The USDA Plant Hardiness Zone Map serves as the standard reference tool for gardeners and growers across the United States. This guide helps determine which perennial plants, trees, and shrubs are most likely to survive the winter conditions in a given location. By mapping temperature averages, the system provides a foundation for selecting species that can tolerate the coldest temperatures a region is expected to experience. Understanding this map is important for successful gardening.
Defining the Hardiness Zone System
The structure of the USDA hardiness zones is based entirely on the average annual extreme minimum winter temperature for a specific area. This is not a measure of the average winter temperature, but rather the single coldest temperature recorded and averaged over a 30-year period. The map divides the country into 13 broad zones, each representing a 10-degree Fahrenheit temperature band.
These zones are further split into two sub-zones, designated ‘a’ and ‘b,’ which represent a 5-degree Fahrenheit difference. For example, Zone 6 is a 10°F temperature band, while Zone 6a and Zone 6b each cover a 5°F range. This subdivision provides more granular detail, allowing for precise planting choices based on localized temperature data. The most recent version of the map, updated in 2023, uses temperature data collected from 1991 to 2020.
Illinois’ Specific Hardiness Zones
Illinois is a climatically diverse state, and its hardiness zones reflect a clear north-to-south gradient, spanning from Zone 5a to 7b. The far northwest corner of the state, particularly the Driftless Area, falls into the colder Zone 5a (-20°F to -15°F). Much of northern Illinois is classified as Zone 5b, with minimums between -15°F and -10°F.
The 2023 map update shifted boundaries significantly, moving the line between Zone 5b and 6a approximately 60 to 70 miles north compared to the previous map. This means many central Illinois communities, including the greater Chicagoland area, are now predominantly classified as Zone 6a (-10°F to -5°F). Central areas south of Interstate 70 often fall into Zone 6b, which indicates a minimum temperature range of -5°F to 0°F.
Southern Illinois is largely Zone 7a, with minimums between 0°F and 5°F. For the first time, a small sliver of the state’s southernmost tip has been designated as Zone 7b (5°F to 10°F). This progression illustrates how the state’s geography influences winter temperatures, with zones becoming warmer toward the Ohio River Valley.
Applying Zone Data to Planting Decisions
The primary use of the hardiness zone data is to select perennial plants that can reliably survive the winter in a specific area. When choosing a species, a gardener should look for the zone rating listed on the plant tag, which indicates the coldest zone where that plant is expected to be hardy. A plant rated as hardy to Zone 6, for instance, should be able to tolerate the cold temperatures of Zone 6 and all warmer zones.
A Zone 6 plant will generally survive a winter in Zone 6a, 6b, 7a, or 7b, but is likely to fail in the colder conditions of Zone 5b or 5a without special protection. Gardeners should select plants with a hardiness rating equal to or lower than their local zone number to ensure winter survival. Selecting a plant rated for a warmer zone risks the loss of the plant during an extreme cold snap.
The zone rating is the most reliable predictor of a plant’s ability to survive the coldest seasonal temperatures. Matching the plant’s requirements to the local zone helps growers save time and resources on species not equipped to handle the local minimum temperature.
Local Climate Factors Beyond the Zone Map
While the USDA map provides a broad overview, it cannot account for the highly specific localized conditions known as microclimates. These small-scale environmental variations can cause a gardener’s actual winter temperatures to differ slightly from the official zone rating. For instance, large urban centers like Chicago create urban heat islands, where concrete and buildings absorb and radiate heat, resulting in warmer nighttime temperatures compared to the surrounding rural areas.
Large bodies of water, such as Lake Michigan, also exert a moderating influence on the climate of adjacent shoreline communities. This effect can keep temperatures warmer during the winter, contributing to the expansion of the Zone 6a classification around the Chicagoland region. Conversely, areas in valleys or at the bottom of slopes can experience cold air pooling, which makes them notably colder than a map might suggest.
Factors like exposure to persistent winter winds or the proximity of a planting bed to a south-facing brick wall can also alter the microclimate of a single yard. The zone map should be used as a general guide, with local knowledge of elevation, wind patterns, and surrounding structures providing the necessary fine-tuning.