Plant hardiness zones: how the USDA, Canadian and RHS systems work

This page describes what the named agencies and the Royal Horticultural Society say about their own systems. It does not say which tree suits a site. The guide on choosing a tree reports what ISA and the Arboricultural Association list about hardiness among other factors.

The three systems side by side

System	What it measures	How it is shown
USDA Plant Hardiness Zone Map (2023 edition)	The average annual extreme minimum winter temperature, over the 30 years 1991 to 2020.	10°F zones and 5°F half zones, with zones 12 and 13 appearing only on the Hawaii and Puerto Rico maps.
Canada's plant hardiness zones (Natural Resources Canada, 1991-2020, 2025 update)	A suitability index built from seven climate variables, not from minimum temperature alone.	Index values from 0 to 100, grouped into zones such as 4a (index 40 to 44) and 4b (45 to 49).
RHS hardiness ratings (since 2012)	The lowest temperature a plant is likely to withstand, rated per plant, not per place.	Nine categories from H1a (warmer than 15°C) to H7 (colder than -20°C).

USDA

The Agricultural Research Service calls its map the standard gardeners and growers use to judge which perennial plants are most likely to thrive at a location. Its Map Creation page says the 1991 to 2020 period was chosen by a group of horticultural, botanical and climatological experts as the best balance between smoothing out year-to-year weather and the fact that perennial plants mostly experience weather rather than climate, and that the same period is the one climatologists use for baseline climate normals. Zones 12 and 13 were added in 2012 for regions with average annual extreme minimum temperatures above 50 and 60°F. The 2023 map is generally about one quarter-zone warmer than the 2012 map across much of the United States, because of the newer averaging period, and in some places the shift to a cooler zone comes from added data sources and better interpolation. The page says zone changes are not reliable evidence of whether there has been global warming, because the map averages what are essentially extreme weather events.

The map is built from station data interpolated to account for elevation and nearness to water. The page gives examples: cities hold more heat than surrounding countryside, mountain tops sit in cooler zones, and a location downwind of a large body of unfrozen water may be milder. Data from 13,625 stations were used. The page also says Canada has developed its own map, and that Mexico has no similar map.

Canada

Natural Resources Canada says the original map was developed in the 1960s for trees and shrubs and combines a variety of climatic conditions into a single map. Its calculations page says the index comes from a formula by Ouellet and Sherk (1967) using seven variables: the mean daily minimum temperature of the coldest month, the length of the frost-free period, rainfall from June to November, the mean daily maximum temperature of the warmest month, a winter factor from January rainfall and the cold of the coldest month, the mean maximum snow depth and the maximum wind gust over 30 years. The agency has remapped the zones for 1961 to 1990, 1981 to 2010 and 1991 to 2020 using thin plate spline interpolation. It also publishes a separate set of Canadian maps on the USDA minimum-temperature approach, with the note that small method differences mean they may not match the USDA's own.

RHS

The RHS says its ratings are determined by the lowest temperature a plant is likely to withstand and are a general guide only, because many other factors affect a plant's overall hardiness. It lists, for example, H4 as minus 10 to minus 5°C ("Hardy, average winter"), hardy through most of the UK apart from inland valleys, high ground and central or northerly places, and H7 as hardy in the severest European continental climates including exposed upland locations in the UK. The RHS says the ranges are meant as absolute minimum winter temperatures, not the long-term average annual extreme minimum used for USDA zones. The 2012 system replaced the H1 to H4 ratings, which are not direct equivalents of the nine categories. The RHS table pairs each rating with a USDA zone, from H1a with 13 down to H7 with 6a-1.

Why the numbers do not convert

Natural Resources Canada says a study found no simple conversions between the Canadian zones and the USDA minimum-temperature zones. When the two maps are overlaid, Canadian zone 4a overlaps nine different USDA zones, from 3a to 7a. The RHS and USDA measures differ on their face: one is an absolute minimum, the other a 30-year average of yearly minimums. The Penn State Extension article on winter burn adds that choosing a plant adapted to a garden's zone is no guarantee against winter desiccation, which is covered on the page about winter burn and frost injury.