On most seed packets and the tags that come with your plants, you'll find useful information, such as how much sunlight your plants need in order to thrive. You'll also notice a line that states in what zones the plants are rated; however, that information is meaningless unless you know which outdoor planting zone you actually live in. Once you determine this, you'll be able to grow perennial plants that can survive your winter weather. Zones are determined by the United States Department of Agriculture and are rated from 1 to 11 based on the lowest average winter temperatures (1 is the coldest).
Look at a basic United States Department of Agriculture plant hardiness zone map. The basic zone map simply shows (in color) the trends the zones follow, which mostly run laterally across the country. Look at where you live and which zone it falls in. However, because zones are intermixed with one another, to know exactly which zone you live in, proceed to the next step.
Visit the United States National Arboretum website and click on the area where you live. A more detailed zone map of your area of the country will result. Pinpoint exactly where you live and notice in which zone it is located.
Use your zip code to determine your outdoor planting zone. On several websites, including the National Gardening Association website and the Jackson and Perkins website, you can enter in your zip code to find out your planting zone.