Almost every plant tag in America lists a hardiness zone. It is the single most useful number in gardening, and also one of the most misunderstood.
What a zone measures
Your USDA Plant Hardiness Zone is based on one thing: the average coldest night of the year where you live. Each zone covers a 10°F range, and each zone is split into an "a" and "b" half covering 5°F. Zone 7a, for example, averages a coldest night between 0°F and 5°F. Zone 7b averages 5°F to 10°F.
When a tag says a plant is hardy in zones 5 to 9, it means the plant can usually survive winters as cold as zone 5. The upper number is a rough guide to where it stops thriving, often because of heat, humidity or lack of winter chill.
The 2023 update
The USDA released a new map in November 2023, built with Oregon State University's PRISM Climate Group from temperature records for 1991 to 2020. About half the country shifted into a warmer half zone compared with the 2012 map. Better data and more weather stations account for some of the change, not just warming.
If your garden moved from 6b to 7a, nothing in your yard changed overnight. It means plants rated for 7a are now a more reasonable gamble.
What zones don't tell you
- Summer heat. Seattle and Dallas share similar zone numbers but grow very different gardens.
- Rainfall and humidity. Many Mediterranean and desert plants survive cold but rot in wet winters.
- Chill hours. Apples, peaches and cherries need a certain number of cold hours to fruit, which warm-zone gardeners have to match carefully.
- Microclimates. A south-facing wall, a low frost pocket or a city's heat can shift conditions half a zone or more.
How to use your zone
Use the zone to rule plants out, then use your climate region to rule plants in. That's exactly how the plant explorer on this site works: pick your state or county on the map and it filters by both.
Look up your exact zone by ZIP code on the USDA Plant Hardiness Zone Map website. Then set "My hardiness zone" above the map to filter plants precisely.