The United States Department of Agriculture (USDA) Plant Hardiness Zone Map provides a general guide to growing conditions in North America. It divides the continent into 11 zones based on the average minimum annual temperatures within each zone. This system has been adapted to other areas of the world. The zones roughly predict which plants will survive in a given area. Because weather varies from year to year, the actual minimum temperatures may be lower or higher than are indicated on the plant hardiness map.

When you’re planning a garden, use the information contained in the map to guide you in making your plant selections. First find your locale on the map, then identify your zone by comparing its color to the legend. Many growers include zone information on their plant tags and bulb packages for your convenience, and the Learn2Grow Plant Database can help as well.

Remember, all plants – from trees to bulbs – grow best in zones where they’ve adapted fully to the climate. It’s possible that plants from warmer hardiness zones than yours may live and bloom in your garden during a series of warm-winter years, only to fail when a cold year is experienced again.

In addition to plant hardiness zones, consider the first and last frost dates in your area. Hardy bulbs that are adapted to survive cold-winter temperatures still require a period of adjustment after planting to ward off winter’s chill. Plant them well ahead of autumn’s first sustained cold temperatures. For tender, warm-season bulbs (those that need planting temperatures of 60 degrees F or higher), wait to plant until spring arrives and the soil warms. Sow seed indoors for cold-season plants (those that tolerate soil temps for germination of 40-50 degrees F) six weeks before the last frost. Wait a few more weeks for warm-season plants (those that prefer planting temperatures of 60 degrees F or higher). Remember, however, that the average first and last frost dates for your area are general guidelines and should be used subject to experience and advice.

Moreover, neither zone maps nor frost charts can account for the effects of thermal belts, nearby bodies of water, topography and other factors that create microclimates within your zone. Only careful observation will give you an accurate picture of the effects due to climatic conditions in your own back yard.