Perennial plants’ ability to survive winter depends on the lowest temperatures they can tolerate. Gardeners use a standardized system to predict this survival, but regional variations require local knowledge. Pennsylvania is geographically diverse, so cold tolerance knowledge is not uniform across all counties. Understanding the zone designation is the first step in successful gardening and landscaping.
Understanding the USDA Hardiness Zone System
The United States Department of Agriculture (USDA) developed the Plant Hardiness Zone Map as the standard guide for determining which perennial plants will thrive. This map is based entirely on the average annual extreme minimum winter temperature, predicting the coldest conditions a plant must endure. The system does not account for other climate factors like summer heat, rainfall, or soil type. It is built around the concept of a plant’s ability to survive the coldest night of the year.
The map divides North America into 13 zones, each representing a 10-degree Fahrenheit difference in average minimum temperature. These zones are further split into ‘a’ and ‘b’ subcategories, representing a finer 5-degree Fahrenheit distinction. A plant hardy to Zone 6 can survive colder temperatures than one rated for Zone 7. This detailed numbering provides a precise guide for gardeners selecting perennial species.
The Range of Zones Across Pennsylvania
Pennsylvania encompasses a wide range of climates, translating to USDA zones from the cold 5a/5b half-zones to the warmer 7a/7b designations. Most of the state falls within the 5b through 7a range. Determining the exact zone requires checking a specific ZIP code, as boundaries often shift over short distances.
The coldest regions (Zone 5a and 5b) are found predominantly in the northern tier and the highest elevations of the Appalachian Mountain ranges. These areas experience the lowest average annual minimum temperatures, posing the greatest challenge to winter survival for less hardy plants. Moving southward and eastward, the zones gradually warm, reflecting a milder winter climate.
The warmest zones (7a and 7b) are concentrated in the southeastern corner of Pennsylvania, particularly around the Philadelphia metropolitan area and the lower Delaware River Valley. This region benefits from its lower latitude and proximity to the Atlantic coastal influence. Zone 7b represents the mildest winter conditions, allowing gardeners to cultivate a broader array of perennial plants.
Local Factors that Shift Zone Boundaries
While latitude and elevation determine the general zone bands, microclimates frequently cause zone boundaries to shift locally. Elevation is a primary factor, with higher altitudes naturally experiencing lower temperatures than valley floors. This means two nearby towns can be in different half-zones if one is situated on a mountain ridge and the other is in a river basin.
The presence of large bodies of water, such as the Great Lakes or major river systems, also exerts a moderating effect on local temperatures. Water retains heat longer than land, which can slightly warm adjacent areas during winter months, often bumping these locations into a warmer half-zone. This thermal effect is observable near Lake Erie.
Another highly localized phenomenon is the Urban Heat Island effect, which causes city centers to register as significantly warmer than the surrounding rural landscape. Densely packed buildings and paved surfaces absorb and retain heat, releasing it slowly at night and preventing temperatures from dropping as low. This can result in a major city center being designated as 7b while surrounding areas are a full zone colder at 6b.
Practical Application: Selecting Plants Based on Zone
Gardeners should use their local hardiness zone number as a foundational guide when purchasing perennial plants. A plant label listing a hardiness range, such as “Zones 4-8,” indicates the species is expected to survive the average annual minimum temperatures experienced in those zones. Selecting plants rated for your specific zone or a colder zone provides the best assurance of winter survival.
It is common practice to select plants rated for one zone colder than the local designation to provide a safety margin against unusually severe winters. While the USDA map is based on long-term averages, a rare cold snap can still occur that exceeds the typical minimum temperature for the area. Relying too heavily on a plant’s ability to survive at the very limit of its cold tolerance can lead to disappointment.
The hardiness zone is only one part of the equation for plant success, as it focuses exclusively on winter cold tolerance. Gardeners must also evaluate other conditions, including a plant’s specific needs for summer heat, available sunlight, and soil moisture levels. The map serves as a starting point, but a cold-hardy plant may still fail if it cannot tolerate the state’s humid summers or heavy clay soils.