UK Hardiness Zones — RHS H1a to H7 ratings explained

UK Hardiness Zones — RHS H1a to H7 ratings explained

If you have searched for "UK hardiness zone" and ended up confused, that is because the question doesn't quite map to how British gardening works. The United States divides its territory into 13 numbered zones and labels plants by the zones they survive in. The UK does the opposite — the climate is mild and uniform enough that the RHS rates each plant with a single hardiness number, and gardeners are expected to know roughly how cold their own garden gets in a hard winter. This guide explains the H1a to H7 system, where each rating applies in the UK, how to convert to USDA zones if you are moving to or from the United States, and how to use the rating when you are standing in a garden centre staring at plant labels.

Skip the lookup: Open Growli's zone finder, enter your UK postcode, and get the equivalent USDA zone plus the realistic RHS hardiness range for your area — useful when you are reading American gardening advice and need to translate it for British conditions.

Why the UK does not use USDA zones

The continental United States spans hardiness extremes from minus 50°C in interior Alaska to plus 20°C in the Florida Keys — a range that genuinely needs 13 named bands to be useful. The United Kingdom is a small, oceanic, mid-latitude island chain. The whole country sits within roughly minus 20°C to plus 5°C average winter minimums, which would translate to only USDA zones 6 through 9.

For a single small country with a moderating ocean influence on three sides, mapping the territory adds little practical value over rating the plants. So the RHS publishes a plant-side hardiness scale rather than a place-side map. You see "H5" on a plant label and translate it to "yes, this should survive an average Birmingham winter."

A second reason: UK winter risk is dominated by wet cold rather than dry cold. A plant that handles minus 10°C in dry continental conditions might rot in minus 2°C with three weeks of wet soil. Per-plant ratings let the RHS account for moisture sensitivity in a way a temperature-only map cannot.

All 9 RHS hardiness ratings explained

The RHS hardiness scale has nine bands. H1 is split into H1a, H1b, and H1c. H2 through H7 are single bands. The numbers describe the minimum temperature the plant tolerates, with hardier plants getting higher numbers.

The full RHS definitions also flag plants that need protection from cold winds, wet soil, or summer heat — those caveats appear in the RHS Plant Finder rather than the number itself.

Where each rating applies in the UK

The UK has no official hardiness map, but the practical pattern is well established. Roughly:

Three caveats that bend this regional map:

• Urban heat islands lift hardiness. Central London routinely behaves a full rating warmer than the surrounding suburbs. Manchester city centre and Glasgow city centre do the same to a lesser degree.
• Coastal proximity softens both summer heat and winter cold. A garden 100 metres from the sea in Devon is meaningfully warmer than one 5 miles inland.
• Altitude drops the effective rating roughly half a step per 150 metres of elevation. A hillside in the Yorkshire Dales is colder than the valley below.

H4 and H5: the band that matters for most UK gardeners

If you live anywhere in mainland UK away from Cornwall or the Highlands, your practical concern is the boundary between H4 and H5. Plants rated H5 or hardier are essentially never at risk from winter cold in your garden. Plants rated H4 are at risk in a hard winter (one in roughly 5-10 years), and H3 plants need either lifting, fleecing, or a sheltered microclimate to survive.

A practical heuristic for the average English Midlands garden:

• H5 and H6: plant freely. These are the workhorse hardy perennials and shrubs.
• H4: plant in a sheltered spot or accept occasional losses in a hard winter.
• H3: treat as borderline; either grow in a pot you can move under cover, or plant against a south-facing wall and mulch heavily.
• H1 and H2: treat as houseplants or summer bedding only. Lift before first frost.

USDA to RHS conversion (for US readers in UK gardens)

If you are American and have moved to the UK, this is the rough translation. Note the conversion is approximate because the rating systems measure different things — USDA averages 30 years of annual minimums, RHS describes the lowest temperature the plant survives.

A worked example. If you used to garden in Boston, Massachusetts (USDA zone 6b), and you now live in Birmingham, you have effectively moved to RHS H5 — about half a rating milder. Plants rated H5 or H6 will do fine. The bigger adjustments are the wet UK winter (more rot risk) and the cooler UK summer (less heat for tomatoes and peppers, more reliable for cool-season crops).

For US gardeners staying in the US, see our companion USDA hardiness zone guide.

How to use RHS hardiness ratings when buying plants

When you pick up a plant at a UK garden centre, look for the H rating either on the plastic label or in the RHS Plant Finder entry. Three steps:

1. Read the rating. If it is H4 or hardier, the plant will survive most UK winters in most regions. If it is H3 or lower, you need a plan.
2. Cross-check the placement. Even a hardy plant fails in wrong conditions — full-shade lavender will not thrive even at H5. Check the light, soil, and moisture requirements alongside the rating.
3. Match the rating to your microclimate, not just your region. A south-facing brick wall might let you grow H3 plants in a typical H4 location. A north-facing waterlogged corner might struggle with H5 plants.

Plants without an RHS rating on the label (common with imports and supermarket plants) are often the riskiest — assume H3 or worse and treat them as tender until you have observed them through a winter.

Diagnose this with Growli: If a plant died last winter and you are not sure whether the rating was wrong or your placement was wrong, open Growli and describe what you saw. The app cross-references the species' RHS rating, your postcode's typical winter minimum, and the symptoms you describe (e.g. blackened crown vs. completely dry) to tell you whether it was cold, wet, or wind.

Common UK hardiness mistakes

1. Trusting "fully hardy" labels. "Fully hardy" is loose retailer language, not an RHS rating. Always look for the H number — "fully hardy" sometimes means H4, sometimes H5, sometimes H6.
2. Forgetting wet-cold is different from dry-cold. Many Mediterranean plants (lavender, rosemary, sage) survive their RHS rating in dry continental climates but rot in UK winter wet at temperatures well above their stated tolerance. Drainage matters as much as the H rating.
3. Assuming the south coast is uniformly mild. Cornwall is H3-H4, but the high ground inland of Bodmin is H5 or colder. Local elevation overrides the regional label.
4. Using American zone advice without converting. Plants labelled "hardy to zone 5" in American sources translate to H6 or H7 in UK terms — usually fine for anywhere on mainland UK. Watch the opposite direction more carefully: "hardy to zone 8" is borderline H4 in the UK and risky outside southern England.
5. Confusing hardiness with cold tolerance of fruit. An apple tree might be rated H6, but its fruit set depends on spring frost timing, not on the tree's overall hardiness.

Related articles

• Browse the UK zone library — RHS-aligned planting calendars by region
• Find your zone in 5 seconds — UK postcode or US ZIP lookup
• USDA hardiness zone map explained — full US zone reference
• Browse the US zone library — every USDA zone with planting calendars
• How to grow tomatoes in the UK — RHS-aligned greenhouse playbook
• When to plant tomatoes — US zone + UK timing chart — both systems side by side
• Get tailored UK plant advice in the Growli app — postcode-aware recommendations

Reviewed and updated by the Growli editorial team. Source: Royal Horticultural Society Hardiness Ratings (H1a-H7), Met Office UK climate normals. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.