The USDA Plant Hardiness Zone Map is the standard tool for gardeners to determine which perennial plants can successfully survive the average minimum winter temperatures in a specific location. By dividing the country into distinct climate zones, the map helps predict the likelihood of plant survival through the coldest months. Understanding the precise hardiness zone is the first step toward selecting appropriate trees, shrubs, and flowers, maximizing the chance of a successful garden. This determination is necessary in Pennsylvania, a state with diverse geography resulting in a wide range of growing conditions.
Understanding the Hardiness Zone System
The USDA system categorizes geographic areas based on the average annual extreme minimum winter temperature, using data collected over a 30-year period. The current map, updated using data from 1991 to 2020, divides the United States into 13 major zones. Each full zone represents a 10°F difference in the average minimum winter temperature, with lower numbers indicating colder climates.
For finer precision, each main zone is split into two subzones, designated ‘a’ and ‘b’. The ‘a’ subzone is the colder half, while the ‘b’ subzone is the warmer half, representing a specific 5°F temperature increment. This division allows gardeners to use a localized number, such as 6a or 6b, to select plants whose cold tolerance matches the average lowest temperatures expected. The zone designation only reflects cold tolerance and does not account for other factors like soil type, rainfall, or summer heat.
The Hardiness Zones Specific to Pennsylvania
Pennsylvania is a climatically diverse state, spanning hardiness zones from 5a to 7b. The coldest areas (Zone 5a and 5b) are typically found in the northern tier and high-elevation plateaus, such as the Poconos and the Allegheny Mountains. Zone 5a indicates an average annual minimum temperature range of -20°F to -15°F, while 5b ranges from -15°F to -10°F.
A large portion of the central and western parts of the state falls within Zone 6, split between 6a (-10°F to -5°F) and 6b (-5°F to 0°F). These zones encompass many mid-latitude valleys and lower-elevation areas. The warmest zones, 7a and 7b, are concentrated in the far southeastern corner of the state, including the greater Philadelphia region and the Delaware River Valley.
Zone 7a features an average minimum temperature range of 0°F to 5°F, while Zone 7b averages 5°F to 10°F as its annual low. The shift from the coldest 5a regions to the warmest 7b areas represents a 30-degree difference in the average minimum winter temperature across the state. This wide variation means a plant suitable for the Philadelphia area may not survive the winter in the northern mountainous regions.
Geographic Factors Influencing PA’s Zones
The variation in Pennsylvania’s hardiness zones is directly linked to its varied geography. Elevation plays a major role; higher altitudes, such as those in the Appalachian Mountain chain, consistently experience colder temperatures, resulting in lower zone numbers. These mountainous areas often act as climatic barriers, influencing wind patterns and temperature distribution.
Proximity to large bodies of water also moderates temperatures, most notably near Lake Erie in the northwest corner. The lake retains heat longer into the fall and winter, which can slightly elevate the zone rating in the immediate coastal area compared to inland locations. This thermal effect helps delay the onset of severe winter cold.
Urban Heat Islands
The presence of major metropolitan areas creates localized phenomena known as urban heat islands. Cities like Philadelphia and Pittsburgh, with dense concentrations of concrete, asphalt, and buildings, absorb and retain heat more effectively than surrounding rural areas. This retained warmth can push a city’s zone rating up by as much as one full subzone compared to the nearby countryside, allowing for the cultivation of slightly less cold-tolerant plants.
Using Hardiness Zones for Successful Planting
Gardeners should use their specific zone designation when selecting perennial plants to ensure they are cold-tolerant enough to survive the winter. If a plant is rated for Zone 6, it should be able to withstand the average minimum temperatures of any Zone 6 or higher location. To provide a buffer against unusually cold winters, it is advisable to select plants rated for one zone colder than your own area (e.g., a Zone 5 plant for a Zone 6 garden).
The hardiness zone only predicts the survival of a plant’s roots and woody structure during the winter; it does not account for summer heat or moisture requirements. Gardeners must also consider their property’s specific microclimates—small areas where temperatures vary from the general zone. A sheltered spot near a south-facing brick wall, for example, will be significantly warmer than an open, windy location, potentially allowing a slightly less hardy plant to thrive.
Conversely, areas that collect cold air or have poor drainage may be effectively one subzone colder than the official rating. Considering microclimates, combined with the official hardiness zone, provides the most accurate assessment of a plant’s likelihood of survival. Successful gardening involves using the zone as a foundational guide and then making practical adjustments based on specific garden conditions.