Why Knowing Your Area's 'Hardiness Zone' Can Keep Your Plants Alive

Have you ever looked at the tag on a plant at your local garden center and been perplexed that it says something like “Zones 5-9”? This number refers to the “hardiness zones” defined by the U.S. Department of Agriculture that act as a guide to which plants will survive best in which regions of the country. While there many factors beyond this number that determine how successful a plant will be, understanding and using hardiness zones is a great first step for planning your garden this spring.

What are plant hardiness zones?

To help gardeners, amateur and pro alike, determine which plants will grow best in which parts of the country, The USDA publishes a map that divides the country into zones based on the lowest temperature each zone is expected to receive in the winter.

There are 13 larger zones separated by 10°F then further divided into two sub-zones of 5°F each. For instance, my neighborhood in Los Angeles is 10b, where nearby Griffith Park is 11a.

To figure out which zone you live in, go to the USDA’s interactive map and either enter your zip code or zoom in on your part of the country.

Once you know what zone you live in, you can compare it to the zone information on the tags of any plants you buy at a garden center or online. If your zone is equal to or higher than the zone listed on the tag, the plant should be sturdy enough to survive the winter.

Growing perennials vs. annuals in hardiness zones

Planting zone information is most useful if you’re planning to grow perennials—plants that are meant to live beyond one planting season. The further you are outside of the plant’s optimum temperature zone, the more likely it is to not make it through the winter.

You can grow plants that are outside of your zone, but they are likely to be annuals, because they probably will need to be replanted in a year. With these plants, the winter temperature isn’t important—they’re not going to make it through anyway—so you should be more concerned with planting at the right time for that plant.

Exceptions to the rules

The zone information on plant tags assume that the plant will be planted in the ground, but if you’re growing it in a pot, the rules change. Ground soil is slightly warmer and won’t freeze as solid as soil in a raised pot. For non-ground plants, a good rule of thumb is to choose plants that are two zones “hardier” than your planting zone.

The USDA’s hardiness map gets very specific, but it’s still a crude tool for determining the exact climate conditions in your garden. Your yard might be more or less exposed to the elements than the areas around it, depending on the geography, which changes which plants will thrive there.

Drilling down even further, there are micro-climates within your garden that you can take advantage of or manufacture that let you “cheat” the zones a bit, too. An area that’s perpetually in shade because of a tree or wall could be used to grow plants for a cooler climate, for instance, where creating a sun trap (or taking advantage of a natural one) can help a plant survive a winter that’s colder than it should be able to handle.

Zones aside, local plants will grow best

Temperature is only one factor that determines whether a plant will be healthy. Two plants that have the same zone rating could have evolved in areas in which other factors (soil composition, humidity, etc) are extremely different. So every 5.5 plant won’t thrive in every 5.5 garden. Ultimately, the most successful plants for your garden are likely to be the plants that are local to your area (especially the weeds). Local plants evolved to live in just the conditions that are natural to your backyard, so if you want the best chance of a thriving garden, stay local.