If you’ve done any gardening, you’ve probably come across a planting zone map depicting the USDA plant hardiness zones. This map will often appear on the back of seed packets, or in the information you find about different plants in seed and nursery catalogs. You may have wondered about the importance of this map and how it can be useful to you when planning your garden.

The planting zone map geographically defines areas based on winter temperatures. This can be used to determine the length of your growing season and help you decide on what plants you can successfully grow. The map can also determine whether a plant will be an annual or a perennial depending on whether or not it will survive through the winter or have to be replanted because it will die in the cold.

The USDA map is divided into 13 major zones based on increments of 10 degrees Fahrenheit. Each zone is subdivided into an “A” and “B” zone separated by 5 degrees Fahrenheit. The continental United States comprises Zones 3 through 10. Alaska spans Zones 1 through 7. Hawaii and Puerto Rico span Zones 9 through 13.

Planting zone map: Prism Climate Group; United States. Agricultural Research Service, Public domain, via Wikimedia Commons

Winter temperatures aren’t the only thing to consider when it comes to garden planning. One of the most important factors to pay attention to is the frost dates. This will determine the length of your growing season and be especially important for vegetables and fruit. The USDA lists the first and last frost dates as follows:

You can get more specific dates by using the calculator on a site like Almanac.com to search by your specific zip code. The last frost date is significant when it comes time to start your seeds, whether it be indoors or outdoors. The first frost date will affect whether or not you will have time to grow something from seed to harvest. For example, watermelons typically take 5 to 9 days to germinate and then need a growing season in soil temperatures over 70 degrees Fahrenheit of 90 days to be fully mature. If you live in Zones 1 through 3, you may not be able to grow watermelon.

There are other important factors to consider when planning your garden. You also need to take into account water, length of daylight, soil pH, heat, and humidity. You should also be aware of microclimates within your zone. You may be in a location that gets more rain or hotter summer days than an area even just a couple of miles away. Only you really know your garden best. The planting zone map and frost dates are helpful, but use your experience with your location to make the best planting decisions.