Gardeners select plants based on their ability to survive local winter temperatures. The United States Department of Agriculture (USDA) developed the Plant Hardiness Zone (PHZ) map as the standard reference for this purpose. This map divides North America into geographic zones based on cold tolerance. Knowing your specific hardiness zone is the most effective way to determine which perennial plants, shrubs, and trees can survive the coldest part of the year. This information is particularly important in Tennessee, which features a wide range of elevations and geographic conditions that create diverse zones.
Understanding the USDA Plant Hardiness Zone System
The USDA Plant Hardiness Zone map measures the cold extremes a location typically experiences each year. This system is based on the average annual extreme minimum winter temperature recorded over a 30-year period. The data helps predict the likelihood of a plant surviving the lowest temperatures in a given area. The map is divided into 13 main zones, numbered 1 (coldest) to 13 (warmest).
Each full zone represents a 10-degree Fahrenheit range in the average annual minimum temperature. For greater precision, each zone is subdivided into ‘a’ and ‘b’ categories, representing temperature increments of five degrees Fahrenheit. For instance, a plant rated for Zone 6 will survive a minimum of 10 degrees colder than one rated for Zone 7. This focus on winter minimums distinguishes the PHZ system from other metrics, such as tracking the first or last frost date.
The Specific Zones of Tennessee
Tennessee encompasses a range of USDA Plant Hardiness Zones, stretching from 6a to 8a, a variation caused by the state’s diverse topography.
The eastern portion of the state, home to the Appalachian Mountains and higher elevations, experiences the coldest temperatures. This mountainous region is generally classified as Zones 6a and 6b, where the minimum average temperature can drop to -10°F.
Moving westward into Middle Tennessee, including the Nashville metropolitan area, the zones transition to the milder 7a and 7b. These areas benefit from a slightly warmer climate, with the coldest temperatures typically staying between 0°F and 10°F. This central region represents a moderate climate for the state’s gardeners.
The warmest zones are found in the far western and southern parts of the state, particularly around the Memphis area and the Mississippi Delta. This region often falls into Zones 7b and 8a, with the coldest average minimum winter temperatures ranging from 5°F to 15°F. Because zones can change rapidly over short distances, gardeners should use a ZIP code tool to verify their exact location.
Using Your Zone Number for Successful Planting
The primary way to use the PHZ number is by comparing it to the rating listed on a plant’s tag or seed packet. Most perennial plants, shrubs, and trees are labeled with a range of zones indicating where they can reliably survive the winter. If a plant is rated for Zones 5 through 9, it means it can tolerate the average annual minimum temperature of Zone 5, which is colder than any zone found in Tennessee. This plant would be expected to survive in all parts of Tennessee.
Conversely, selecting a plant rated for a higher zone than your own presents a significant risk of winter loss. A plant rated for Zone 8 will likely not survive the average winter minimums experienced in Zone 6 or 7. A strategy known as “planting down” is often used for extra safety, which involves choosing plants rated for one or two zones colder than your own.
The PHZ map is not a complete guide to plant health, as it only addresses winter survival. It does not account for microclimates, which are localized variations in temperature caused by factors like proximity to a large building or a body of water. Furthermore, the PHZ system does not consider a plant’s tolerance for summer heat and humidity. While the PHZ is crucial for perennials, planting schedules for annuals and vegetables are still primarily determined by the expected local frost dates.