How To Use The Plant Hardiness Zone Map
*As of 2017-18, the 2012 USDA Plant Hardiness Zone Map(above) is still the standard by which gardeners and growers can determine which plants are most likely to thrive in which locations. I will stay on top of any changes!
Start by determining your own zone numbers for hardiness by looking at the map on this page below. Each of the map's colored zones is separated by 10 degrees and divided into subzones A and B, separated by five degrees.
To get more specific about your hardiness zone, you can type in your ZIP code at the U.S. Dept. of Agriculture website, or for even more precision, use the interactive map there that lets you click down to within a half mile of your home.
Typically, a plant tag will have a range of numbers — say, Zones 6 through 9 — rather than a single number. In many cases, the farther south you go in terms of the zones, chances are the more shade the plant will need. If there's no number at all on the tag, the plant is probably not hardy in your area and is intended to be grown as an annual (for one growing season).
The American Horticulture Society's "Heat Zone Map"
Because cold isn't the only factor determining whether our plants will survive and thrive, The American Horticulture Society has now developed a "Plant Heat Zone Map". Use the AHS Plant Heat-Zone Map in the same way that you do the Hardiness Map. Start by finding your town or city on the map on the American Horticulture Society's Website. The larger versions of the map have county outlines that may help you do this.
The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).
Where To Find Plant Hardiness And Heat Zone Information And How To Use It
Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. You will see the heat zone designations joining hardiness zone designations in garden centers, references books, and catalogs.
Used in combination with the AHS heat map, the updated USDA hardiness map now allows plants to be assigned four codes—two hardiness codes and two heat codes. Many nurseries are now using the four number zone system in their catalogs and plant labels.
Soon vegetables, annuals, aquatic plants, and even turf grasses will have both of these codes on their labels. It's an easy way to learn about the optimum growing season for individual plants.
On each plant, there will be four numbers. The first two numbers in the series indicate hardiness: the initial number is the coldest zone to which the plant is rated and the second number is the “least” cold zone in which a plant will thrive—this often reflects how intense a period of “chilling” or dormancy a plant such as an apple tree or daffodil needs in order to grow successfully.
The second set of numbers indicate heat tolerance and requirement: The initial number in the series indicates the hottest zone in which a plant will thrive, while the second number reflects the “least heat” zone in which the plant will grow—this is often an indication of the minimum number of warm days a plant needs to fruit or flower successfully.
For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can leave tulips outdoors in your garden year-round. An ageratum may be 10-11, 12-1. It can withstand summer heat throughout the United States, but will over winter only in our warmest zones. An English wallflower may be 5-8, 6-1. It is relatively cold hardy, but can't tolerate extreme summer heat.
The zone ratings have always been intended to indicate excellent adaptability of the plants. Many plants may survive in warmer or colder zones, but survival alone is not considered satisfactory garden performance.