The USDA Plant Hardiness Zone Map is the standard resource for gardeners and farmers seeking to understand which perennial plants will survive the winter in a specific location. This map divides the country into distinct geographical areas based on climate data, providing a framework for selecting trees, shrubs, and flowers that can tolerate the coldest temperatures of the season. Knowing the designated zone for East Tennessee is the first step toward cultivating a successful, long-term garden.
Understanding the USDA Plant Hardiness Zones
The USDA system is based on the average annual extreme minimum winter temperature recorded over a 30-year period. This specific metric is used because the lowest temperature a plant experiences is the primary factor determining its survival through the winter. The map is broken down into 13 major zones, each representing a 10-degree Fahrenheit temperature range.
For finer precision, each major zone is further divided into two subzones, labeled ‘a’ and ‘b’, which narrow the range to 5 degrees Fahrenheit. For example, Zone 7 has a full range of 0°F to 10°F. Zone 7a covers the colder half (0°F to 5°F), and Zone 7b covers the warmer half (5°F to 10°F).
The Specific Zones Applicable to East Tennessee
East Tennessee is characterized by a significant range of hardiness zones due to its varied topography, spanning from Zone 6a in the highest peaks to Zone 7b in the warmest valley floors. The most populated areas, including cities like Knoxville and Chattanooga, typically fall within Zone 7a and 7b.
However, the mountainous regions of the Great Smoky Mountains and the Cumberland Plateau include pockets of colder zones. These higher elevations are predominantly designated as Zone 6b, where the minimum winter temperature averages between -5°F and 0°F, and even Zone 6a in the coldest spots. This variability means that a gardener in a valley community may be in 7b, while a neighbor living just a few miles away on a mountain slope could be in 6b.
How Elevation and Geography Create Microclimates
The significant variation in East Tennessee’s zones is directly attributable to the Appalachian Mountains and the resulting geological features, which create numerous localized microclimates. These are small areas whose climate differs from the general surrounding region due to localized factors like elevation, proximity to water, and air drainage. The difference in elevation between the valley floors and the mountain ridges dictates the temperature patterns.
The phenomenon of cold air drainage causes the coldest air to sink and pool in the lowest-lying areas, such as river valleys and hollows, which can sometimes result in frost pockets that are significantly colder than nearby slopes. Conversely, slopes, especially those facing south or west, often experience warmer temperatures because the cold air drains away, making them slightly warmer than the valley floor below. Large bodies of water, such as the Tennessee River system, can also temper the surrounding climate by releasing heat slowly, which slightly raises the winter minimums in their immediate vicinity. This interplay of mountain peaks, valleys, and river systems prevents the region from having a single, uniform hardiness designation.
Translating Zone Data into Planting Success
For residents in the 6b to 7b range of East Tennessee, the hardiness zone information should be a primary reference point when purchasing perennial plants, trees, and shrubs. Always check the plant tag or catalog description for its stated USDA hardiness rating before making a purchase. A plant rated for Zone 7 is expected to survive the average minimum winter temperatures in both Zone 7a and 7b.
For long-term plantings, a conservative strategy is to choose plants rated for a zone slightly colder (a lower number) than the area’s specific zone. For example, a gardener in Zone 7b might select a plant rated for Zone 6 or 7a to provide an extra margin of safety against an unusually severe winter. The hardiness zone is less relevant for annual flowers and vegetables, as these complete their lifecycle within a single growing season and are not expected to survive the winter.