The USDA Plant Hardiness Zone Map is the standard resource gardeners use to determine which perennial plants can survive winter temperatures. Based on the latest revisions, the St. Louis metropolitan area is largely designated as Zone 7a. This zone indicates the minimum cold tolerance a plant must possess to be reliably grown outdoors year-round. Understanding this zone is the first step in successful gardening, especially when choosing trees, shrubs, and other plants that return each spring.
Understanding How Growing Zones Are Determined
The USDA Plant Hardiness Zone system is based on the average annual extreme minimum winter temperature. This is the coldest temperature recorded each year, averaged over a 30-year period, which defines the zone. The system predicts the lowest temperature a perennial plant will likely need to endure.
The latest 2023 map revision moved the central St. Louis metro area from Zone 6b into the warmer Zone 7a. Zone 7a is defined by an average annual extreme minimum temperature range between 0°F and 5°F. This means a plant rated for Zone 7 is expected to survive the typical coldest night in St. Louis.
The zones are divided into 10-degree Fahrenheit increments, with half-zones like ‘a’ and ‘b’ representing a 5-degree difference. Zone 7a is slightly colder than 7b, which spans 5°F to 10°F. The shift to Zone 7a reflects a trend of warming minimum winter temperatures recorded between 1991 and 2020.
Using the St. Louis Zone for Planting Success
The Zone 7a designation dictates the selection of perennial plants. Gardeners should select trees, shrubs, and ornamental grasses with a listed hardiness rating of Zone 7 or lower. Choosing a plant rated for a colder zone, such as Zone 6 or 5, provides a greater margin of safety against unusually harsh winters.
The hardiness rating measures cold tolerance but does not account for other factors like soil type, moisture, or summer heat. Plants rated for Zone 7 are generally safe from the cold, but their success depends on proper placement and care. Plants rated for Zone 8 are unlikely to survive a typical St. Louis winter, as they cannot handle temperatures near 0°F.
The zone helps gardeners avoid planting species that may die back during a cold snap. Some plants previously considered borderline, like certain varieties of Crape Myrtle or Cherry Laurel, may now thrive more reliably in Zone 7a. It is advisable to choose cultivars that can handle at least Zone 6 for added security against unpredictable temperature drops.
St. Louis Specific Climate Factors
While the hardiness zone addresses cold tolerance, successful gardening requires considering other local climate variables. The region’s growing season is determined by average frost dates. The average last spring frost in St. Louis occurs around April 15, and the average first fall frost is near October 15, providing a growing period of approximately six months.
The urban environment introduces the Urban Heat Island Effect. This effect causes the dense city core, with its concrete and buildings, to absorb and radiate more heat than the surrounding suburbs. This microclimate can push temperatures several degrees warmer, effectively creating pockets that behave more like a Zone 7b.
This localized warming can benefit those attempting to grow slightly less cold-tolerant plants, but it also presents summer challenges. St. Louis often experiences high heat and humidity, with around 35 to 40 days per year reaching 90°F or higher. Gardeners must select varieties that are not only cold-hardy but also tolerant of intense, prolonged summer heat and moisture stress.