RHS hardiness rating
H1c — Warm temperate (cool greenhouse / bright porch)
minimum 5-10 °C · minimum 41-50 °F
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H1c sits at minimum 5-10 °C. H1c is a glasshouse rating — these plants cannot be grown outdoors year-round anywhere in the UK. There is no direct USDA-zone equivalent because this rating describes indoor minimum temperatures rather than outdoor cold tolerance.
What grows at H1c
Plants that need H1c minimums (typically under glass in the UK):
- Pelargoniums (overwintered inside)
- Citrus in pots — lemon, lime, kumquat
- Olives (young plants — mature trees handle colder)
- Cordyline australis (young plants)
- Strelitzia (with winter shelter)
- Plectranthus
- Salvia "Hot Lips" (in mild south coast only)
- Echeveria and tender succulents
- Brugmansia
- Tetrapanax papyrifer
Climate notes for H1c
Treat H1c plants as movable container specimens — outdoors May to October, indoors November to April. Direct outdoor cultivation only works in the Isles of Scilly and parts of west Cornwall, and even there a hard winter will catch you out.
Growing conditions under glass
H1c plants need a heated environment in the UK from autumn through spring. Aim for the minimum temperature given above as an overnight target, and accept that British winter light levels (short days, low sun angle, frequent cloud) will slow growth even with adequate warmth. Supplementary lighting helps for actively-growing species.
Source and methodology
Temperature thresholds from the RHS hardiness rating reference. Frost-date typicals are taken from Met Office regional climate summaries for the geographies each rating describes. Plant lists curated by the Growli editorial team from the RHS Find a Plant database and UK extension references. USDA-zone equivalents are approximate cross-references for readers comparing American catalogues.