As you plan a garden consider your planting zone and frost dates. You can usually rely on your local garden centers and growers to be selling seedlings with any eye on what grows best in the zone they are selling in. More knowledge on your part of the world is needed however, when you order from catalogs or from the internet.
Plant Hardiness Zones divide the United States (and the rest of the world!) into 11 planting zones based on a 10 degree Fahrenheit difference in the average. Basic laboratory testing can easily determine the lowest sustained temperature a particular plant species can withstand, but how does that relate to your garden?
In an attempt to answer this question for gardeners, years ago botanists and horticulturists (God love 'em!) started gathering weather records throughout North America to compile a database to show the average coldest temperatures for each region. These records were condensed into a range of temperatures and transformed into various zones of plant hardiness. Maps were then constructed to delineate these temperature zones.
The climatic studies translated later into maps were undertaken by two independent groups: The Arnold Arboretum of Harvard University in Cambridge, Massachusetts, and the United States Department of Agriculture (USDA) in Washington, D.C. The two maps reflected some variances, but in recent years, the differences between the Arnold Arboretum and the USDA have narrowed. Today, the USDA map, which was last updated and released in 1990 (based on weather records from 1974-1986), is generally considered the standard measure of plant hardiness throughout much of the United States.
But many gardening enthusiasts in the West have long been dissatisfied with the USDA's system of averaging temperatures. Western gardeners are well aware that the combined influences of the Pacific Ocean, desert regions, and the Sierra Nevada and Cascade mountain ranges provide the West with unique weather conditions and patterns. Naturally these conditions affect what gardeners can grow in their local areas. The differences between neighboring regions are substantial enough that information for baseline zones in the Midwest, South, and Northeastern states often do not apply in western states.
There are also different climates and frost dates within planting zones in a region due to the topography, lakes and rivers, canyons or mountains. These can cause altered airflows which can raise or lower the temperature, changing the zone in your area. The chemical balance and texture of the soil, exposure, altitude, rainfall, humidity, sun light levels, and wind, and wind chill factors can also alter the effects of plant hardiness zones. If you are not sure of the difference between frost and freeze look at definitions of frost and freeze.
A plant's winter hardiness is critical in deciding whether it is a good choice for your garden. Some plants will not live through severe winters; others will wither in heat; and many spring-flowering bulbs and trees need a cold period to stimulate their growth cycles. Plants usually survive the cold better in a dry area. Deciduous plants tolerate a more exposure, and the summer heat. Evergreens prefer a sheltered area with more humidity, and cooler summers.
See more on US Frost Dates
Extremely detailed PDF maps are found at Better Homes and Gardens.
Not in North America? Try this site.