The success of any garden is tied to understanding the local climate, a concept encapsulated by the planting zone system. This framework offers gardeners a clear, data-driven method for selecting perennial plants that can withstand the typical winter cold of their region. Knowing the specific conditions of a location prevents planting trees, shrubs, or flowers that are destined to fail during the coldest part of the year.
Identifying Tulsa’s Specific Hardiness Zone
Tulsa, Oklahoma, is primarily situated in USDA Plant Hardiness Zone 7b, according to the latest 2023 map update. This designation indicates that the average annual extreme minimum temperature in the area ranges from 5°F to 10°F (-15°C to -12.2°C). The shift from the previous 7a classification reflects a slight warming trend in the region’s winter lows. The Tulsa metropolitan area is large enough to contain minor variations, and small sections, particularly in the northeastern edges of the county, may still fall into the slightly colder Zone 7a. Zone 7a has an average annual extreme minimum temperature of 0°F to 5°F (-17.8°C to -15°C).
Understanding the USDA Hardiness Zone System
The USDA Plant Hardiness Zone Map is the standard measure used by gardeners across the United States to determine plant survivability. The system calculates the average coldest temperature recorded each year over a long-term period, typically 30 years. This metric is used because a plant’s ability to tolerate the winter’s lowest temperature is the primary factor determining survival.
The system is organized into major zones, numbered 1 through 13, with each full zone representing a 10-degree Fahrenheit range of minimum temperatures. Each major zone is further subdivided into two smaller subzones, ‘a’ and ‘b’, which represent a narrower 5-degree Fahrenheit temperature difference. This allows for a more accurate selection of plants for a specific location. The system is a tool for perennial selection, but it does not account for other climatic factors like summer heat, rainfall, or soil type.
Local Factors Beyond Temperature
Successful gardening in Tulsa requires managing several local environmental variables beyond cold tolerance. The region is known for its heavy clay soil, often classified as silty clay loam, which is dense and holds water for long periods. This characteristic can lead to poor drainage and root rot, making soil amendment with organic matter a regular necessity.
Microclimates and Urban Heat
Tulsa’s urban environment creates distinct microclimates that can locally alter the hardiness zone. Areas in the dense downtown core experience the Urban Heat Island (UHI) effect, where concrete and pavement absorb and re-emit heat. This results in summer and nighttime temperatures that can be several degrees warmer than surrounding rural areas. Conversely, the Arkansas River and local topography can create small, colder pockets where frost may linger longer in the spring.
Wind and Moisture
The climate is characterized by strong, prevailing southerly winds for much of the year, especially during the dry summer months. These persistent winds increase the rate of moisture loss from plant leaves, a process called desiccation. Desiccation is detrimental to broadleaf evergreens and young specimens. Gardeners must account for this wind exposure when siting plants, especially in open areas.
The area receives ample rainfall, with spring being the wettest season. However, this moisture is often delivered in heavy thunderstorms, creating a risk of flash flooding.
Using Zone Data for Planting Success
Applying Tulsa’s Zone 7b data starts with selecting perennial plants rated for Zone 7 or a lower, colder zone number. Plants rated for Zone 8 or higher will likely perish during a typical Tulsa winter unless protected. A plant rated for Zone 5 is a safe choice as it tolerates temperatures much colder than the expected minimum for the area.
Planting schedules must also be guided by typical frost dates. The average last spring frost, which signals the safe time to plant tender annuals and vegetables, occurs between April 5 and April 20. Conversely, the average first frost in the fall is observed between November 1 and November 10, marking the end of the growing season.
Many popular Zone 7 plants thrive in Tulsa, including the native Eastern Redbud, Purple Coneflower, and drought-tolerant Russian Sage. Occasional severe cold snaps can test the limits of the zone, causing dieback in plants like Crape Myrtles during rare, prolonged freezes. Simple season extension techniques, such as applying a thick layer of mulch or using row covers, can help protect roots and extend the growing period.