Understanding which plants will survive the winter is the foundation of successful gardening. The official guide is the USDA Plant Hardiness Zone Map, which separates the country into distinct geographical areas based on climate data. Knowing your zone number for Illinois is the first step in selecting trees, shrubs, and perennial flowers that can withstand the local cold temperatures. This information helps avoid choosing plants that are not suited to the state’s climate.
How the Hardiness Zone System Works
The USDA Plant Hardiness Zone (PHZ) map is the standard tool used by gardeners to determine where perennial plants are most likely to survive the winter. This system is based entirely on the average annual extreme minimum winter temperature recorded over a specific period. The most recent map update, released in 2023, utilizes data collected from 1991 through 2020 to establish these averages.
The map is divided into 13 primary zones, with each zone representing a 10-degree Fahrenheit temperature band (e.g., Zone 6 covers -10°F and \(0^{\circ}\text{F}\)). To provide greater precision, each primary zone is split into two subzones, designated by ‘a’ and ‘b’.
These subzones represent a \(5^{\circ}\text{F}\) difference. Zone 6a, the colder half, covers average minimums from -10°F to \(-5^{\circ}\text{F}\), while Zone 6b, the warmer half, covers \(-5^{\circ}\text{F}\) to \(0^{\circ}\text{F}\). The system provides a standardized numerical rating that can be directly compared to the hardiness rating found on plant tags.
Illinois’ Specific Hardiness Zones
The state of Illinois spans a wide range of zones due to its significant north-south length, covering areas from the colder Midwest to regions approaching the mid-South. The 2023 USDA map update shows Illinois ranging from Zone 5a in the coldest parts to Zone 7b in the warmest areas.
The most northern region, including the far northwest corner, falls within Zone 5a (\(-20^{\circ}\text{F}\) to \(-15^{\circ}\text{F}\)). Much of the rest of Northern Illinois is categorized as Zone 5b (\(-15^{\circ}\text{F}\) to \(-10^{\circ}\text{F}\)). The Chicagoland area benefits from the moderating influence of Lake Michigan and urban development, leading to a warmer designation of Zone 6a.
Central Illinois generally sits in the Zone 6 range. Areas north of Interstate 70 are typically in Zone 6a, and regions slightly further south are in Zone 6b, indicating average minimum temperatures between \(-5^{\circ}\text{F}\) and \(0^{\circ}\text{F}\). The shift in zone boundaries was noticeable, with the \(-10^{\circ}\text{F}\) line moving north toward the Peoria area in the latest map.
Southern Illinois experiences the mildest winters, with most of the region classified as Zone 7a (\(0^{\circ}\text{F}\) and \(5^{\circ}\text{F}\)). A small section at the very southern tip of the state has been designated as Zone 7b, reflecting the continued trend of warming winters.
Local Factors That Affect Plant Survival
The hardiness zone map provides a broad overview but does not account for localized conditions that significantly impact a plant’s survival. These hyper-local environments, known as microclimates, can create pockets that are either warmer or colder than the surrounding area. Recognizing these factors is important for predicting how a plant will fare.
Large bodies of water, like Lake Michigan, have a substantial moderating effect, which is why the immediate Chicagoland area is often warmer than adjacent inland suburbs. Similarly, urban heat islands, where concrete and buildings absorb and retain heat, can elevate winter temperatures in city centers. Urban areas frequently carry a warmer zone designation than the surrounding countryside.
Other site-specific conditions, such as wind exposure, greatly affect a plant’s ability to tolerate cold. Plants exposed to harsh, drying winter winds can suffer desiccation damage, even if the temperature remains within their hardiness zone. Poor soil drainage can also lead to root damage from freezing water, regardless of the air temperature rating.
Using Zone Data for Planting
The hardiness zone rating helps gardeners select perennial plants, trees, and shrubs that can withstand the average minimum winter temperatures in their location. When purchasing plants, look for a tag that lists a zone number the same as or lower than your own zone. For instance, a gardener in Zone 6a should choose a plant rated for Zone 6 or a colder zone, such as Zone 5.
Selecting a plant rated for a warmer zone carries a greater risk, often termed “zone-pushing.” If you choose a marginally hardy species, you may need to take protective measures, such as applying a thick layer of mulch around the base to insulate the roots. These actions help mitigate the risk posed by occasional extreme cold snaps that fall below the published average minimum.
While hardiness zones focus only on winter cold, they also influence planting timing for spring starts. The zone helps determine the typical last frost date, which guides when tender annuals and vegetables can safely be placed outdoors. Successful gardening also requires considering other factors like sun exposure, soil type, and summer heat, which are not reflected in the hardiness zone data.