USDA Growing Zone Calculator

Each full USDA zone spans a 10°F range, and the "a"/"b" split divides it into two 5°F halves -- "a" is the colder half of the zone, "b" is the warmer half. Zone 6a covers average annual extreme minimums from -10°F up to (but not including) -5°F, while 6b covers -5°F up to 0°F. The half-zone distinction matters most for borderline-hardy plants, where that 5-degree difference can be the difference between reliable overwintering and occasional winterkill.

A typical, average annual extreme -- not a one-time record cold snap. The official USDA Plant Hardiness Zone Map is built from decades of weather station data averaged this way specifically so a single unusually harsh or mild winter doesn't skew a location's zone. Enter the coldest temperature your area typically reaches most winters for the most useful result.

Zone numbers are a standardized shorthand the whole nursery and seed industry uses so growers don't need to compare raw temperatures themselves -- "hardy to Zone 6" means the plant is expected to survive winter minimums typical of Zone 6 and any warmer zone, but is at risk in a colder zone like 5 or 4. Using your looked-up zone lets you quickly cross-reference plant tags, seed catalogs, and extension office guides that all speak the same zone language.

Yes -- USDA zones are ordered directly by average annual extreme minimum temperature, from Zone 1 (below -55°F, the coldest) up through Zone 12+ (above 50°F, essentially frost-free). Entering a colder minimum temperature always produces the same or a lower zone number, and entering a warmer minimum always produces the same or a higher one -- there's no non-linear jump or exception in how the ranges are assigned.