Growli UK planting calendar
UK gardener? Here are your RHS hardiness ratings
— H1a to H7, what they mean, and what to plant.
British gardeners use the Royal Horticultural Society's hardiness rating, not the American USDA zone system. Each rating describes the lowest winter temperature a plant will reliably survive, so you pick plants whose rating suits your own coldest winter. Most of England and Wales sits at H4-H5; Scotland reaches H6 and high ground H7; the Scilly Isles and Channel Islands touch H2-H3.
How RHS ratings differ from USDA zones:USDA splits the country into geographical zones; RHS rates each plant against an absolute minimum temperature. There is no "your zone" in the UK system — there is "the rating my coldest winter will tolerate." See the official reference at rhs.org.uk. For the deeper walkthrough, see our UK hardiness zones guide. For US gardeners cross-referencing British plant catalogues, our USDA zones page has equivalent ranges.
All nine RHS hardiness ratings
H1a
under glassminimum above 15 °C · minimum above 59 °F
Tropical species that need constant warmth. Pure indoor growing in the British climate.
Heated greenhouse and conservatory growing only — never reliably outdoors anywhere in the UK
H1b
under glassminimum 10-15 °C · minimum 50-59 °F
Subtropical species that tolerate a cool night but still need above-freezing winters.
Heated greenhouse, conservatory, and bright indoor positions. Outdoor cultivation not viable anywhere in the UK.
H1c
under glassminimum 5-10 °C · minimum 41-50 °F
Warm-temperate plants that need a frost-free winter but tolerate cool nights.
Cool conservatory, frost-free greenhouse, sheltered porch. A handful of microclimate pockets in coastal Cornwall and the Channel Islands brush this band outdoors in mild winters.
H2
250 daysminimum 1-5 °C · minimum 34-41 °F
Frost-tender plants that survive in the UK only where the sea moderates winter lows to above 1 °C.
Isles of Scilly, Channel Islands, the warmest sheltered pockets of west Cornwall and south Devon
H3
230 daysminimum -5 to 1 °C · minimum 23-34 °F
Plants that tolerate a brief light frost but suffer when temperatures sit below freezing for more than a few hours.
Coastal Cornwall, south Devon, the south coast of England, mild parts of Pembrokeshire and west Wales
H4
180 daysminimum -10 to -5 °C · minimum 14-23 °F
The default rating for "British garden hardy". Plants in this band cope with most UK winters but can be damaged in unusually cold years.
Most of southern England, the south Midlands, south Wales, and coastal areas across the rest of the UK
H5
150 daysminimum -15 to -10 °C · minimum 5-14 °F
Solidly cold-hardy plants. The default working assumption for a "British hardy perennial" away from the south coast.
The Midlands, northern England, inland Wales, lowland Scotland — the band most UK gardens actually sit in once you leave the south coast
H6
120 daysminimum -20 to -15 °C · minimum -4 to 5 °F
Cold-resistant plants that shrug off hard Scottish and northern winters. Suitable for upland and exposed gardens.
Upland Scotland, the Cairngorms foothills, the Pennines, high ground in the north of England and north Wales
H7
90 daysminimum below -20 °C · minimum below -4 °F
Genuinely cold-hardy alpine and boreal plants — what you would plant in upland Aberdeenshire or Caithness.
High Scottish mountains, exposed glens, alpine gardens above 500 m. A tiny fraction of UK gardens experience true H7 conditions.
Which rating fits my garden?
Most of the UK is H4-H5
Southern England, the south Midlands, south Wales, and most coastal areas are H4 — average UK winter, minima around -5 to -10 °C. The Midlands, northern England, inland Wales, and lowland Scotland are H5 — colder winters down to -15 °C. Pick H5-rated plants if you are unsure; you will lose fewer.
Scotland and uplands sit at H6-H7
The Scottish Highlands, Cairngorms, and high ground in the Pennines and Snowdonia are H6. Genuine H7 territory — Aberdeenshire uplands, the high mountains — is rare but real. The growing season here is short and exposure matters more than absolute temperature.
Coastal pockets touch H2-H3
The Isles of Scilly, Channel Islands, and warm coastal nooks of Cornwall, south Devon, and west Wales are H2-H3. This is where you see Echium pininana, banana, tree ferns, and the "exotic" UK gardens (Tresco, Tregrehan, Inverewe).
H1a-H1c are indoor only
Anything rated H1a, H1b, or H1c needs a heated greenhouse or warm bright room year-round. Most common houseplants — Monstera, rubber plant, orchids, Calathea — fall in this band. They are never reliable outdoors anywhere in the UK.
When to plant — by crop and UK rating
Each crop has its own frost rules. Pick a crop below for sowing, transplant, and harvest dates calibrated to your hardiness band. We default the crop links to the band most UK gardens sit in (H4 for warm-season tender crops, H5 for hardy crops like garlic and peas).
Plant tomatoes in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H4 →
Plant peppers in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H4 →
Plant basil in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H4 →
Plant garlic in your rating
fall plantedPlant in autumn, October to November in most of the UK.
See timing for rating H5 →
Plant lettuce in your rating
cool seasonSow well before the last spring frost — as soon as soil can be worked.
See timing for rating H5 →
Plant bush beans in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H5 →
Plant cucumbers in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H4 →
Plant summer squash in your rating
warm seasonWait until after the last spring frost — late April to mid-May in most UK gardens.
See timing for rating H5 →
Plant peas in your rating
cool seasonSow well before the last spring frost — as soon as soil can be worked.
See timing for rating H5 →
Plant carrots in your rating
cool seasonSow a couple of weeks before last frost under cloches.
See timing for rating H5 →
What rating matters for
When to plant
Frost dates drive the planting calendar. H4 gardens (southern England) plant tomatoes out late May; H5 gardens (most of the UK) wait until early June; H6 (Scottish Highlands) needs a polytunnel or wait till mid-June.
When to plant tomatoes (H4) →What grows here
Apples, pears, plums and most temperate fruit thrive in H4-H5. Heat-loving crops (aubergine, melon, sweet peppers) need a polytunnel or south wall in H5 and above. Tropical plants are H1 only.
Start a vegetable garden →Pest pressure
Wet UK summers drive slug, snail, and fungal pressure across H4-H6. Cooler, drier H7 gets less fungal disease but a shorter window for outdoor sowing.
Get rid of fungus gnats (UK) →Cross-references
- USDA hardiness zones 1-13 — for US readers and cross-checking American seed catalogues. We list the approximate USDA equivalent on each UK rating page.
- Fungus gnats in UK houseplants — full guide
- How to start a vegetable garden
- RHS hardiness rating reference (official)
Cross-reference table (RHS rating to USDA zone, approximate):
| RHS rating | Temperature minimum | Approx USDA zone |
|---|---|---|
| H1a | minimum above 15 °C | greenhouse-only (no USDA equivalent) |
| H1b | minimum 10-15 °C | greenhouse-only (no USDA equivalent) |
| H1c | minimum 5-10 °C | greenhouse-only (no USDA equivalent) |
| H2 | minimum 1-5 °C | ~zone 10 |
| H3 | minimum -5 to 1 °C | ~zone 9 |
| H4 | minimum -10 to -5 °C | ~zone 8 |
| H5 | minimum -15 to -10 °C | ~zone 7 |
| H6 | minimum -20 to -15 °C | ~zone 6 |
| H7 | minimum below -20 °C | ~zone 5 |