• Online Exclusive: Zone Map Resources
Before you add another plant to your garden, ask yourself: Will it survive both the lowest and highest temperatures in your region? If not, are there any cultivars or species of the plant that are more cold- or heat-tolerant? Refer to the USDA Plant Hardiness Zone Map and the AHS Plant Heat-Zone Map—they can help you in your quest for the best herbs to grow in your region.
USDA Plant Hardiness Zone Map
This map divides the United States into 11 zones and is based on average minimum temperatures. Plants are assigned a zone based on how low a temperature they can survive. Each zone differs by 10 degrees. The map was created in 1960 and included 10 zones. The 11th zone was added during a 1990 update; at this time, each zone was further subdivided into “a” and “b” regions.
Pros: This map is the most commonly used zone map and is very easy to use. It helps gardeners identify which plants will survive in their region’s lowest temperatures and is a great starting point for beginning gardeners.
Cons: This map only takes into account minimum temperatures; it doesn’t consider maximum temperatures, which can make or break a plant; degrees of dryness or moisture; light requirements; or plants’ preferred soils.
(Click here to view the USDA Plant Hardiness Zone map.)
AHS Plant Heat-Zone Map
This map, created in 1997, divides the United States into 12 zones and is defined by the average number of days the region’s temperature exceeds 86 degrees. Plants’ zones are based on how high a temperature they can tolerate. In Zone 1, the temperature reaches 86 degrees less than one day a year; in Zone 12, the temperature exceeds 86 degrees more than 210 days per year.
Pros: Since cold hardiness is not the only factor in a plant’s survival, this map was created for gardeners interested in heat-tolerant garden design. It also addresses temperature increases and global climate change concerns. It helps gardeners identify which plants can survive in their region’s highest temperatures.
Cons: This map is not as widely used as its predecessor. It also doesn’t relate well to plants that require winter climates to thrive, such as cilantro or tulips, and faces similar challenges as the hardiness map: It doesn’t address light requirements, unusual weather patterns or moisture conditions.
(Click here to view the AHS Plant Heat-Zone map.)
The best way to choose a plant for your region is to use both maps congruently. But remember that these maps apply only to your general geographical area. They might not apply to your microclimate. According to Jim Long, an Herb Companion contributing editor, most gardeners don’t kill enough plants. He follows the advice of J.C. Raulston, the late curator of North Carolina State University Arboretum: Instead of planting one precious little plant and hoping it will grow, plant three or five of everything in different locations. You’ll learn more about what that plant needs if you kill two of the three you’ve planted than you ever will by guessing at a map.
Check with your local extension service office for more information about your specific region.
• The Garden Primer by Barbar Damrosch (Workman Publishing Company, Inc., 2008)
• Heat-Zone Gardening by Dr. H. Marc Cathey (Time-Life Books, 1998)
Gina DeBacker is editorial assistant at The Herb Companion.