There are 11 planting zones on the USDA Plant Hardiness Map in the contiguous United States and southern Canada. The regions are defined by a 10 degree Fahrenheit difference in the average annual minimum temperature. To put the definition in layman's terms, the higher the numbers, the warmer the temperatures for gardening in those areas.
It is standard practice for seed dealers and nurseries to label their products according to their USDA Plant Hardiness Zones -- that is, the planting zones in which you'll be most successful at growing those particular plants.
Enthusiasts of horticulture plan their gardens carefully, and part of that planning means consulting maps showing the USDA planting zones. Growing plants not suited to your region's climate is sometimes possible, but not recommended for beginners. Those experienced in gardening and landscaping, however, often make use of what are known as "microclimates."
For a look at just where the various planting zones fall, consult the USDA maps I supply here, by region.
"USDA" stands for United States Department of Agriculture, the institution that published the original map showing USDA planting zones (1960). The publication was sponsored by the American Horticultural Society, in conjunction with the U.S. National Arboretum.
Examples of cold-hardy plants (numbers in parentheses indicate zones):
- Peonies (2-9)
- Goldenrod (2-8)
- Adonis Plants (3-7)
- Ninebark (3-7)
- Oriental Poppy (3-7)
- PeeGee Hydrangeas (3-8)
Examples of plants that are not very cold-hardy: