zones 12 through 1. This indicates that corn is primarily an annual (perhaps surviving longer than one year in USDA zone 11, although this is more theoretical than practical from a vegetable-gardening standpoint) and is a heat-loving plant that will bear crops in all of the AHS heat zones. In the very coolest regions of the country, gardeners may need to select corn varieties that produce crops in a short growing season.”
Pushing Zone Limits
The AHS and USDA zone maps are guidelines, not meant to say that a particular plant will never grow outside of its rated zone.
You can push the boundaries of your USDA or AHS zone using microclimates—a small area where the climate differs from surrounding areas. In cold climates, the microclimates offered by season extenders—think low tunnels, high tunnels and floating row cover—can be good investments for small-scale farmers. In both hot and cold climates, structures around your farm and the contours of your land also offer microclimates that heat up faster, stay cooler longer and allow you to tweak your zones just a bit.
Likewise, just because a plant is rated for a zone doesn’t mean an odd weather event can’t take it out. The USDA Plant Hardiness Zone website explains, “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.”
Your microclimates and unusual weather patterns are not taken into account for the USDA and AHS zones. Also, as the climate warms over time, the zones are shifting. As of this writing, the USDA’s most recent Plant Hardiness Zone Map update was in 2012, and the AHS is currently seeking funding for development of an updated version of its Heat Zone Map.