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. The higher the numbers, the warmer the temperatures for gardening in those areas. The zones are further broken down into an "a" section and a "b" section, representing 5-degree Fahrenheit differences (for greater precision), with "a" being colder than "b."
Why Do We Have These Zones?
It is standard practice for seed dealers and nurseries to label their products according to their USDA Plant Hardiness Zones. These are the planting zones in which you are most likely to be successful at growing those particular plants. As such, these "zone" designations serve as guides.
Enthusiasts of horticulture plan their gardens carefully. Part of that planning means understanding the USDA planting zones. An important part of the plant-selection process when thumbing through garden catalogs is targeting shrubs, perennials, etc. that are cold-hardy enough to survive a winter where you live.
Growing plants not suited to your region's climate is sometimes possible, but it is not recommended for beginners. Those experienced in gardening, however, often make use of "microclimates." A microclimate is a location on a property where the climate differs from the average climate on that property. For example, a location against a house's south-facing wall will usually be warmer than other spots in that yard. An experienced gardener could take advantage of this fact by growing a plant there that normally would be a bit too tender for the region.
Digging Deeper Into USDA Zones
"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. Updates are occasionally made but are of greater relevance to professionals than to average gardeners (the zones are, after all, only rough guides).
As you might expect, parts of the state of Alaska lie in USDA planting zone 1. Parts of northern Minnesota are considered to be in planting zones 2 and 3. Central and southern Florida lie in zones 9 to 11. The bulk of America lies in planting zones 4 to 8. Zone 11 is not, technically, the hottest USDA zone: There are special zones 12 and 13 for Hawaii and Puerto Rico.
There is also a competing system known as the "Sunset climate zones." This system is popular in the West, whereas the USDA system is predominant in the East. Sunset justifies the existence of its separate system by saying that its zones, unlike the USDA's, factor in all of the following important considerations:
- Length of growing season
- Timing and amount of rainfall
- Winter lows
- Summer highs
Examples of Hardy and Tender Plants
As a beginner, all of these numbers may confuse you, at first. But once you get used to them, you will become adept at immediately classifying properly labeled plants as being either cold-hardy or on the tender side. Examples of cold-hardy plants include:
- Peonies (Paeonia lactiflora): 2 to 9
- Goldenrod (Solidago): 2 to 8
- Adonis amurensis: 3 to 7
- Ninebark (Physocarpus opulifolius): 3 to 7
- Oriental poppy (Papaver orientale): 3 to 7
- Common lilac shrubs (Syringa vulgaris): 3 to 7
- PeeGee hydrangeas (Hydrangea paniculata): 3 to 8
Examples of plants that are not very cold-hardy include:
- Bougainvillea: 9 to 11
- Bird of paradise (Strelitzia reginae): 9 to 11
- Gerbera daisy (Gerbera jamesonii): 9 to 11
- Aloe vera (Aloe barbadensis): 9 to 11
- Purple fountain grass (Pennisetum setaceum Rubrum): 9 to 11
- Elephant ear (Colocasia esculenta): 8 to 11
- Snake lily (Amorphophallus konjac): 8 to 10
- Tropicanna canna (Canna Phasion): 8 to 11
Northern gardeners often single themselves out as being constrained by planting zones, wishing they lived in a climate warm enough that they could overwinter a plant such as purple shamrocks (Oxalis regnellii) outdoors. But the constraints do work the other way, too. For example, gardeners in very hot climates have trouble growing plants with chilling requirements, such as Crocus bulbs, which northern gardeners grow with ease.