Understanding the local climate is a foundational requirement for successful gardening and landscaping. A plant’s ability to survive long-term depends heavily on the seasonal weather extremes it will face. Knowing the climate profile helps a gardener choose species that can endure the coldest temperatures of winter or the harshest heat of summer.
Understanding the USDA Hardiness Zone System
The USDA Plant Hardiness Zone Map is the standard tool used by growers to evaluate a plant’s cold tolerance. This map divides North America into distinct zones based on the average annual minimum winter temperature. This metric is calculated using weather data collected over a 30-year period.
The zones themselves are delineated in 10-degree Fahrenheit increments, with lower numbers representing colder climates. For greater precision, each zone is further subdivided into a warmer half, designated ‘a’, and a colder half, designated ‘b’. These sub-zones represent a smaller 5-degree Fahrenheit range, providing gardeners with a more detailed classification for their specific location.
Missouri’s Specific Zone Designations
Missouri’s transitional climate results in a range of hardiness zones, reflecting a clear temperature gradient from north to south. The state generally encompasses USDA Plant Hardiness Zones 5b through 7a. Northern counties, such as those near the Iowa border, are the coldest at Zone 5b, where the average annual minimum winter temperature falls between -15°F and -10°F.
Moving southward, the majority of the state, including central areas like Columbia and Kansas City, falls into Zone 6a (-10°F to -5°F) and Zone 6b (-5°F to 0°F). These zones cover the largest portion of Missouri. The southern half of the state, including the Springfield and St. Louis metropolitan areas, transitions into the milder Zone 7a (0°F to 5°F).
The warmest zones are confined to the state’s extreme southeastern tip, known as the Bootheel region. Areas along the Mississippi River can reach Zones 7b and even small pockets of 8a, reflecting the mildest winter conditions. This north-to-south variation means a plant suitable for the Bootheel may not survive the average winter in the northern plains.
Applying Zone Information to Gardening and Landscaping
Gardeners primarily use hardiness zone information to select perennial plants, trees, and shrubs rated to survive the coldest expected temperatures. A plant with a zone rating matching or lower than the local zone is considered a safe investment for long-term survival. For instance, a gardener in Zone 6b should select plants rated for Zone 6 or a colder zone like Zone 5.
The zone designation acts as a guide for ensuring a plant can withstand the lowest temperatures without suffering winter damage or death. Gardeners sometimes choose plants rated for a slightly warmer zone, a practice known as “zone pushing,” but this requires extra precautions. Protecting these borderline plants often involves applying a thick layer of mulch around the base or providing windbreaks to insulate them from extreme cold snaps.
The hardiness zone also influences the timing of specific planting activities, especially for long-lived species. While frost dates are more relevant for annual vegetables, the zone number helps determine if a plant can be successfully established in the fall. This ensures a perennial has enough time to develop a strong root system before the onset of the average annual minimum winter temperature.
Beyond Hardiness: Other Climate Classifications
While the USDA hardiness zone focuses on winter cold, it is only one of many factors determining a plant’s success. The American Horticultural Society (AHS) developed the Heat-Zone Map to address the summer climate, which is an equally important consideration in Missouri. This system classifies regions based on the average number of days per year that the temperature exceeds 86°F (30°C).
The 86°F threshold is used because many plants begin to experience physiological damage and heat stress at this temperature. Utilizing both the USDA Hardiness Zone and the AHS Heat-Zone rating provides a comprehensive picture of a plant’s climatic needs. Gardeners in Missouri must often select species that can withstand both sub-zero winter lows and intense summer heat waves.
Furthermore, localized variations in terrain, proximity to water bodies, or the presence of buildings create what are called microclimates. A small, sheltered area in a backyard, such as one near a south-facing brick wall, may be slightly warmer than the surrounding area. Conversely, a low-lying, exposed area prone to frost pockets may be cooler than the official USDA designation, illustrating the need for site-specific observation.