For a perennial, shrub or tree to successfully grow year after year, it must be able to withstand year-round conditions in an area. One major condition that must be taken into account is temperature extremes. Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well.
That's why climate zone maps were created, says Tony Kahtz, with University of Illinois Extension and Land of Lincoln AmeriCorps. Climate zone maps show the minimum and maximum temperature extremes a plant can withstand.
The USDA plant hardiness zone map is the most used resource of any map. It divides the United States into 11 zones. On average, each zone is 10 degrees Fahrenheit warmer or colder than the adjacent zone. Excluding zones 1 and 11, all other zones are divided into two sub-zones such as 2a and 2b, 3a and 3b. The letters "a" and "b" further break down temperature variations by increments of 5 degrees. For example, zone 3a's lowest temperature is 35 to 40 degrees below zero; while zone 3b's lowest temperature is 30 to 35 degrees below zero.
A plant hardiness zone map is not a "one size fits all needs" resource. For instance, Carbondale is located in USDA 6a, but so are portions of New Mexico and Pennsylvania. Does Carbondale really have similar weather to New Mexico? Fortunately for people living in the eastern half of the U.S., the USDA plant hardiness map is more applicable than for gardeners living in the western United States. In the western U.S., one popular guide is the Sunset Climate Zones, which takes into account some of the unique conditions found in that area of the country.
Zones 4, 5 and 6 occur in Illinois. Zones 5b and 6a make up the vast majority of Southern Illinois. This means that zone 5b has the potential to reach 10 to 15 degrees Fahrenheit below Zero, while zone 6a has the potential to be as cold as 5 to 10 degrees below zero.
Granted, some plants may be capable of growing outside of their hardiness zone range. Kahtz says this may be primarily due to microclimates which are areas of your garden or neighborhood that are protected from climate extremes. For example, a south- or west-facing brick wall may provide extra warmth during the winter to a small area of your garden/landscape. Other examples of situations that may create a microclimate include being near a body of water, between two structures, or in courtyards.
The USDA plant hardiness zone map only defines where a plant may survive the winter. In 1997, the Plant Heat-Zone Map was developed. It is intended to supplement the hardiness map. Heat-related problems to plants can be harder to identify. High summer temperatures only told part of the story. Plants don't usually react to a day or two of heat the way they might respond to a frost or deep freeze. However, plants subjected to a three-week heat wave could succumb. Other variables that were not considered include humidity levels, nighttime temperatures, soil moisture and monthly rainfall.
The USDA plant hardiness zone map is a basic tool for gardeners. The weather and its effect upon the earth is an unpredictable part of nature. However, the map does provide basic guidelines for selecting plants. Ultimately, the usefulness of any hardiness zone map depends upon how well plants undergo hardiness trials and how they are accordingly labeled. In addition, hardiness zone maps are probably the greatest help when you are dealing with an unfamiliar plant or area of a given state.
