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. 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 -- that is, 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, 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 it is not recommended for beginners. Those experienced in gardening and landscaping, however, often make use of what are known as "microclimates."
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 (remember, 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 9-11. The bulk of America lies in planting zones 4-8. 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. Here are some examples of each:
Examples of cold-hardy plants include (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 include:
While northern gardeners often single themselves out as being constrained by planting zones (as in, "I wish I lived in a climate warm enough that I could overwinter purple shamrocks outdoors"), sometimes the constraints do work the other way. For example, gardeners in very hot climates have trouble growing plants with chilling requirements, such as crocus bulbs, which northern gardeners grow with ease.
Now that you know what the USDA planting zones are, you can read a garden catalog and understand which plants are likely to be suitable for your climate and which are not. Armed with this information, you can begin the plant-selection process with greater confidence.