USDA Plant Hardiness Zones: How the Number and Letter Actually Work

The **USDA Plant Hardiness Zone** system classifies locations by the *average annual extreme minimum temperature* — the coldest night a place typically experiences in a year, averaged over a 30-year period. It is the standard reference gardeners use to judge whether a perennial will survive the winter where they live. ## How the scale works Zones run from 1 (coldest) to 13 (warmest). Each whole zone spans **5.6 °C** (10 °F), and each is split into an **a** (colder) and **b** (warmer) half of **2.8 °C** each. The label reads as *[number][half]*: "6b" means zone 6, upper half. It is a single ordered sequence — …5a, 5b, 6a, 6b, 7a, 7b… — where stepping up either moves within a number (a → b) or rolls over to the next (6b → 7a), like minutes rolling into the next hour. Some reference points on the scale: | Zone | Average annual extreme minimum | |---|---| | 4b | −32 to −29 °C | | 5b | −26 to −23 °C | | 6a | −23 to −21 °C | | 6b | −21 to −18 °C | | 7b | −15 to −12 °C | | 9a | −7 to −4 °C | The zones **tile the temperature scale edge to edge** — each zone's warm boundary is the next zone's cold boundary, with no gaps and no overlap. A location falls in exactly one zone. ## What the number actually is Two clarifications that account for most misuse: It is an **extreme**, not an average. Zone 6b does not mean winter averages −20 °C; it means the coldest night of a typical year lands around there. A place can have mild winters overall and a cold zone if it gets occasional hard nights. It is a **historical average of extremes**, not a floor. An unusual winter can go well below the zone's range and kill a "zone-appropriate" plant. The zone is a probability statement about a typical year. ## Revisions The map is periodically rebuilt on updated climate normals. The 2023 revision, based on 1991–2020 data, moved roughly half of the United States into the next half-zone warmer — partly genuine warming, partly better station coverage and improved interpolation. Older plant tags, books and maps therefore disagree with current maps in many places. The system is widely borrowed but **not universal, and not interchangeable across borders** — see Canadian Plant Hardiness Zones Are Not USDA Zones. For what the number leaves out, see What Hardiness Zones Don't Tell You, and for common misreadings, Hardiness Zone Misconceptions: Notation, Boundaries, and Cross-Border Comparison.