The success of a garden, especially one containing plants that must survive multiple winters, depends on understanding the local climate. For gardeners in regions with cold seasons, the Plant Hardiness Zone system serves as a foundational tool for making informed planting decisions. This system provides a standardized way to measure the lowest temperatures a plant can reliably endure. Knowing your location’s designation is the first step toward determining which perennial trees, shrubs, and flowers are most likely to survive the winter without significant damage.
Decoding the Plant Hardiness Zone System
The United States Department of Agriculture (USDA) developed the Plant Hardiness Zone (PHZ) map to provide a consistent framework for gardeners and commercial growers. Zones are determined exclusively by the average annual extreme minimum winter temperature recorded at a specific location. This calculation uses 30 years of temperature data, which smooths out yearly weather fluctuations to create a reliable long-term average.
The system is divided into 13 major zones, running from Zone 1 (coldest) to Zone 13 (warmest). Each whole zone represents a 10-degree Fahrenheit difference in the average minimum temperature. For increased precision, each zone is further broken down into ‘a’ and ‘b’ subzones. The ‘a’ subzone represents the colder half of the temperature band, while the ‘b’ subzone indicates the warmer half, with each subzone covering a 5-degree Fahrenheit increment.
Mapping Pennsylvania’s Specific Zones
Pennsylvania’s varied topography results in a broad range of hardiness zones across its borders. The state currently spans from the colder Zone 5a to the warmer Zone 7b, with some highly localized areas reaching 8a. This wide span means a plant that thrives in one part of the state may quickly perish in another.
The state’s coldest designations, typically Zone 5a and 5b, are found in the northern tier counties and high-elevation areas of the Allegheny Plateau. These regions experience the most severe winter temperatures, limiting the palette of plants that can reliably survive. Moving toward the central part of the state, most areas fall within the transitional range of Zone 6a and 6b, including the Ridge and Valley regions.
The warmest zones, including 7a and 7b, are concentrated in the southeastern portion of the state, particularly the Philadelphia metropolitan area. These warmer zones benefit from the urban heat island effect, which elevates minimum winter temperatures. Areas along the Lake Erie shoreline in the northwest also experience a moderating effect from the large body of water, pushing those coastal regions into warmer zone designations than immediate inland areas.
Beyond the Zone: Accounting for Local Factors
While the USDA hardiness zone provides an excellent baseline, it is a regional average and does not account for many localized variables. These small-scale variations, often called microclimates, can shift the effective hardiness of a garden plot by a half-zone or more. Elevation is one such factor, as cold air is denser than warm air and tends to pool in low-lying valleys, creating colder pockets that can harm sensitive plants.
Proximity to structures, such as a building’s south-facing wall or a paved patio, can create a small-scale “heat island” effect that radiates warmth and protects plants from extreme cold. Conversely, constant, high-speed winter wind exposure can cause desiccation and cold damage, even if the temperature is within the plant’s survival range. The map focuses only on the coldest temperature and ignores other factors like soil type.
Soil drainage is another non-temperature factor that significantly influences plant survival. A plant that is perfectly hardy for a given zone may fail if its roots sit in poorly draining, saturated soil during the winter, which makes the roots more susceptible to freezing and rot. Gardeners must consider the site’s sun exposure, wind patterns, and moisture levels in conjunction with the official zone number. Using the PHZ map as a suggestion rather than an absolute rule allows for greater success and experimentation.
Applying Zone Data to Your Garden
The primary function of the hardiness zone rating is to guide the selection of perennial plants (including trees, shrubs, and herbaceous plants) that must tolerate the entire winter cycle. When purchasing plant material, gardeners should look for a zone number that matches their location. For added security, selecting a plant rated for one zone colder (e.g., Zone 5 for a Zone 6b location) provides a buffer against unusually harsh winters.
The zone number is less relevant for annual plants, such as most vegetables and seasonal flowers, which complete their life cycle within a single growing season. For annual plantings, information regarding the last expected spring frost and the first expected fall frost is more useful. However, the zone data still contributes to the overall climate picture by helping determine the length of the growing season. By cross-referencing a plant’s hardiness with the unique conditions of a specific garden spot, growers maximize the chance of long-term establishment.