by Marcia Kirinus, EMGV

Gardeners often rely on seasonal cues or the calendar to determine when to perform specific gardening tasks. For example, they may plant nasturtiums on St. Patrick’s Day or set out tomato plants on Mother’s Day. However, what happens when you move to a new climatic zone and your gardening timing is thrown off? And what if weather patterns change to the extent that plants that once thrived now struggle?

Unpredictable weather patterns or a relocation to a different climate can make it challenging to assess a plant’s adaptability to the local conditions. Fortunately, there are two valuable tools at your disposal: the USDA Cold Hardy Zone designation and the Horticultural Society’s Heat Zone designation.

These designations provide vital information about a plant’s ability to tolerate both winter cold and summer heat. Most gardeners are familiar with the USDA hardiness zone system, which rates zones on a scale of 1 to 12. The Horticultural Society’s Heat Zone system parallels this scale, with higher numbers representing regions experiencing more hot days.

In Durham, for instance, the heat zone map places it at a temperate 7, while the hardiness zone map designates it as 7b. The USDA Plant Hardiness Zone indicates the lowest winter temperature a plant can withstand before facing potential death. In Durham’s case, being in Hardiness Zone 7b means plants in this zone can survive temperatures as low as 5-10°F. If you travel south, the numbers increase, and if you go north, they decrease.

However, the USDA hardiness map does not provide insight into a plant’s ability to endure the heat of summers. For that, the Horticultural Society’s Heat Zone number comes into play. It indicates a plant’s tolerance to heat and specifies the temperature threshold at which it starts to suffer and becomes unable to efficiently process water for normal functions. In Durham, our Heat Zone is 7, suggesting that plants should not be exposed to a cumulative total of 61 to 90 heat days above 86°F. Given that we experience many days higher than 86°F, it is crucial to choose perennials wisely based on the heat zone index number.

To accurately describe Durham’s local climate, we can use the format 7b/7, representing the cold hardiness zone followed by the heat zone designation. This combination of numbers provides a clear picture of the climatic conditions suitable for your plants. You can typically find these numbers listed on informational tags at reputable plant nurseries.

Consider incorporating native plants into your landscape. Native plants are well adapted to the local environment, offering numerous benefits such as enhanced climatic resilience, support for local wildlife, and reduced maintenance requirements. Here are some native shrubs that can thrive in Durham’s summers and winters, typically associated with USDA Zone 7b and Heat Zone 7:

• Summersweet Clethra (Clethra alnifolia)
• Fragrant Sumac (Rhus aromatica)
• Sweetshrub (Calycanthus floridus)
• Dwarf Fothergilla (Fothergilla gardenii)
• Virginia Sweetspire (Itea virginica)

This is a short list, and it is advisable to consult your local Cooperative Extension Master Gardeners office for a comprehensive selection of suitable native plants, shrubs, and trees. You can also use the “Find a Plant” tool on the North Carolina Extension Gardener Plant Toolbox to narrow down native plants by your site conditions and needs.

While the heat zone and hardiness zone systems are essential factors to consider when choosing plants for your garden or landscape, other variables like soil type, sunlight exposure, humidity, and drought tolerance should also be considered to ensure successful plant cultivation.

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References:

• USDA Plant Hardiness Map: https://planthardiness.ars.usda.gov/
• American Horticultural Society https://ahsgardening.org/gardening-resources/gardening-maps/
• “Site Considerations When Selecting Plants”: https://hgic.clemson.edu/factsheet/choosing-a-planting-location/
• University of Arizona Cooperative Extension: “Heat Zone Map for Arizona” (https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1673-2021.pdf)
• Dr. Bryce Lane lecture