Our Place on the Map

These maps, representing various climatic conditions, take some of the guesswork out of choosing plants for your garden. If you know your zone, you can decide — with a glance at the zone information on a plant label — whether that plant will thrive in your climate.

This year, I'm planning to take a closer look at climate zones and maps and explore planting advice for each zone. Just for example, in January, while gardeners in Madison, Wisconsin, are shoveling snow and reading seed catalogs in cozy chairs by their fireplaces, gardeners in southern California are planting tomato seeds and down in Louisiana, they are picking camellias. No wonder it's hard to compare gardening experiences with friends and family who live in different parts of the country. But it all adds up to a beautiful natural mosaic.

Three nationwide zone maps have been developed for gardeners:

— The USDA Plant Hardiness Zone Map draws its lines based on the average minimum winter temperatures across the country. The new map, issued in 2012, reflects data collected from nearly 8,000 weather stations over 30 years, from 1976 through 2005. Try the "interactive map" feature to find the details in your own zone, or download the Climate Wise Plant Hardiness Zone finder (99 cents), which includes quick climate information for the U.S. and Puerto Rico. If you live in the lower 48 states, plugging in your ZIP code also tells you the date of the first fall frost and last spring frost and the number of days in the growing season.

— The American Horticultural Society's Heat-Zone Map provides a guideline for how much heat plants can take; it divides the country into 12 heat zones, based on the number of days with temperatures over 86 degrees. Zone 1 has less than one such "heat" day in the average year, and Zone 12 has more than 210 heat days.

— By far the most ambitious gardening map is the sophisticated Sunset Garden Climate Zone map, which divides the United States into 45 zones. The map contains information about the length of the growing season, the amount of rainfall and its seasonal distribution, winter lows, summer highs, and humidity. Sunset zones were originally developed for the magazine's primary distribution area, in the southwest, west, and northwest, but the latest map covers the country from coast to coast.

Looking at your garden on zone maps is a great exercise, especially when the weather keeps you indoors. Studying gardening zones and comparing experiences from year to year will help you develop a sense for what you can and can't grow in your area. It takes some of the guesswork out of gambles with tender plants and experiments with early and late planting. The difference between one zone and the next may be insignificant for some plants, but crucial for others. In many cases, you can push the zone limitations in your own garden by finding (or making) a place for a plant that would otherwise be unable to survive in your garden. Cool-climate gardeners around the world are old hands at this trick.

While you're getting to know your zone, it is important to remember that even detailed zone maps necessarily embody very broad generalizations, and that zone systems are not foolproof. With a rain gauge, a thermometer, and a notebook to record minimum and maximum temperatures, you can keep track of temperature ranges and changes in your own garden from year to year, find the hot and cold spots, and make decisions accordingly. With good notes, you can make a microclimate zone map of your own back yard. Let's give it a whirl.