Hardiness Zone
A geographic region defined by average annual minimum winter temperatures, used to determine which perennial plants can survive winter in a given location. Published by the USDA as the Plant Hardiness Zone Map.
The USDA Plant Hardiness Zone Map divides North America into 13 numbered zones based on average annual minimum winter temperatures. Zone 1 is the coldest (below -50°F average annual minimum). Zone 13 is the warmest (above 60°F). Each zone spans 10°F; each zone is divided into “a” (colder half) and “b” (warmer half), each representing 5°F.
What Zones Tell You
Hardiness zones are a single data point: average minimum winter temperature. This tells you whether a perennial plant can survive winter in your location. A lavender rated “hardy to zone 5” will typically survive winters in zone 5 or warmer (zones 5-13), but may die in zone 4 winters.
For perennial plants - trees, shrubs, perennial herbs, and some perennial vegetables - zone ratings are among the most important selection criteria. A fig tree that’s hardy to zone 7 is a poor investment in zone 5 without winter protection.
For annual vegetables grown within a single season (tomatoes, beans, squash), hardiness zones are largely irrelevant. These plants don’t survive winter anywhere. Their relevant constraint is frost dates, not minimum winter temperature.
What Zones Don’t Tell You
Zones are a minimum temperature average, not a full climate description. Two locations in zone 7 can have dramatically different growing conditions:
- Portland, Oregon (zone 8b) and Raleigh, North Carolina (zone 7b): similar hardiness zones but very different summer temperatures, humidity, and rainfall patterns. A Mediterranean herb like lavender thrives in Portland’s dry summers and struggles in Raleigh’s humidity.
- The western and eastern sides of a mountain range often differ by 2-3 zones across a distance of 30 miles.
Summer heat, humidity, precipitation patterns, soil drainage, and wind exposure all affect plant survival and performance in ways that hardiness zones don’t capture.
Current USDA Zone Map
The USDA updated the Plant Hardiness Zone Map in 2023, the first update since 2012. The 2023 map reflects recent temperature data and shows shifts primarily in the western US and parts of the East, with many locations moving into a warmer half-zone designation.
The current interactive map is available at planthardiness.ars.usda.gov. Enter your zip code for a precise zone designation based on the nearest weather station data.
Microclimates and Zone Modification
Your garden’s zone rating is an average. Specific spots in your yard may be warmer or colder by 1-2 zones:
- South-facing walls collect and radiate heat, creating warmer microclimates
- Low spots collect cold air drainage and may be colder than the surrounding area
- Areas with good overhead cover from deciduous trees experience fewer hard frosts
Understanding your specific microclimates allows you to push zone limits on marginally hardy plants by siting them in protected spots.