Hardiness zones are a shorthand for winter cold. Each zone is defined by the average annual minimum temperature, and plants are labelled with the coldest zone they can survive. The system was developed by the USDA and first published in 1960.

The USDA map splits North America into 13 zones, each 10°F colder or warmer than the next, with half-zone subdivisions of 5°F. Canada, Europe, Australia, and China have their own versions. A gardener in zone 5 can plant a apple variety rated to zone 4 but should think twice about a zone 7 fig.

What the zones do not tell you

• Summer heat, humidity, and rainfall
• Snow cover, which insulates roots
• Soil type and drainage
• Day length and seasonal wind

Hardiness zones are a starting point, not a verdict. Growers in marginal areas often push the limits with microclimates: a south wall, a windbreak, or a raised bed can carry a plant a zone or two beyond its official rating.