The classification of a region’s climate is a foundational step for any gardener or agricultural planner, determining which perennial plants can survive the local conditions. For East Tennessee, understanding these climate zones is paramount for cultivating a successful landscape. The most widely used system for this purpose is the USDA Plant Hardiness Zone map, which focuses specifically on winter temperatures.
The Specific Plant Hardiness Zones of East Tennessee
East Tennessee falls predominantly within USDA Plant Hardiness Zones 6a through 7b, with the specific designation depending heavily on the local elevation and topography. The USDA map is based on the 30-year average of the lowest annual winter temperature. Zone 6a represents the coldest average minimums, ranging from -10°F to -5°F, while Zone 7b indicates the mildest, with average minimums between 5°F and 10°F.
The majority of the lower-lying valleys and the Knoxville area are typically classified as Zone 7a, where the average annual extreme minimum temperature falls between 0°F and 5°F. Moving into the foothills and higher elevations of the Appalachian Mountains, the zone drops to 6b, signifying a slightly colder range of -5°F to 0°F. These zone designations are segmented into 10°F increments, with the “a” and “b” suffixes further dividing them into 5°F sub-increments for greater precision.
How Elevation Creates Microclimates
The significant variation in East Tennessee’s hardiness zones over short distances is a direct result of its dramatic changes in elevation, a phenomenon known as a microclimate. The region’s defining features, including the Great Smoky Mountains and the Ridge-and-Valley Province, create distinct thermal pockets that influence the local climate. Higher altitudes in the Great Smoky Mountains and the Cumberland Plateau retain the state’s coldest zones, with areas reaching down to 6a. These elevated areas experience longer periods of cold air retention and a shorter growing season compared to the surrounding lowlands.
Conversely, the valley floors and areas adjacent to large bodies of water experience a warming effect, pushing their classifications toward the warmer 7a and 7b zones. This geographical layout causes cold air drainage, where the dense, cold air slides down the mountain slopes and settles in the lower valleys and coves. However, the largest and deepest valleys, which include major metropolitan areas, can experience a slight tempering effect from urban heat and surrounding geography, resulting in their warmer zone classifications.
Secondary Climate Considerations: Heat Zones
While the USDA Plant Hardiness Zone map focuses solely on cold tolerance, the American Horticultural Society (AHS) Heat Zone map offers a complementary metric for East Tennessee gardeners. This map classifies areas based on the average number of “heat days” annually, with a heat day defined as a period where the temperature exceeds 86°F (30°C). The heat zone map helps identify which plants can tolerate prolonged periods of high heat stress.
The general AHS Heat Zones for East Tennessee typically range from 6 to 8, depending on the specific location’s exposure and proximity to urban centers. A plant rated for Heat Zone 6 can tolerate between 45 and 60 days above 86°F, while a Zone 8 rating indicates tolerance for 90 to 120 such days.