The Plant Hardiness Zone Map (PHZM), developed by the United States Department of Agriculture (USDA), is the established standard for comparing the winter survival capacity of plants against local conditions. It provides gardeners and agricultural professionals with a guide for selecting perennial plants most likely to survive the winter temperatures in a specific geographical area. Using this metric is a practical step toward ensuring gardening success and preventing the loss of landscape investments.
The Basis of Plant Hardiness Zones
The PHZM is built on a scientific methodology centered on the average annual extreme minimum winter temperature. This metric is the average of the lowest temperatures recorded in a location over a 30-year period. A plant’s cold-hardiness is determined by its ability to endure this specific low temperature.
The map is divided into 13 major zones, each representing a 10-degree Fahrenheit temperature range. Each major zone is split into two sub-zones, “a” (the colder half) and “b” (the warmer half), which represent a 5-degree difference for more localized readings. The latest map, updated in 2023, uses weather data collected from 1991 to 2020, reflecting recent climatic shifts.
Mapping Illinois’ Specific Hardiness Zones
Illinois spans a wide range of latitudes, causing a significant difference in winter temperatures from north to south, which is reflected in its hardiness zones. The state encompasses zones 5a in the far northwest to 7b in its southernmost tip. This variation means planting conditions can be drastically different depending on the specific location within Illinois.
The coldest parts of the state, including the northwestern corner, fall into Zone 5a, where the average annual minimum temperature ranges from -20°F to -15°F. Much of northern Illinois, including areas like Rockford, is classified as Zone 5b, with minimums between -15°F and -10°F. The greater Chicagoland area has largely shifted into Zone 6a (-10°F to -5°F), a warming attributed partly to the moderating influence of Lake Michigan and the urban heat island effect.
Central Illinois, generally situated north of Interstate 70, is predominantly Zone 6a. The area between Interstates 70 and 64 is categorized as the milder Zone 6b, where temperatures drop no lower than -5°F to 0°F. The majority of southern Illinois, including the Carbondale area, is Zone 7a (0°F to 5°F). A small part of the state’s extreme southern point now reaches into Zone 7b, indicating minimums of 5°F to 10°F.
Applying Zone Information to Gardening Success
Once a gardener knows their specific zone, they can use this number to interpret plant labels and make informed purchasing decisions. Perennial plants, which are expected to survive and return year after year, are typically labeled with the coldest zone in which they can reliably survive the winter. For instance, a plant labeled “Hardy to Zone 6” should be able to withstand the average annual minimum temperatures of a Zone 6 location.
This zone information is primarily a guide for perennial survival and does not apply to annual plants, which complete their life cycle in a single growing season. Gardeners should also consider the presence of microclimates, which are localized environmental variations that can create pockets of warmer or colder conditions within a yard.
A sheltered area near a south-facing brick wall, for example, can absorb and radiate heat, allowing a gardener to successfully grow plants rated for a slightly warmer zone. Conversely, low-lying areas in a landscape, often referred to as frost pockets, can trap colder air, sometimes making a location effectively half a zone colder than the map suggests. Factors like wind exposure, proximity to large bodies of water, and urban development can also slightly shift the effective hardiness zone for a specific site. Recognizing these fine-scale differences allows for strategic plant placement and can extend the variety of plants a gardener can successfully cultivate.