Half-zone

A half-zone is the a or b subdivision of a USDA zone — a 5°F slice of the average annual extreme minimum. The 2023 map added about half a zone of warmth versus the 2012 edition for much of the country, which is why gardeners who 'know their zone' sometimes find it moved. Half-zones matter at the margins: a plant rated hardy to zone 6 often means 6a's colder edge is the gamble.

See also: Plant hardiness zone