Planting zones are also referred to as “hardiness zones”. This refers to how cold-hardy a plant must be to thrive in a particular region. The zones run in numbers from 1-11 and the lower the zone number, the lower the average temperatures of that region. For example, Zone 3 has temperatures that get below -30°F in the winter so the plants grown there (for example, in upper Minnesota) must be sown or transplanted later in spring and must be cold-hardy to survive the winter. So, the lower the zone number is where you live, the more your plants and trees should be cold-tolerant.
These zones were mapped and developed in 1927 and have been continuously revised ever since, to accommodate the specific temperatures of particular regions and to take into account the general warming trend experienced all over the earth. Newer zone maps also take into account heat-hardiness, which is how high average temperatures reach in a region. This can also greatly affect what plants can thrive in a particular area. For example, a fern (Polypodium sp.) will not do well in the open desert of southern Arizona (Zone 11) but does wonderfully well farther north, where average high temperatures are much lower.
Here, in northern Illinois, we are in right in the middle of Zone 5, where average temperatures can range from -20°F to 105°F. We must take both cold and heat-tolerance into consideration, when planting our gardens. Many perennials, such as clematis (Clematis sp.) and peony (Paeonia sp.), can survive the cold winter temperatures just fine, if properly cared for. Perennials can be cut back and mulched over, to insulate them from the winter cold. Snow also creates a layer of insulation that, believe it or not, actually protects plant roots from freezing.
The Zone 5 growing season for most annuals and vegetables is between late April and early October. Many plants, like peas (Pisum sativum) and forget-me-nots (Myosotis sp.), can be planted as soon as the soil can be worked in the spring. Others, like tomatoes (Solanum lycopersicum) and impatiens (Impatiens sp.), are more sensitive to the cold and must be planted well after any danger of frost, usually around mid to late May in northern Illinois.
There are ways, however, to extend the growing season and beat out the high and low temperatures. Starting seeds inside is a great way to get a jump on the season. Many people also use the aid of a greenhouse to protect plants until the last frost date. A cold frame, a small enclosure of mostly glass, has a similar, warming effect and can be placed right on top of plants in the soil or over seedlings in flats. Adding mulch to the soil acts as insulation to protect root systems from extreme temperatures.
Zone 5 gardeners must be aware of both cold and heat-tolerance in the plants they select but, luckily, there are many plants and trees will thrive in Zone 5.