Plant Hardiness Zone Map

The Plant Hardiness Zone Map was developed by the US National Arboretum to aid gardeners in selecting plants that will thrive in their regions.

The map defines zones by their average winter low temperature. It builds on an older system, which used 10 zones that each represented a 10 degrees range in average winter low temperatures. When a gardener says a plant will grow in zone 8, she means it will survive winters where the average winter low temperature is 10 degrees.

The current map divides each of those zones into two zones, calling them 8a and 8b. Zone 8a temperatures get as low as 10 degrees. Zone 8b get as low as 15 degrees. Find out more at the National Arboretum’s site, Plant Hardiness Zone Map.

Another plant adaptation map commonly referred to is the Sunset Garden Map. Long a staple of Western gardening, Sunset has recently extended it to include the entire country. It is more detailed, and takes more climate factors into account than the USDA map, but is not as widely accepted and used. See the Western map at Sunset’s Climate Zones.