Understanding the local climate is paramount for successful gardening. The primary system used across the United States is the USDA Plant Hardiness Zone Map (PHZ), which guides the selection of suitable perennial plants. Pennsylvania is geographically diverse, encompassing a range of zones rather than a single classification. Gardeners must pinpoint their exact location on the map for the most accurate planting advice.
Understanding the USDA Plant Hardiness Zones
The USDA Plant Hardiness Zone Map is the standard reference tool for determining the long-term winter survival of perennial plants. This system is based on one specific metric: the average annual extreme minimum winter temperature. A lower zone number indicates a colder climate, while a higher number signifies a warmer climate.
The map is divided into zones, with each major zone representing a 10-degree Fahrenheit range of minimum temperatures. Each full zone is further split into two sub-zones, labeled ‘a’ and ‘b’, which narrow the temperature range to a 5-degree Fahrenheit difference. This provides a more precise classification; for example, 6b is slightly warmer than 6a.
The most recent PHZ Map is based on 30-year averages of observed extreme cold temperatures. This long-term average helps gardeners select plants likely to thrive over many years. The map is designed as a guide for perennial plants that must survive the winter, and is not a factor for annual plants.
Pennsylvania’s Specific Hardiness Zones
Pennsylvania’s diverse landscape results in a planting zone range of 5a to 7b, with a small portion of 8a appearing in the latest data. The coldest regions are found in the northern tier and the high-elevation areas of the Allegheny Plateau and the Poconos. These areas are primarily classified in the 5a and 5b zones.
The majority of the state, including the central, western, and southwestern regions, falls predominantly within the intermediate zones of 6a and 6b. Cities like Pittsburgh often fall into these moderate zones, which offer a longer growing season. This allows for a wider variety of plant life compared to the north.
The warmest classifications are concentrated in the southeastern corner, particularly around the Philadelphia metropolitan area and along the Delaware River. This region is generally found in zones 7a and 7b, though a tiny sliver of 8a has been identified. This warmer climate is influenced by the proximity to the Atlantic Ocean and the urban heat island effect, which elevates winter minimum temperatures.
Applying Your Zone for Successful Gardening
Gardeners use their specific zone designation to select perennial plants that have the best chance of surviving local winter conditions. Every perennial plant sold by a reputable nursery is labeled with a hardiness zone range. The goal is to ensure the plant’s minimum hardiness zone matches or is lower than the local area’s zone.
For instance, a gardener in zone 6b should choose plants rated for zones 6, 5, or lower. Plants rated for zone 7 or higher may not tolerate the expected minimum temperatures. Using a zip code search tool on the official USDA map is the most direct way to confirm a specific location’s zone and sub-zone.
The PHZ Map is a broad guide, and localized conditions known as microclimates can cause variations within a small area. Factors like a south-facing wall, a sheltered valley, or proximity to a large body of water can create a pocket of warmer air. Conversely, a windy, exposed hilltop may experience colder conditions than the surrounding map suggests. Gardeners can account for these localized temperature shifts by observing their specific landscape.