RHS hardiness rating
H1b — Subtropical (heated greenhouse / sheltered indoor)
minimum 10-15 °C · minimum 50-59 °F
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H1b sits at minimum 10-15 °C. H1b 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 H1b
Plants that need H1b minimums (typically under glass in the UK):
- Ficus elastica (rubber plant)
- Monstera deliciosa
- Philodendron varieties
- Calathea
- Bougainvillea (indoor only)
- Hibiscus rosa-sinensis
- Citrus (lemon, calamondin) under glass
- Coleus
- Tradescantia
- Plumbago auriculata
Climate notes for H1b
These plants do well as conservatory specimens or summer-outdoor / winter-indoor patio plants. Move them outdoors only between June and September in mild UK summers, and never let them sit below 10 °C overnight.
Growing conditions under glass
H1b 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.