The USDA Plant Hardiness Zone Map divides North America into geographical areas based on the lowest winter temperatures. Knowing your local zone is necessary for selecting plants that can withstand the coldest temperatures of a typical winter. For gardeners in Philadelphia, this zone designation determines which perennials will reliably survive and return year after year.
Philadelphia’s Current USDA Hardiness Zone
Philadelphia currently falls primarily into USDA Hardiness Zone 7, confirmed by the 2023 map update. Most of the city and its immediate surroundings are now considered Zone 7b, shifting from the previous 2012 map where the city was split between 7a and 7b. This change reflects a warming trend in the region’s winter climate.
The update means local gardeners can choose from a broader selection of plants, as the average annual minimum temperature has increased. While a small section of far northeastern Philadelphia may still be designated 7a, the majority is established in the warmer 7b subzone. This allows for the successful cultivation of plants once considered marginally hardy for the region.
What the Zone Number Signifies
The USDA Plant Hardiness Zone system is based solely on the average annual minimum winter temperature, excluding factors like snowfall or summer temperatures. Each full zone, such as Zone 7, represents a 10-degree Fahrenheit band of minimum temperature data.
Zone 7 indicates that the average coldest temperature recorded each year falls between 0°F and 10°F. The zone is then divided into two 5-degree sub-designations: 7a (0°F to 5°F) and 7b (5°F to 10°F). This temperature range is the primary factor determining a perennial plant’s ability to survive the winter.
A plant labeled “hardy to Zone 7” should survive the coldest expected temperatures in that zone. Although the map provides a standard, other local factors like soil drainage, wind exposure, and snow cover also influence a plant’s winter survival.
Planting and Timing Strategies for Zone 7
The Zone 7 designation, particularly 7b, allows Philadelphia gardeners to successfully grow a wide range of popular perennials and shrubs. Reliable choices include Coneflowers (Echinacea spp.), Black-Eyed Susans (Rudbeckia spp.), and Daylilies (Hemerocallis). The warmer winters also make it possible to grow plants previously considered challenging, such as certain varieties of Crape Myrtle or Southern Magnolia.
Frost Dates and Growing Season
Planting timing is guided by typical frost dates, which signal the start and end of the growing season for tender vegetables and annuals. The average last spring frost date for Philadelphia is typically around April 4th, aligning with the Zone 7 range of late March to early April. Tender plants, such as tomatoes and peppers, should not be transplanted outdoors until after this date, generally in early to mid-May, to avoid unexpected cold damage.
The growing season extends until the average first fall frost date, which usually occurs around November 11th in Philadelphia. This long growing period provides ample time for both spring-planted crops and fall crops like garlic, which can be planted in October.
Microclimates
Gardeners should also consider the effect of microclimates within the city. Large areas of concrete and buildings create an “urban heat island” effect. Downtown Philadelphia often experiences slightly warmer minimum temperatures, leaning more strongly into the 7b classification, while outlying suburban areas may be closer to 7a. Observing specific local conditions, such as a sheltered courtyard versus an exposed rooftop garden, helps refine planting choices beyond the general zone map.