The USDA Plant Hardiness Zone Map is the standard tool for gardeners to assess which trees, shrubs, and perennial plants are most likely to survive winter in a specific location. This map divides the country into zones based on winter coldness. Understanding St. Paul, Minnesota’s designation is the first step toward successful local gardening, as it defines the lower temperature limit plants must tolerate.
What Hardiness Zones Measure
The hardiness zone system is based on the average annual extreme minimum winter temperature. This is calculated by averaging the lowest temperature recorded each winter over a 30-year period (1991 to 2020 for the most recent map update). This measurement is an average, not the absolute coldest temperature that may ever occur. Each full zone represents a 10-degree Fahrenheit temperature range.
For more granular detail, the USDA divides each full zone into two half-zones, ‘a’ and ‘b’. Each half-zone covers a narrower 5-degree Fahrenheit temperature band. The ‘a’ designation is the colder half, and ‘b’ is the warmer half. This classification helps gardeners select plants with closer tolerance for specific local climate conditions.
St. Paul’s Official Growing Zone
The city of St. Paul, Minnesota, is predominantly classified as USDA Plant Hardiness Zone 5a according to the 2023 map update. This designation means the average annual extreme minimum winter temperature falls within the range of -20°F to -15°F. This is a change from the previous 2012 map, which generally placed the city in the colder Zone 4b (-25°F to -20°F).
This shift reflects long-term weather trends and improved data resolution. As a major urban center, St. Paul experiences the urban heat island effect, where buildings and asphalt retain more heat than surrounding rural areas. The 2023 map utilized a higher density of weather stations and sophisticated mapping techniques to better capture this localized warmth. While much of the surrounding region remains in Zone 4, the Twin Cities metro core has warmed enough to cross the 5a threshold.
Applying Zone Knowledge to Your Garden
Knowing your hardiness zone directly informs the selection of perennial plants, trees, and shrubs that will survive the local winter. Gardeners in St. Paul should look for plants rated as hardy to Zone 5 or a colder zone, such as Zone 4 or 3. Plants rated for Zone 6 are unlikely to survive without significant winter protection, as their cold tolerance is insufficient for the average St. Paul winter.
The official zone acts as a guideline, but local conditions, known as microclimates, can modify the temperature in small areas. A garden space next to a brick wall or large building may retain enough heat to successfully host a plant rated for a slightly warmer zone. Conversely, an exposed, windy location may be colder than the official zone. For plants that are considered marginally hardy, simple techniques like applying a thick layer of mulch or providing a windbreak can offer insulation to help them survive the winter months.