Plant hardiness zones define the geographical areas where specific plants are most likely to survive the winter season. These zones guide plant selection, ensuring the chosen species can tolerate the coldest temperatures expected in a given location. Consulting this information is the first step in garden planning, especially for perennial plants expected to return year after year. Understanding your region’s zone maximizes the chances of a successful landscape.
What Plant Hardiness Zones Measure
The USDA Plant Hardiness Zone Map (PHZM) is the standard reference for determining plant survival in the United States. This map is built upon an area’s average annual extreme minimum winter temperature, which is the single coldest temperature expected during the year. The latest version of the map is based on thirty years of weather data, providing a robust estimate of winter conditions.
The system is divided into thirteen zones, with each full zone representing a 10-degree Fahrenheit temperature range. Each zone is further subdivided into ‘a’ and ‘b’ sections, which narrow the temperature range to 5 degrees Fahrenheit. This allows gardeners to select plants with a greater degree of accuracy regarding their cold tolerance. A plant’s listed hardiness zone indicates the coldest zone where it is reliably expected to survive the winter.
The Specific Zone for Southern Illinois
Southern Illinois is the warmest region of the state, primarily falling within USDA Hardiness Zones 7a, with pockets of 6b and 7b in the extreme south. The majority of the region, including cities like Carbondale, is classified as Zone 7a, which indicates that the average annual coldest temperature is between 0 and 5 degrees Fahrenheit. This classification reflects a warming trend in winter temperatures over the last few decades compared to earlier maps.
The northern border of Southern Illinois, closer to Interstate 64, may still contain areas within Zone 6b, where the minimum extreme temperature averages between -5 and 0 degrees Fahrenheit. Conversely, the southernmost tip of the state, near the confluence of the Mississippi and Ohio Rivers, now features small slivers of Zone 7b. This designation signals the mildest winter conditions in the state, with the average annual minimum temperature remaining between 5 and 10 degrees Fahrenheit. The presence of these warmer zones expands the possibilities for planting species previously limited to more southern climates.
Using Zone Data for Successful Planting
A gardener uses the hardiness zone data by selecting perennial plants whose recommended survival zone is the same as, or lower than, their local zone. For instance, a plant rated for Zone 6 will survive comfortably in the warmer Zone 7a of Southern Illinois, but a plant rated for Zone 8 would be unlikely to survive a typical winter there. This method ensures that the plant’s root structure can endure the coldest part of the year.
Gardeners should also consider microclimates, which are localized conditions that can modify the effective hardiness zone. Factors such as a garden plot sheltered by a large building or a windbreak, or one situated near a large body of water, can create a slightly warmer environment. Hardiness zones only predict a plant’s ability to survive winter cold. They do not account for summer heat tolerance, soil quality, or rainfall, which are also important considerations for plant health.