Determining a location’s growing zone is a foundational step that helps gardeners select perennial plants capable of surviving the coldest winter temperatures. This geographical classification is a practical tool for assessing plant viability and is used by nurseries and growers across the United States.
The Hardiness Zone for Salt Lake City
The Salt Lake City metropolitan area is primarily designated as USDA Hardiness Zones 6b and 7a on the most recent 2023 map update. This designation indicates the average annual extreme minimum winter temperature the area experiences. A zone of 6b corresponds to a minimum temperature range of -5°F to 0°F, while a zone of 7a covers 0°F to 5°F.
Gardeners in the core city areas are often categorized as 7a, suggesting a slightly milder winter minimum. Those living in the surrounding suburbs and higher bench areas may find themselves closer to the 6b designation.
Defining the USDA Hardiness Zone System
The United States Department of Agriculture (USDA) Hardiness Zone Map is the official standard for determining where specific perennial plants can grow. The map divides North America into 13 zones based on long-term weather data collected from thousands of stations. Its single, defining metric is the average annual extreme minimum winter temperature, not the lowest temperature ever recorded.
Each of the 13 zones represents a 10-degree Fahrenheit difference in this minimum temperature. Every zone is split into two sub-zones, designated ‘A’ and ‘B,’ which narrow the range to a 5-degree Fahrenheit difference; for instance, Zone 7a is 5 degrees colder than Zone 7b. The map is regularly updated; the 2023 version incorporated 30 years of temperature data, leading to warmer zone designations for many areas, including parts of Utah.
Local Climate Factors and Microclimates in the Salt Lake Valley
While the USDA zone provides a broad classification, local geographic features create microclimates that cause temperature variability within Salt Lake City. The Wasatch Mountain range to the east and the Great Salt Lake to the west significantly influence local weather patterns. This topography results in temperature differences based on elevation, with higher bench areas experiencing colder minimums than the valley floor.
The Great Salt Lake acts as a thermal buffer, moderating temperatures in nearby areas during the winter months. Conversely, the urban core is subject to the urban heat island effect, where dense infrastructure absorbs and retains more heat. This phenomenon can cause the city’s temperature to be several degrees warmer, particularly at night, effectively pushing some downtown locations into a higher sub-zone. Temperature differences of up to 7.2°F at night have been observed between the city center and the outskirts.