The Plant Hardiness Zone Map is a foundational tool for gardeners and growers across North America, designed to help predict which perennial plants can survive the winter in a specific location. By dividing regions into distinct zones, the map serves as a standard reference for understanding the limits of cold tolerance for various species. This system allows individuals to choose plants best suited to their local climate, improving the chances of long-term gardening success.
The Purpose of Plant Hardiness Zones
The Plant Hardiness Zone Map was developed by the United States Department of Agriculture (USDA) to provide a geographical representation of where perennial plants can endure the coldest part of the year. This map is based entirely on the average annual extreme minimum winter temperature recorded over a 30-year period. This minimum temperature is the primary factor determining the survivability of perennial species.
The map is divided into 13 zones, with each major zone representing a 10-degree Fahrenheit difference in the average annual minimum temperature. Each zone is further split into two sub-zones, designated ‘a’ and ‘b’, to provide a more accurate reading of temperature variations. These sub-zones, such as 5a and 5b, represent a narrower 5-degree Fahrenheit increment, allowing for a finer distinction in cold tolerance requirements. Gardeners should select plants with a hardiness rating that matches or exceeds their zone’s coldest expected temperature.
The Specifics of Zone 5
Zone 5 is defined by an average annual minimum winter temperature that falls within the range of -20°F to -10°F (-28.9°C to -23.3°C). This temperature band indicates a climate with cold winters, requiring plants to possess a significant degree of cold tolerance to survive. Zone 5 covers various regions across the United States, including parts of the Northeast, Midwest, and Mountain West.
The zone is further specified by its two sub-divisions. Zone 5a is the colder half, with an average annual minimum temperature between -20°F and -15°F. Conversely, Zone 5b is the warmer half, covering areas where the average lowest temperature ranges from -15°F to -10°F. Knowing the specific sub-zone helps gardeners make precise decisions when selecting perennial plants.
Successful Gardening Using Zone Data
Selecting Plants for Zone 5
Zone 5 data guides the selection of perennial plants, trees, and shrubs that can withstand the region’s intense winter cold. When choosing a plant, a gardener in Zone 5 should look for varieties labeled as hardy to Zone 5 or a lower, colder number, such as Zone 4 or 3. Selecting a plant rated for a warmer zone, like Zone 6, means it is likely to be killed by the average minimum temperatures experienced in a Zone 5 winter.
Limitations of the Hardiness Map
The hardiness zone designation is not the only factor determining a plant’s success, as the system does not account for local environmental variables. It does not factor in summer heat, which is measured separately by systems like the American Horticultural Society’s Heat-Zone Map. Other localized conditions significantly influence plant health, including:
- Soil type
- Drainage
- Rainfall
- Humidity
- Frequency of snow cover
Understanding Microclimates
Microclimates are small areas within a zone that have slightly different growing conditions and can affect plant survivability. For example, a sheltered area near a south-facing wall or a location protected from winter wind may be a half-zone warmer, enabling a gardener to successfully grow a plant rated for Zone 6. Conversely, low-lying areas where cold air pools might experience colder temperatures than the official zone designation.
Common Zone 5 Plants
For Zone 5, many cold-tolerant plants thrive. Popular perennial examples include hostas, peonies, daylilies, and coneflowers. Reliable trees and shrubs for this zone include lilacs, certain hydrangeas, and maples like the paperbark maple. Gardeners must consider the plant’s overall requirements for sun and moisture, recognizing that the hardiness zone only addresses the single, most limiting factor of winter cold.