The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is the primary reference tool for gardeners and growers across the country. This map divides North America into defined geographic areas, providing a standardized way to determine which plants can survive the winter cold in a specific location. Understanding this designation is necessary for Philadelphia residents to select appropriate trees, shrubs, and perennial flowers that will thrive year after year.
Defining the Hardiness Zone System
The USDA Plant Hardiness Zone Map is based on the average annual minimum winter temperature. The map is divided into 13 major zones, progressing from the coldest (Zone 1) to the warmest (Zone 13) across the United States. Each ascending zone number represents a 10-degree Fahrenheit increase in the average minimum winter temperature.
For greater precision, each major zone is divided into two sub-zones, “a” and “b.” These sub-zones represent a narrower temperature difference of 5 degrees Fahrenheit. This classification system allows gardeners to compare a plant’s cold tolerance with the typical winter conditions of their local area.
Philadelphia’s Specific Hardiness Designation
Based on the latest USDA Plant Hardiness Zone Map, updated in 2023, the majority of Philadelphia, Pennsylvania, is classified as Zone 7b. This designation indicates that the average annual minimum winter temperature falls within the range of 5°F to 10°F.
The 7b classification represents a shift from the previous 2012 map, which had split the city between Zone 7a and 7b. The current 7b sub-zone allows for the successful cultivation of plants that would have been marginally hardy under the slightly colder conditions of Zone 7a. Only the farthest northeastern edges of the city may still retain a Zone 7a designation.
Using the Zone for Plant Selection
The practical application of knowing the Zone 7b designation is to select plants that are labeled as hardy to Zone 7 or a lower number. Plant tags and nursery catalogs typically list the coldest hardiness zone a species can reliably survive. Choosing plants rated for Zone 7 means they can withstand a minimum temperature down to 0°F to 10°F, ensuring their survival through a typical Philadelphia winter.
The zone number is particularly important when selecting perennial species, which are expected to return year after year, as well as trees and shrubs. Annual flowers, which complete their life cycle in a single season and must be replanted, are generally not affected by the hardiness zone rating. Gardeners in Zone 7 can successfully cultivate a wide range of popular species, including roses, hostas, coneflowers, asters, and hydrangeas. The moderate winter temperatures also permit the growth of plants previously considered tender in the region, such as certain varieties of camellias and crape myrtle.
Local Variations and Map Updates
Although the Zone 7b designation provides a reliable baseline, local conditions can create significant variations within the city, known as microclimates. The most prominent of these is the urban heat island effect, where the dense concentration of buildings, asphalt, and concrete absorbs and retains heat more effectively than surrounding suburban or rural areas. This effect can make the core of Philadelphia measurably warmer than its outlying neighborhoods, sometimes by as much as 5 to 12 degrees Fahrenheit during high-temperature periods.
These warmer microclimates mean that a garden in Center City might function more like a Zone 8 area than the official Zone 7b. Other local factors, such as proximity to a large body of water like the Delaware River, or the protection offered by a building or fence, can also moderate winter temperatures. The shift of Philadelphia’s classification to a solid Zone 7b in the 2023 map update reflects a broader trend of warming winters, which continues to expand the planting possibilities for local gardeners.