The USDA Plant Hardiness Zone Map is the standard tool used by gardeners and agricultural professionals to determine which perennial plants are most likely to survive the winter in a specific location. This system links climate science directly to plant survival by mapping the lowest temperatures a region typically experiences. The map divides the United States into 13 zones, providing a scientific framework for selecting appropriate trees, shrubs, and flowers. Understanding this designation for St. Louis, Missouri, is the first step toward successful, long-term gardening. This knowledge helps prevent the loss of plants that cannot tolerate the average annual minimum winter temperatures of the area.
The Official USDA Hardiness Designation for St. Louis
The most recent update to the USDA Plant Hardiness Zone Map, released in 2023, places the city of St. Louis, Missouri, within Zone 7a. This designation marks a shift from the previous 2012 map, which had classified the area as Zone 6b. This change reflects a long-term analysis of weather data, specifically the average of the coldest annual winter temperature recorded over a 30-year period. The map is not based on the single coldest night ever recorded but on this long-term average, which provides a reliable baseline for plant hardiness.
While the city proper is designated as Zone 7a, the surrounding St. Louis County remains split between Zone 6b and the warmer 7a. This variation highlights the geographical nuances within the metropolitan area. Gardeners in the region must consider their specific location to ensure they are using the most accurate zone rating for their property.
Translating the Zone into Winter Temperatures
The USDA hardiness zones are defined by specific bands of temperatures. Each full zone number represents a 10-degree Fahrenheit range of the average annual minimum winter temperature. For greater precision, each of these zones is further subdivided into two sub-zones, designated ‘a’ and ‘b,’ which represent a 5-degree Fahrenheit band. The ‘a’ sub-zone is the colder half of the full zone, while the ‘b’ sub-zone is the warmer half.
St. Louis’s Zone 7a designation translates to an average annual minimum winter temperature range of 0°F to 5°F. This means that the average of the lowest temperature recorded each winter falls within this five-degree window. In Celsius, this temperature range is approximately -17.7°C to -15.0°C. Understanding this precise temperature metric is necessary because a plant rated only for Zone 7b may not survive the occasional drop to the 0°F threshold that defines the colder 7a sub-zone.
Using Zone Data for Successful Planting
Gardeners use the Zone 7a designation as a guide for selecting plants, especially perennials, trees, and shrubs that are meant to survive year after year. A plant must be rated for Zone 7a or any lower, colder zone (like Zone 6 or 5) to have the best chance of surviving the St. Louis winter. For instance, a plant labeled as hardy to Zone 5 will easily tolerate the cold in Zone 7a, while a plant rated for only Zone 8 is likely to perish during a typical winter cold snap in St. Louis.
Localized environmental factors, known as microclimates, can modify the official zone rating for a specific yard. The urban heat island effect, common in large cities like St. Louis, often causes the inner city to be marginally warmer than the surrounding suburban or rural areas. Elements like large bodies of water, south-facing walls that radiate heat, or dense paving can create pockets that are effectively a half-zone warmer. Conversely, low-lying areas that collect cold air or spots exposed to harsh winter winds may experience conditions closer to the colder 6b zone.