The temperatures shown on the Planting Zones map are referred to as "average annual minimum temperatures" and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows 10 different planting zones, each of which represents an area of winter hardiness for the plants of agriculture and the natural landscape. It also introduces planting zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.
How to Use the Map
Planting Zones 2-10 in the map have been subdivided into light- and dark-colored sections (a and b) that represent 5 F (2.8 C) differences within the 10 F (5.6 C) planting zone. The light color of each planting zone represents the colder section; the dark color, the warmer section. Planting Zone 11 represents any area where the average annual minimum temperature is above 40 F (4.4 C). The map shows 20 latitude and longitude lines. Areas above an arbitrary elevation are traditionally considered unsuitable for plant cropping and do not bear appropriate planting zone designations. There are also island planting zones that, because of elevation differences, are warmer or cooler than the surrounding areas and are given a different planting zone designation. Note that many large urban areas carry a warmer planting zone designation than the surrounding countryside. The map-contains as much detail as possible, considering the vast amount of data on which it is based and its size.
In using the Planting Zone map to select a suitable environment for a landscape plant, today's gardeners should keep in mind the following:
New Plant Management Systems. New techniques of planting, transplanting, watering, fertilizing, and providing pest control measures have done much to increase the vigor of landscape plants. But used unwisely, these same measures can reduce plant hardiness.