The USDA Plant Hardiness Zone (PHZ) map is an important tool for gardeners determining which perennial plants are most likely to survive the winter in a specific location. This map divides the country into distinct zones based on long-term weather data, providing a standardized measure of cold tolerance. Oklahoma encompasses a wide range of these zones due to its large size and varied geography, meaning a single plant may thrive in one corner of the state but perish in another. Understanding the local hardiness zone is the first step toward successful gardening within Oklahoma’s dynamic climate.
Understanding the USDA Plant Hardiness Zone System
The Plant Hardiness Zone system measures the average annual minimum winter temperature over a 30-year period. This measurement provides a reliable indicator of the coldest temperature a plant must endure to survive the winter and return the following spring. The system is divided into 13 main zones, each representing a 10°F difference in the average minimum temperature.
Each major zone is further refined into two subzones, labeled ‘a’ and ‘b,’ which narrow the temperature range by 5°F. For example, Zone 7 is split into 7a and 7b, with 7a being the colder portion. Gardeners use this information to select perennial plants, trees, and shrubs that are rated to survive the lowest expected winter temperatures in their specific region. The USDA map, recently updated using data through 2020, reflects the most current climate averages.
The Specific Hardiness Zones of Oklahoma
Oklahoma’s hardiness zones span primarily from Zone 6a to 7b, with a small pocket of Zone 8a in the far south, increasing in warmth from the northwest to the southeast. The coldest areas are the Panhandle and the northern tier of the state, falling into the 6a and 6b categories. Zone 6a represents an average annual extreme minimum temperature range of -10°F to -5°F, while 6b ranges from -5°F to 0°F.
The central portion of Oklahoma, including the major metropolitan areas of Oklahoma City and Tulsa, is largely situated within Zone 7a. This zone experiences average annual minimum temperatures between 0°F and 5°F. While the OKC metro area is mostly 7a, some urban areas and parts of the Tulsa metro may trend toward the slightly warmer 7b designation.
The warmest zones are concentrated in the southern and southeastern corners of the state, including cities like Lawton and Ardmore, which are predominantly in Zone 7b. Zone 7b has an average annual minimum temperature range of 5°F to 10°F, allowing for a wider selection of cold-sensitive plants. A small area along the state’s southern edge has recently shifted to Zone 8a, indicating average minimum winter temperatures are now between 10°F and 15°F.
Applying Zone Information to Planting Decisions
Gardeners must use their specific hardiness zone to ensure the long-term survival of perennial plants. When selecting a plant, the listed hardiness rating on the tag should be equal to or lower than the gardener’s zone number. For instance, a gardener in Zone 7a should only purchase a perennial rated for Zone 7 or a lower number (such as Zone 6 or 5) to ensure it withstands the expected average low temperature of 0°F to 5°F.
Selecting plants rated for a zone warmer than the local area, such as a Zone 8 plant in Zone 7a, risks winter mortality. For plants considered “borderline” hardy (where the rating is the same as the local zone), gardeners often provide winter protection. This protection involves applying a thick layer of organic mulch around the base or covering the plant with burlap to insulate the root system from severe cold snaps.
The hardiness zone map is strictly a guide for perennial plant survival during winter and does not provide information about spring planting times. The PHZ map is distinct from first and last frost dates, which determine the safe planting window for annual vegetables and flowers. While the hardiness zone tells a gardener which plants can survive the winter, local frost dates dictate when to plant seeds or transplants and when to expect the end of the growing season.