Hardiness zone

A geographic category based on a location’s average annual extreme minimum winter temperature. Gardeners use it to estimate which plants can survive outdoors through winter.

Average annual extreme minimum temperature: The mean of the lowest temperatures recorded in each year over a specified climate period. This statistic is the basis for assigning a location to a USDA hardiness zone.
Perennial plant: A plant that lives for more than two years, often regrowing after seasonal dormancy. Winter survival is central to choosing perennials for a location’s zone.
United States Department of Agriculture: The United States federal department responsible for agriculture, food, and related policy and research. The USDA developed and publishes the widely used United States hardiness-zone map.
Plant hardiness rating: A rating that describes the lowest temperatures a plant is expected to tolerate. This plant-centered rating is compared with the geographic zone assigned to a site.
USDA Plant Hardiness Zone Map: A United States map dividing locations into zones according to average annual extreme minimum winter temperatures. It turns the temperature statistic into mapped categories used across the United States.
USDA Plant Hardiness Zone Map 2023: The 2023 revision of the USDA map, based on weather data from 1991 through 2020. Its updated boundaries can change planting expectations compared with earlier editions.
Alfred Rehder: A German-American botanist and horticulturist who worked at the Arnold Arboretum. Rehder produced an early United States map of plant hardiness zones in 1927.
Cold injury in plants: Damage to plant tissues caused by exposure to freezing or near-freezing temperatures. Winter minima predict only part of the risk; injury also depends on duration and conditions.
Plant cold hardiness: A plant’s ability to survive exposure to low temperatures without serious injury or death. A zone is a rough geographic proxy for this biological trait.
Planting zone: An informal label for a climate region used to guide plant selection, sometimes based on different measures. The phrase can mean a hardiness zone or a distinct system based on heat or growing season.

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