The term hardiness refers to a plant's ability to survive outdoors year-round without protection. How do you know whether a plant is hardy? Buy from a local nursery or home-and-garden center, where practically all of the plants on display are hardy, unless, of course, they're annuals or tropicals. Or consult a USDA Plant Hardiness Zone Map, often printed in gardening books and seed and plant catalogs. The map was updated several years ago, and you may find that your zone has changed according to the new map. The map divides North America into 11 zones in which average minimum winter temperatures range from 50 degrees below zero in Zone 1 to 40 degrees in Zone 11. Sellers of plants indicate the hardiness of plants according to this map. Consult the map to determine which hardiness zone you're in. When you shop, keep in mind that plants that are winter-hardy in your zone won't necessarily be able to endure your region's summers. [Editor's note: In 1997 the American Horticultural Society introduced the Plant Heat-Zone Map, which designates regions of the United States according to the average number of days per year in which the temperature exceeds 86 degrees Fahrenheit. The book Heat-Zone Gardening (Time Life, 1998) by H. Marc Cathey and Linda Bellamy includes a copy of the map and provides heat-zone ratings for hundreds of herbaceous and woody plants.] In many cases you can grow plants that are rated hardy one zone south of yours if you protect them during the winter. Plants that aren't hardy in your area may also be grown in containers and stored in a sufficiently warm spot during the winter. And if you live near a lake, in the mountains or along the margins of a big city -- areas where the climate may vary half a zone within a few miles -- you may be able to grow a greater variety of plants than a gardener who lives across town.
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