RHS hardiness rating
H1a — Heated greenhouse (tropical)
minimum above 15 °C · minimum above 59 °F
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H1a sits at minimum above 15 °C. H1a 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 H1a
Plants that need H1a minimums (typically under glass in the UK):
- Anthurium
- Heliconia
- Cacao
- Tropical orchids (Vanda, Phalaenopsis)
- Bird of paradise (Strelitzia reginae, indoor)
- African violets
- Plumeria
- Cordyline fruticosa
- Bromeliads
- Stephanotis
Climate notes for H1a
H1a plants live indoors year-round in the UK — typically in a heated greenhouse, conservatory, or warm bright room. They will not tolerate any UK outdoor winter, and most struggle below 15 °C even briefly. Treat them like permanent houseplants.
Growing conditions under glass
H1a 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.