Gardening in Northern Illinois requires understanding the region’s climate limitations to ensure perennial plants, trees, and shrubs survive the cold winter months. This understanding is provided by the USDA Plant Hardiness Zone Map, which serves as the standard reference for determining a plant’s ability to tolerate cold. Northern Illinois encompasses a diverse area, stretching from the rural far northwest to the densely populated Chicago metropolitan area along Lake Michigan. For gardeners in this region, knowing the specific hardiness zone is the first step in successful, long-term landscape planning.
Understanding the Hardiness Zone System
The United States Department of Agriculture (USDA) developed the Plant Hardiness Zone Map as a standardized guide for plant survival across the country. This system is not based on the average yearly temperature, but rather on the average annual extreme minimum winter temperature recorded over a 30-year period. The current map, for instance, uses weather data collected from 1991 through 2020 to define the zones.
The map is divided into 13 primary zones, each representing a 10-degree Fahrenheit difference in the minimum temperature. For greater precision, each zone is subdivided into two sections, designated by the letters ‘a’ and ‘b’. These subzones reflect a 5-degree Fahrenheit temperature increment, allowing for a more accurate assessment of a location’s winter minimum.
Nurseries and seed packet labels use this zone system to indicate the coldest temperature a plant species can reliably endure. A plant rated for Zone 5, for example, is expected to survive the minimum winter temperatures typical of all Zone 5 locations. This tool has become the industry standard, providing a common language for discussing cold tolerance.
The Specific Zones of Northern Illinois
Northern Illinois is primarily situated within two half-zones: 5b and 6a, though a small area in the far northwest corner of the state falls into the colder 5a designation. Zone 5b is defined by an average annual extreme minimum temperature range of -15°F to -10°F, representing the majority of the region’s rural and outlying suburban areas. The slightly warmer Zone 6a has a corresponding temperature range of -10°F to -5°F, which is seen in the most populous parts of the state.
The geographical variation across the region is significant, with the Chicago metropolitan area and its immediate suburbs largely classified as Zone 6a. This warmer designation results from two primary climatic influences that moderate winter temperatures compared to the surrounding open land. The first factor is Lake Michigan, which tends to keep adjacent areas warmer in the winter.
The other major influence is the urban heat island effect, where the dense concentration of concrete, asphalt, and buildings absorbs and retains heat more effectively than rural landscapes. This effect prevents temperatures from dropping as low as they do in less developed areas nearby. As a result, a gardener in downtown Chicago may be in Zone 6a, while a counterpart 30 or 40 miles west may be in the colder Zone 5b.
Applying Your Zone Knowledge for Gardening Success
Understanding your specific hardiness zone provides the foundation for selecting perennial plants that will successfully return year after year. The most direct application of this knowledge is to select trees, shrubs, and perennial flowers rated for your zone or a colder one. For instance, a gardener in Zone 6a should choose plants rated for Zone 6 or colder, such as Zone 5 or Zone 4, to ensure winter survival.
Conversely, attempting to grow a perennial rated for a warmer zone, such as Zone 7, is high-risk because the plant is unlikely to survive the typical extreme winter lows of Northern Illinois. The zone map is intended as a guide for cold tolerance, and it does not account for other factors that influence plant health, such as summer heat, soil drainage, or moisture levels.
Gardeners can also leverage microclimates, which are small areas within a garden that are slightly warmer or cooler than the surrounding landscape. A sheltered area against a south-facing brick wall, for example, often absorbs and radiates heat, creating a pocket that can mimic conditions of a slightly warmer half-zone. Utilizing these warmer microclimates may allow for the successful cultivation of plants rated for one zone warmer than the official designation, but this practice involves a degree of risk.
Beyond plant selection, zone knowledge informs specific winter preparation. Plants newly installed in the fall, even those rated for the proper zone, benefit from a layer of protective mulch applied after the ground freezes. This mulch helps to insulate the soil and maintain a consistent cold temperature, preventing the plant from being pushed out of the ground by alternating freeze-thaw cycles. Recognizing that the zone is based on a long-term average, gardeners must also prepare for the occasional extreme cold snap, which can still damage or kill even the hardiest plants.