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Gardening glossary

Hardiness zone

A hardiness zone tells you the coldest temperature a location typically reaches in an average winter, which is the single biggest factor in whether a perennial, shrub, or tree will survive year-round outdoors.

The **USDA Plant Hardiness Zone Map** divides North America into 13 zones based on the average annual extreme minimum temperature, each 10°F wide and split into "a" (colder half) and "b" (warmer half) subzones. For example, USDA Zone 5a runs from -20°F to -15°F, while Zone 9b runs from 25°F to 30°F. The map was last refreshed in 2023 using 1991–2020 climate data, which shifted roughly half of the US about a half-zone warmer than the 2012 edition.

In the UK and Ireland, the **RHS Hardiness Rating** (H1a through H7) plays the same role but is calibrated to milder maritime winters. H4 ("Hardy — average winter") covers most of the UK lowlands and corresponds loosely to USDA Zone 8. H7 ("Very hardy") matches roughly USDA Zone 5 or colder. Because UK winters are wet rather than deeply cold, RHS ratings also factor in tolerance of damp soil at low temperatures, which the USDA scale ignores.

In the Growli app I default to USDA for US users and RHS for UK users, but both systems share the same logical purpose: filtering out plants that can't realistically overwinter in your patch.

A few caveats worth knowing:

- Hardiness zones only describe **winter low**. They say nothing about summer heat, rainfall, humidity, or season length — that's why I cross-reference them with frost dates and chill hours. - **Microclimates** inside your garden can shift the effective zone by half a step in either direction. - Container plants experience roots that are roughly one zone colder than the ground, so an in-ground Zone 6 shrub may need Zone 5 hardiness when grown in a pot.

Where this comes up in our guides

Related terms

Frost date

A frost date is the average calendar date when air temperature at your location is expected to drop to 32°F (0°C). Gardeners use two: last spring frost and first fall frost, which together define the frost-free growing window.

Microclimate

A microclimate is a small area whose temperature, wind, humidity, or sun exposure differs noticeably from the surrounding regional climate. A south-facing brick wall, a frost pocket, or a sheltered courtyard can shift the effective hardiness zone by half a step or more.

Chill hours

Chill hours measure the cumulative time a fruit tree spends between roughly 32°F and 45°F (0–7°C) during winter dormancy. Deciduous fruit trees like apples, peaches, and cherries need a specific minimum to break dormancy and bloom normally in spring.

Growing season

The growing season is the stretch of days each year when conditions allow plants to actively grow — most commonly defined as the frost-free window between last spring frost and first fall frost. It ranges from about 90 days in cold climates to year-round in subtropical zones.