Nashville, Tennessee, is currently designated as a split hardiness zone, falling primarily into USDA Plant Hardiness Zones 7a and 7b, following the most recent 2023 map update. A plant hardiness zone is a geographical area defined by the average annual extreme minimum winter temperature, which determines which perennial plants are most likely to survive the winter cold. While Nashville was previously categorized solely as Zone 7a, warming winter temperatures have pushed a significant portion of the area into the slightly milder 7b classification. This designation is a foundational tool for gardeners and landscapers, guiding the selection of plants that can survive the winter cold.
Understanding the USDA Plant Hardiness Zone System
The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is the standard measure for a plant’s ability to survive winter temperatures. This system divides the country into 13 zones, with each zone representing a 10-degree Fahrenheit range of the average annual extreme minimum temperature. Each main zone is further subdivided into a five-degree Fahrenheit range, denoted by ‘a’ and ‘b’. The designation is based only on the average coldest winter temperature over a 30-year period.
For Nashville’s Zone 7, the average annual extreme minimum temperature falls between 0°F and 10°F. Zone 7a represents the colder half of this range, with minimums between 0°F and 5°F. Zone 7b is the slightly warmer half, with minimum winter temperatures averaging between 5°F and 10°F. This five-degree difference can impact the survival of plants that are borderline hardy.
Gardening Strategies Based on Nashville’s Zone
The Zone 7a/7b designation means gardeners can confidently select plants rated for USDA Zone 7, ensuring they possess the necessary cold tolerance to survive a typical Nashville winter. This includes a wide array of perennials, shrubs, and trees, such as certain varieties of apples, cherries, black-eyed Susans, and hostas. Selecting plants based on the hardiness zone helps avoid planting species that cannot endure the deep freeze.
With the shift toward Zone 7b, the growing season is lengthening. The average last spring frost date for Nashville is typically April 1 to 10, while the average first fall frost occurs between November 1 and 10. Plants rated for Zone 8 may not survive the coldest periods in Nashville’s winter without protection, especially in 7a areas. Techniques like applying thick layers of mulch or wrapping plants in burlap can provide the necessary insulation for vulnerable species.
Accounting for Heat and Local Variations
While the USDA zones focus only on winter cold, successful gardening in Nashville also requires accounting for summer heat, which the American Horticultural Society (AHS) Heat Zone Map addresses. The AHS system measures the average number of “heat days” per year, defined as days when the temperature exceeds 86°F (30°C). Nashville experiences a substantial number of these hot days, which can cause stress for plants not adapted to high temperatures.
The city’s environment also features microclimates, which are small areas where the climate differs from the surrounding region. The urban heat island effect is a prominent example, where dense concentrations of pavement and buildings in downtown Nashville absorb and re-radiate heat. This effect can make the central city several degrees warmer than surrounding areas, often pushing those locations into the warmer 7b subzone. Gardeners near large bodies of water, on slopes, or in courtyards may find their specific location behaves slightly warmer or colder than the general zone designation.