When describing or categorizing plants based on their ability to tolerate and survive the temperature extremes of the place where they are growing, horticulturists commonly refer to the plants as hardy or tender. It would be illogical to select a plant for outdoor production or use if it was intolerant of the local temperatures. While the plant might flourish during the summer, it might die during the winter. In other instances, it might survive in vegetative form, but never set fruit or reproduce because of overly warm temperatures.
The United States Department of Agriculture (USDA) has prepared and periodically updates a Hardiness Zone Map (Figure 2-2) that shows the average annual minimum temperatures of the 50 states and much of Canada and Mexico. It divides the continent into 11 hardiness zones. On the most detailed maps, every county within the 48 adjacent mainland states is shown along with its hardiness zone rating.
Each hardiness zone has an average annual minimum winter temperature variation of 10 degrees Fahrenheit. As the hardiness zone numbers increase, the temperature minimum warms. For example, northern Kansas temperatures (Zone 5) drop to between -10 and -20 degrees F in the winter, but southern Kansas (Zone 6) drops only to between 0 and -10 degrees F during the same months. Zones 1 and 11 represent the coldest and warmest regions of the country and are the least common zones within the United States. On color coded and detailed Hardiness Zone Maps, Zones 2 through 10 are further divided into a and b zones. The a zone represents the coldest half of the 10-degree variance, and the b zone represents the warmest half of the 10-degree variance.
NOTE: The a and b zones do not show on the map in this text.
When plants are described on the basis of temperature tolerance, they are identified by their hardiness zone rating. For example, in addition to one of the descriptive categories listed earlier, the plant might be described as a “zone 6 evergreen” or a “zone 4 perennial.” A plant’s hardiness zone rating indicates that the plant may survive the winter in that zone or in zones with a higher numbered rating. Thus, a tree or shrub rated as a Zone 6 plant may survive the winters in Zones 6 through 11, but not in Zones 5 through 1. However, there are also upper temperature
limits to a plant’s hardiness. Certain plants cannot survive or grow normally in climates that are too warm. That is why, for example, there are no apple trees in the Florida Keys. Still other plants can have unreliable hardiness zone ratings when grown in microclimate areas, regions with atypical growing conditions. For example, some plants will survive the winter because they are buried each year under a protective snow cover
where they escape the severe cold and drying winds that would otherwise kill them. If a winter fails to provide the protective snow cover, the plants would be unable to survive.