International Growing Zones vs. USDA Zones: What Compares

By Growing Home Greens Team · July 28, 2026

Every zone number on this site is a USDA hardiness zone, the system used across the United States, based entirely on one measurement: the average coldest winter temperature a location experiences, in 10°F bands (split further into “a” and “b” 5°F half-zones). It’s a useful shorthand for US gardeners, which is why our planting calendar and Garden Designer both run on it. But it’s a US-specific system, and gardeners outside the US often run into the confusing fact that other countries use zone numbers too, numbers that don’t mean the same thing.

Canada’s zones aren’t USDA zones, even though the numbers look similar

Canada has its own hardiness zone system, developed by Natural Resources Canada, and it predates and differs from the USDA map in an important way: the original Canadian system isn’t based on winter minimum temperature alone. It factors in several variables together (minimum temperature, length of the frost-free period, summer rainfall, maximum temperature, and snow cover), since those additional factors affect what survives a Canadian winter beyond just “how cold does it get.”

The practical result: a Canadian zone 5 and a USDA zone 5 do not describe the same climate. A location can carry a noticeably different number under each system for the same conditions. If you’re gardening in Canada and using this site, don’t assume your Canadian zone number is the number to enter here, the more reliable approach is to look up your area’s actual average winter minimum temperature and match it to the USDA scale directly, rather than converting zone numbers.

The UK and much of Europe use a completely different scale

The Royal Horticultural Society’s hardiness rating (the system most UK gardening references use) runs on an entirely different scale, H1a through H7, and it runs in the opposite direction from USDA zones: H7 is the most cold-hardy rating (surviving below -20°C/-4°F), while H1a is the least hardy, describing plants that need a heated greenhouse to survive a UK winter at all. There’s no simple arithmetic conversion between an H-rating and a USDA zone number. They’re built around different climate assumptions (the UK’s maritime climate has much milder winter extremes but far less summer heat than equivalent-latitude US regions), so a plant’s H-rating and its USDA zone range have to be looked up separately, not derived from each other.

Some European gardening references also publish USDA-style zone maps adapted to European weather data, which adds a second, differently-numbered system circulating alongside RHS ratings for the same region, worth checking which one a given source is using before assuming a number means what it would mean on a US map.

Australia doesn’t map cleanly onto either system

Australia’s main climate-zone reference, from the Australian National Botanic Gardens, isn’t a cold-hardiness scale at all in the USDA sense. Australia’s growing challenges skew toward heat, drought, and rainfall variability far more than winter cold across most of its populated regions, so its zone system is built around different variables entirely. Some retailers publish rough USDA-equivalent numbers for Australian regions as a convenience for gardeners used to that scale, but treat those conversions as approximate at best, not an authoritative match.

What to do if you’re outside the US

If you’re gardening outside the US and want to use this site’s calendar or designer, the most reliable approach is to skip zone-number conversion entirely and instead find your location’s average annual extreme minimum winter temperature (many national meteorological services publish this, or a regional gardening extension/society equivalent to a US university extension office) and match that number directly to the USDA zone scale, the same one this site’s zone pages already use. That sidesteps every mismatched-numbering problem above, since temperature in °F or °C converts cleanly even when zone numbers don’t.

One more US-specific system worth knowing about if you’re comparing sources: the American Horticultural Society’s Plant Heat Zone map, which rates heat tolerance (days per year above 86°F) rather than cold hardiness, a completely separate axis from USDA zones, and the reason some US plant tags list two different zone numbers side by side.

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International Growing Zones vs. USDA Zones: What Compares: The UK, Canada, and Australia all rate cold-hardiness differently, and their zone numbers don't line up with USDA zones the way it looks like they should.

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