The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is the standard tool for determining which perennial plants can survive the winter cold in a specific location. For gardeners and growers in Lancaster, Pennsylvania, the current official designation is USDA Hardiness Zone 7a. This classification provides the foundational knowledge necessary to select trees, shrubs, and perennial flowers that can reliably withstand the region’s average annual extreme minimum winter temperatures.
The Specific USDA Hardiness Zone for Lancaster
The current USDA Plant Hardiness Zone Map, updated in 2023, places the city of Lancaster and much of Lancaster County into Zone 7a. This zone is defined by an average annual extreme minimum winter temperature range of 0°F to 5°F.
This designation represents a significant shift from the previous 2012 map, which had classified most of Lancaster County as the slightly colder Zone 6b. Zone 6b has an average annual extreme minimum temperature of -5°F to 0°F. The change to Zone 7a indicates that the region’s winters have become measurably warmer over the past few decades, aligning with broader climate trends observed across Pennsylvania.
The USDA system is specifically designed to help gardeners determine a plant’s cold tolerance. When a plant is rated for Zone 7, it means it should be able to tolerate temperatures down to 0°F without needing special winter protection. While the map provides a reliable average, it does not predict the coldest temperature that may ever occur in a single, unusually severe winter.
Accounting for Local Climate Variations
While the official designation is Zone 7a, the actual conditions on a specific property can vary due to localized climate factors, creating small-scale “microclimates”. The urban center of Lancaster City, for example, experiences the urban heat island effect. This phenomenon occurs because dense concentrations of pavement, buildings, and impervious surfaces absorb and retain heat more effectively than vegetated rural areas.
As a result, the city’s built-up areas can be measurably warmer, especially at night, compared to the surrounding farmlands of Lancaster County. This effect can sometimes push the minimum temperature in the densest parts of the city closer to the warmer end of Zone 7a, or even occasionally into Zone 7b. Conversely, properties located at higher elevations or in low-lying areas where cold air settles may experience temperatures closer to the historical Zone 6b.
The influence of the Susquehanna River, which forms the western border of Lancaster County, is also a factor. Large bodies of water can slightly moderate temperatures, often preventing the most extreme cold snaps immediately along the banks. Gardeners should observe their specific property’s conditions, noting factors like wind exposure, proximity to walls or structures, and elevation, to fine-tune the general Zone 7a guidance for their yard.
Practical Planting Guidance Based on the Zone
The Zone 7a classification offers a wide range of planting possibilities for trees, shrubs, and perennial flowers. Gardeners should choose varieties rated for Zone 7 or any lower (colder) zone, such as Zone 6 or 5, to ensure winter hardiness. For instance, certain varieties of shrubs like Azalea, Boxwood, and Crape Myrtle are now a more reliable choice.
Suitable perennial flowers include favorites such as Peonies, Irises, Daylilies, and Coneflowers. For trees and other woody plants, options such as certain hardy Camellias, Holly, and specific varieties of Magnolia can be successfully grown. However, plants rated for Zone 8 or higher should be treated as annuals or require substantial winter protection, like heavy mulching, to survive.
Beyond cold tolerance, planting schedules are guided by the average frost dates for the Lancaster area. The typical last spring frost occurs around April 27, which is the general benchmark for safely planting tender annuals and vegetables outdoors. The average first fall frost is expected between October 21 and October 31, marking the approximate end of the growing season.