USDA Plant Hardiness Zone Map

The USDA Plant Hardiness Zone Map is the reference point behind many American plant labels, nursery recommendations and perennial-selection tools. Its interactive map translates thirty years of extreme-minimum-temperature records into zones 1 through 13, with half-zone detail, and lets a gardener look up a ZIP code or inspect a location directly. State maps are also available for download, which makes the site useful beyond a quick lookup.

Its strength is authority and precision, not garden design. A hardiness zone describes the coldest part of a typical winter; it does not account for summer heat, soil, moisture, wind or the small microclimates inside a yard. Used with that limitation in mind, this is the best first stop for deciding whether a perennial is broadly suited to a U.S. location and for checking claims made by commercial gardening tools.