Gardening successfully in Georgia requires understanding the state’s diverse climate, which ranges from mountainous terrain to coastal plains. A powerful tool for any gardener is the concept of growing zones, which provides a framework for plant selection based on temperature tolerances. This system helps ensure that perennial plants can survive the winter cold and thrive throughout the year.
Understanding the USDA Plant Hardiness Zone System
The standard used for determining a plant’s winter survival across the United States is the USDA Plant Hardiness Zone (PHZ) system. This map divides North America into zones based exclusively on the average annual extreme minimum winter temperature. The current standard is based on the 2023 map revision, which uses data from 1991 to 2020 and reflects a general warming trend. Each full zone represents a 10°F difference in the average minimum temperature. For a more precise measure, each zone is further divided into ‘a’ and ‘b’ subzones, where ‘a’ is the colder half and ‘b’ is the warmer half of the 10°F range.
Defining the Growing Zones Across Georgia
Georgia’s distinct geography results in a significant range of USDA Hardiness Zones, typically spanning from Zone 6b to Zone 9a. The state’s zones are dictated by elevation, latitude, and proximity to the Atlantic Ocean. These distinct regional climates mean that a plant thriving in one part of Georgia might not survive in another.
The coldest zones are found in the highest elevations of North Georgia, specifically in the Appalachian Plateau and Blue Ridge regions. Areas here fall predominantly into Zone 6b and 7a, where average annual minimum temperatures can drop to 0°F to 5°F.
Moving south into the Piedmont region of Central Georgia, which includes the Atlanta metropolitan area, zones generally transition to 7b and 8a. Atlanta is predominantly situated in Zone 8a, meaning the average annual minimum temperature is between 10°F and 15°F.
The warmest zones are located in the Coastal Plain and along the Atlantic shoreline in South and Coastal Georgia. Cities like Savannah fall into the warmer range of 8b and 9a. Zone 9a, found in the extreme southeast and barrier islands, has an average minimum temperature that rarely drops below 20°F to 25°F.
Using Hardiness Zones for Successful Plant Selection
The Hardiness Zone rating listed on a plant tag serves as a guide for successful long-term gardening. This rating specifies the coldest zone in which the plant can reliably survive the winter. To ensure a perennial plant survives, its required zone number must be equal to or lower than the zone of your garden.
For example, a plant rated hardy for Zone 8 will likely survive in a Zone 8a or 8b garden. However, planting it as a perennial in a colder Zone 7a area risks its death during a typical winter. When selecting plants, gardeners should always choose those with a PHZ number that matches their local zone or is a lower number, indicating greater cold tolerance.
Understanding your specific subzone, such as 8a versus 8b, is important, as the difference can be a full 5°F in winter minimums. While the zone map provides a broad framework, small areas known as microclimates can exist, such as a sheltered courtyard or a south-facing wall. These localized conditions can sometimes be slightly warmer than the official zone, but relying on this for plant survival carries risk.
The Importance of Considering Summer Heat Zones
While the USDA PHZ map addresses winter cold, it is not the only factor for successful gardening in Georgia’s climate. The state’s long, intensely hot, and humid summers introduce a separate set of challenges for plant survival. The PHZ system does not account for a plant’s ability to tolerate high temperatures.
To address this, gardeners should consult the American Horticultural Society (AHS) Heat Zone Map. This complementary system measures the average number of “heat days” per year, defined as days when the temperature exceeds 86°F (30°C). This threshold is the point at which many plants begin to experience physiological stress and damage.
The AHS system divides the country into 12 zones, with Zone 1 experiencing the fewest heat days and Zone 12 the most. Successful planting in Georgia, especially in the central and southern regions, requires checking both the PHZ for winter survival and the AHS Heat Zone for summer tolerance.