USDA Plant Hardiness Zones are the standard measure for determining which plants will survive winter in a given location. Understanding zones is essential for successful gardening.
What Hardiness Zones Measure
Hardiness zones are based on the average annual minimum winter temperature. The USDA divides North America into 13 zones, each representing a 10°F temperature range. Zone 1 is coldest (-60°F to -50°F), Zone 13 is warmest (60°F to 70°F). Most of the continental US falls between zones 3 and 10.
Zone Subdivisions: A and B
Each zone is divided into "a" (colder half) and "b" (warmer half), representing 5°F increments. Zone 7a (-5°F to 0°F) is colder than Zone 7b (0°F to 5°F). This subdivision helps gardeners make more precise plant selections, especially for borderline species.
What Zones Don't Tell You
Zones only indicate minimum winter temperatures. They don't account for summer heat, humidity, rainfall, soil type, or microclimates. A plant may survive your winter but fail due to summer humidity or drought. Use zones as a starting point, not the complete picture.
Climate Change and Zone Shifts
The USDA updates the hardiness zone map periodically as climate data evolves. The 2012 update showed zones shifting northward compared to the 1990 map. Many areas are now one half-zone warmer. This affects long-term planting decisions for trees and perennials.