Gardeners often face the challenge of selecting plants that can successfully withstand the local winter climate. The Plant Hardiness Zone (PHZ) map provides a standard guide for plant survival. This system, developed by the United States Department of Agriculture (USDA), is the standard reference used across the country to classify regions based on their cold tolerance. Understanding the specific zones of Western Pennsylvania is the necessary first step for any gardener planning a thriving landscape. This information allows for the informed selection of perennial plants, trees, and shrubs best suited to survive the area’s winter conditions.
Defining the USDA Plant Hardiness Zone System
The USDA Plant Hardiness Zone Map is a geographical framework designed to help growers determine which perennial plants are most likely to survive the coldest winter temperatures. The entire system is based exclusively on the average annual minimum winter temperature recorded over a 30-year period. It does not account for factors like summer heat, rainfall, or soil type.
The map divides North America into 13 distinct zones, numbered 1 through 13, with Zone 1 being the coldest. Each successive zone represents a 10-degree Fahrenheit increase in the average minimum winter temperature. For instance, a plant rated for Zone 6 can survive a colder winter than a plant rated for Zone 7.
To provide greater precision, each 10-degree zone is further subdivided into two 5-degree increments labeled ‘a’ and ‘b’. Zone ‘a’ represents the colder half, while Zone ‘b’ represents the warmer half. For example, Zone 6 spans from -10°F to 0°F, meaning Zone 6a covers -10°F to -5°F, and Zone 6b covers -5°F to 0°F. This specific distinction allows gardeners to make more accurate choices.
Mapping the Zones of Western Pennsylvania
Western Pennsylvania contains a range of hardiness zones, primarily spanning from Zone 5b to Zone 6b, with a few smaller areas classified as Zone 7a. The specific zone a location falls into is heavily influenced by its elevation and proximity to large bodies of water or urban centers. Gardeners in this region should use their specific ZIP code on the latest USDA map to find their exact classification.
The coldest zones, typically Zone 5b, are found in higher elevations, such as the Laurel Highlands and northern counties like McKean and Elk. These areas experience average annual minimum temperatures between -15°F and -10°F. Much of the central and western parts of the region, including the Pittsburgh metropolitan area, fall into the Zone 6 range (6a and 6b).
Zone 6a is characterized by average lows between -10°F and -5°F, covering a significant portion of the western counties. The warmer Zone 6b, with minimum temperatures from -5°F to 0°F, is found in southwestern valleys and areas benefiting from the urban heat island effect of dense urban development. Small pockets of Zone 7a, where temperatures rarely drop below 0°F, can be found along the shore of Lake Erie or in the most concentrated metropolitan cores. The large volume of water in Lake Erie helps moderate temperature extremes in the adjacent coastal areas.
Applying Zone Information to Gardening Success
The primary use of the hardiness zone is to select perennial plants, trees, and shrubs that can survive the lowest winter temperatures. When examining a plant tag, a gardener should choose a variety rated for their zone or any zone with a lower number, which signifies greater cold tolerance. For example, a plant labeled as hardy in Zones 4 through 9 will successfully overwinter in Zone 6b, but a plant rated only for Zones 7 through 10 would likely not survive.
It is important to recognize that the published zone is an average guide and not an absolute predictor of plant survival. Within any yard, microclimates exist that can alter the effective hardiness zone by as much as a half or full zone. Areas near south-facing brick walls or building foundations often retain heat, creating a warmer microclimate that might allow for slightly less hardy plants. Conversely, low-lying areas where cold air settles or spots exposed to persistent winter wind can be colder than the official zone suggests.
Gardeners must also consider local environmental factors that the hardiness zone system does not address. The PHZ does not offer information about soil quality, which is particularly relevant in Western Pennsylvania where heavy clay soils are common and can affect root health and drainage. Additionally, the planting zone is not a guide for when to plant annual vegetables or flowers.
For annual planting, the local last and first frost dates are the controlling factors, as they determine the length of the growing season. Combining the PHZ data with information on soil composition, sun exposure, and local frost dates provides a comprehensive strategy for successful gardening in Western Pennsylvania.