RHS Hardiness Ratings vs USDA Zones

The RHS bands in Celsius

Each band is defined by the lowest temperature a plant is likely to survive.

• H1a: warmer than 15C; needs a heated glasshouse (tropical)
• H1b: 10 to 15C; can go outside in summer but best under glass or as a houseplant
• H1c: 5 to 10C; warm temperate, outdoors in summer across most of the UK
• H2: 1 to 5C; tender, tolerates low temperatures but not freezing
• H3: -5 to 1C; half-hardy, coastal and mild areas, needs shelter
• H4: -10 to -5C; hardy through most of the UK
• H5: -15 to -10C; hardy in most of the UK even in severe winters
• H6: -20 to -15C; hardy throughout the UK and northern Europe
• H7: colder than -20C; hardy in the severest continental climates

Why the UK uses ratings, not zones

The USDA system sorts places by average annual minimum temperature alone. That works well for a large continent with cold, dry winters, but the UK's maritime climate is milder and wetter than its latitude suggests, and its summers are cool. Winter wet (rot) and lack of summer heat to ripen growth often decide whether a plant survives a UK winter as much as the temperature does, so the RHS rating is built around UK-relevant conditions rather than a single minimum.

Converting between the systems, honestly

You can approximate a match by comparing temperature ranges: H5 (-15 to -10C) sits around USDA zone 7, and H6 (-20 to -15C) around zone 6, for example. But this is a guide, not an equation. Because the two systems weight climate differently, a plant rated hardy on one scale may behave differently in the other's conditions, so use conversions to get in the right ballpark and then check the plant against your actual local climate.

✔ Rule of thumb: match the temperature ranges to get close, but never assume an exact RHS-to-USDA equivalence. See our USDA hardiness zones guide for the North American side.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• RHS — Hardiness ratings — The RHS rates hardiness in nine bands from H1a (warmer than 15C, heated glasshouse) through H4 (-10 to -5C, hardy through most of the UK), H5 (-15 to -10C), H6 (-20 to -15C) to H7 (colder than -20C), based on the lowest temperature a plant is likely to withstand, and notes UK survival is also affected by winter wet and cool summers.

Keep going