The survival of perennial plants, trees, and shrubs depends heavily on a region’s climate, particularly the severity of its winter cold. Knowing the local climate zone is a necessary first step for anyone planning a garden or landscape project. This geographical classification provides a reliable guide for selecting plants that can withstand typical winter conditions. Understanding the specific zones that cover Western Pennsylvania is the foundation for successful gardening in the region.
Defining the USDA Plant Hardiness Zone System
The recognized standard for determining a plant’s cold tolerance is the USDA Plant Hardiness Zone (PHZ) Map, which divides North America into distinct regions. This system is based solely on the average annual extreme minimum winter temperature, derived from the coldest night expected each year over a 30-year period. The system does not account for the absolute lowest temperature ever recorded, only the long-term average minimum.
The map is structured into 13 major zones, with each zone representing a 10-degree Fahrenheit range of minimum temperatures. To offer greater precision, each of these zones is further divided into two sub-sections, labeled ‘a’ and ‘b,’ which represent 5-degree increments. For instance, Zone 6 covers an average minimum temperature range from -10°F to 0°F. Zone 6a represents the colder half (-10°F to -5°F) and Zone 6b the warmer half (-5°F to 0°F).
The Hardiness Zones of Western Pennsylvania
Western Pennsylvania is not a single uniform zone but rather a transitional region spanning three hardiness designations: 5b, 6a, and 6b. The most recent USDA map (2023) shows that the vast majority of the western half of the state falls within the warmer Zone 6 categories. Zone 5b is the coldest designation found in the region, with an average annual extreme minimum temperature between -15°F and -10°F. This zone is typically restricted to the highest elevations and northernmost counties, such as portions of McKean and Elk Counties.
Moving southward and westward, the primary zones become 6a and 6b, indicating milder average minimum winter temperatures. Zone 6a (-10°F to -5°F) covers a large portion of the central and northwestern counties, including Erie, Crawford, and Somerset Counties. Zone 6b (-5°F and 0°F) is prevalent in the central to southwestern region. This warmer zone includes all of Allegheny County (Pittsburgh), as well as Beaver and Washington Counties. A small portion of the extreme southwestern corner may even touch Zone 7a (0°F and 5°F).
Understanding Regional Climate Factors
The variation across Western Pennsylvania, from Zone 5b to 7a, results from distinct geographical features that create localized microclimates. One major influence is the effect of Lake Erie on the northwestern counties. The large body of water moderates winter temperatures along the immediate shoreline, acting as a heat sink that prevents extreme cold and can push areas like the city of Erie into the warmer 6a or 6b zones.
Conversely, elevation plays a significant role in determining the coldest zones. The Allegheny Plateau includes mountainous regions where altitude causes temperatures to drop significantly. These higher areas, particularly in the east and north, are typically classified as the colder Zone 5b due to the cooling effect of the landscape. The presence of large metropolitan areas also affects the local climate through the Urban Heat Island Effect. Cities like Pittsburgh retain heat more effectively than surrounding rural areas, often resulting in a zone “bump” where the city center may be 6b while nearby countryside areas remain 6a.
Using Zone Data for Plant Selection
The zone number serves as a practical tool for predicting the likelihood of a plant surviving the winter. When purchasing perennial plants, gardeners should check the label, which lists the recommended hardiness zone range. Selecting plants rated for Zone 6 or lower is generally a safe choice for most of Western Pennsylvania, ensuring the plant’s root system can tolerate the average minimum cold.
While the PHZ map is a useful guide, it has limitations, as it only measures cold tolerance. It does not account for other factors that affect plant health, such as summer heat, rainfall, soil type, or micro-environmental conditions. Experienced gardeners may attempt to “push the zone” by planting a species rated for a slightly warmer area, like a Zone 7 plant in a Zone 6b location. This requires careful mitigation strategies, such as planting in a sheltered location, providing additional winter protection, or using a container that can be moved indoors during the coldest periods.