An ambiguous, generalized term, "planting zones" refers to either the USDA Hardiness Zones, AHS Heat Zones or Sunset Climate Zones, which gardeners use to determine what plants to grow. Regardless of zone system used, labels and literature normally lists at least one zone rating per plant species.
USDA Hardiness Zones
Beginning with data in the 1940s, the United States Department of Agriculture (USDA) compiled winter low temperature averages across the country and created 12 zones. The USDA Hardiness Zone map became the standard "planting zone" resource for decades, as gardeners often found that the degree of winter cold markedly affected plants' abilities to return the following spring. Individual plants receive a USDA zone rating, informing gardeners where plants consistently survive winters. Recently, these zones expanded to 15, which includes tropical regions, so that now all of North America falls into a USDA Hardiness Zone.
AHS Heat Zones
A relative newcomer to the horticultural world is the American Horticultural Society's (AHS) Heat Zones. These 12 zones delineate areas by the number of days annually daily high temperatures exceed 86 degrees F. Many plant species perform well in all zones, but other species begin to falter if summer heat persists or remains too high. Heat Zones prove particularly useful in the southern United States.
Sunset Climate Zones
Gardeners in the western United States and Canada in 1954 first saw a climatic map pertinent for the varied elevation, latitude, wind patterns and influences of ocean and mountain ranges. The Sunset Climate Zones provide more precise information than the USDA Hardiness Zones regarding plant performance, as rainfall and soils divided the continent's West into roughly 30 zones. More recently, the publishers at Sunset expanded to 45 zones, now describing all of the United States and Canada. Ratings on plants reveal a list of all Sunset Climate Zones where they grow successfully and persist.
Some local authorities make references within a governmental boundary, such as state or county, when discussing planting zones. The zone names may generalize climate or soil conditions and correlate to common regions like coastal plain, piedmont and mountains, or overlay native ecological habitats like deciduous forest, tundra, alpine and prairie. Of course, cardinal directions form understandable zone names, too.