An overhead view of two open gardening reference books side by side, one showing a UK hardiness chart and the other a USDA zone map of the United States, with a notebook and pen between them in soft natural light
ClimateWritten by Dedrab9 min read

RHS Hardiness vs USDA Zones: A Quick Translator for British, Irish and American Gardeners

You're looking at a plant in a US online catalogue. It's listed as "hardy in USDA Zone 8." You're in Cork, or Manchester, or Galway, or Bristol, and you have no idea whether that plant will live through your winter or not. The catalogue assumes you know your zone. You don't, because the zone system was designed for someone else's climate.

Same problem in reverse. You're an American gardener moving to London or Dublin, or just reading British gardening books for ideas, and every plant is rated H4 or H5 with no USDA equivalent. The two systems exist in parallel, mostly ignore each other, and the conversion between them is roughly translatable but quietly imperfect in ways that catch people out.

Here's the practical translator, plus the climate quirks that make a direct translation fail more often than it works.

Why Two Systems Exist

The Royal Horticultural Society publishes the RHS hardiness scale, which runs from H1 (frost-free glasshouse only) to H7 (hardy in the most extreme British conditions). It's calibrated against the maritime climate of Ireland and the UK: mild but consistently wet winters, cool damp summers, the moderating influence of the Atlantic.

The United States Department of Agriculture publishes the USDA Plant Hardiness Zone Map, running from Zone 1 (sub-Arctic Alaska) to Zone 13 (Hawaii and parts of Puerto Rico). It's calibrated against average annual minimum temperatures across North America, a continental climate dominated by extremes: hot dry summers in much of the west, cold dry winters in much of the north, humid heat in the southeast.

Both systems answer the same headline question — will this plant survive the cold here? — but they're measuring different climates and were never designed to translate between each other. The conversion table below is approximate, and the section after it explains why the approximation matters.

The Conversion Table

USDA ZoneApprox. RHS equivalentTypical North American exampleNotes
Zone 5H6Chicago, Denver, much of New EnglandContinental winters down to around -20°C
Zone 6H5 to H6Boston, mountain Pacific NorthwestDown to around -15°C
Zone 7H5Mid-Atlantic states, parts of the southDown to around -12°C
Zone 8H4 to H5Seattle, lower mid-Atlantic, parts of TexasDown to around -7°C
Zone 9H3 to H4Coastal California, Gulf Coast, north FloridaDown to around -1°C
Zone 10H2 to H3Southern California, central FloridaFrost-rare to frost-free
Zone 11H1c to H2South Florida, HawaiiEffectively frost-free

Most of Ireland and the milder parts of the UK sit somewhere around USDA Zone 8 to 9 in raw winter minimum temperature terms. Most of England is broadly Zone 8. The Scottish Highlands and exposed parts of the Pennines edge toward Zone 7. Western Cork and the Beara peninsula push toward Zone 9.

If you want the full regional breakdown, our UK hardiness guide and our Irish hardiness guide cover it properly, region by region.

Why the Translation Quietly Fails

The conversion table works for raw winter minimum temperature. It misses three other things that, in practice, determine whether a plant lives or dies. This is where the cross-Atlantic confusion starts.

The drainage trap (for US gardeners reading British advice)

A plant rated H5 in the UK can survive a typical Irish or British winter. The problem isn't usually the cold. It's the constant wet. Mediterranean-origin plants — lavender, rosemary, many salvias, certain cistus — are often rated H4 or H5 (good down to around -10°C) but die in Atlantic gardens because the soil is saturated from October through March. The roots rot.

In a USDA Zone 8 garden in California, the same plants thrive because Zone 8 there means dry winters and very dry summers. The hardiness number is identical, the temperature is comparable, but the soil moisture profile is completely different. American gardeners moving to Ireland or the UK and assuming "Zone 8 is Zone 8" learn this the hard way.

The wet-versus-cold paradox (for UK and Irish gardeners ordering from US catalogues)

The mirror problem also bites. A plant listed in a US catalogue as "Zone 8 hardy" was assessed in a climate where Zone 8 means dry winters and hot dry summers. Bring that same plant to an Irish or western British garden and it has to survive the same minimum temperature but in soil that's effectively swampy from autumn through spring. Lots of supposedly "hardy" plants from American catalogues never make it through their first wet winter in Ireland not because of the cold, but because of the cold combined with sustained wet.

Summer heat (for UK and Irish gardeners reading US gardening books)

The USDA system doesn't measure summer heat at all (the American Horticultural Society has a separate Heat Zone Map for that). A plant rated USDA Zone 8 might be fine through the cold but might require a hot dry summer to set flower buds or ripen wood properly. American hybrids bred for the Pacific Northwest or the mid-Atlantic often struggle in Irish or British summers — not because of cold, but because they never get the heat they evolved with.

What we've found is that the most successful cross-Atlantic transplants are plants from comparable climate zones — Pacific Northwest cultivars for western Ireland, mid-Atlantic for eastern England, southern English varieties for the Pacific Northwest — rather than direct zone-number swaps from sun-belt America.

Practical Rules of Thumb

If you're an American gardener moving to Ireland or the UK:

  • Most of the country is roughly equivalent to USDA Zone 8 to 9 in temperature, but treat anything rated USDA Zone 9 with caution if it specifies "well-drained" or "drought-tolerant" — that's a warning flag for Atlantic winter conditions.
  • Look for plants of British, Irish, Western European or Pacific Northwest origin first. They were bred or selected against conditions closer to what you have now.
  • Use the RHS Plant Finder rather than your American catalogues for the first couple of years. The cultural advice is calibrated for your new climate.

If you're a UK or Irish gardener ordering from US sources or reading American gardening books:

  • USDA Zone 7 or below means the plant can handle real cold. Almost certainly fine for anywhere in Ireland or the UK. Zone 8 is also probably fine.
  • USDA Zone 9 or 10 plants from California or the southwest are likely too tender for most British and Irish gardens, even though the temperature numbers look comparable, because they evolved for hot dry summers and dry winters.
  • USDA Zone 9 or 10 plants from the southeast US (Florida, Gulf Coast) handle humidity better but want serious summer heat to perform. Risk-assess accordingly.

And Briefly — Australia and New Zealand

Australia uses its own hardiness scale (zones 1 to 7, where Zone 1 is the coldest alpine and Zone 7 is tropical north), which translates very roughly to USDA Zones 7 through 12. New Zealand mostly maps to USDA Zone 8 to 10 depending on island and elevation. The same drainage and summer-climate caveats apply. If you're working from Australian or New Zealand cultivars, treat them with the same care as American ones: zone is a useful starting point, microclimate is the answer.

The Bigger Point

Hardiness zones tell you whether a plant might survive your average winter. They tell you nothing about whether your specific site will let it thrive. Aspect, soil drainage, exposure to wind, summer moisture, and microclimate within your garden all matter as much as the zone number, sometimes more.

The cross-Atlantic gardener's lesson is the same as the local gardener's: read your site as carefully as you read the label. Zone is the first conversation, not the last. And if reading your own site is the bit you find hard, that's the work we put into every plan dedrab produces.

Related Notes

Whatever scale you're working from, the right plant for your specific garden depends on more than a number. Dedrab turns a single photo of your garden into a planting plan that factors in hardiness, soil, aspect and exposure — calibrated for your specific site, not someone else's climate. Try it at dedrab.com.

References

Written by Dedrab