H1a - Under glass (heated)

Plants needing reliably warm, heated indoor conditions all year, with no tolerance of cool winter rooms, frost, or outdoor British weather.

What it means

H1a is the warmest and most tender RHS plant hardiness rating. It describes plants best grown in a heated greenhouse, conservatory, or warm room where winter temperatures remain above 15 degrees C. In the UK they are houseplants or specialist glasshouse subjects, not garden plants. RHS ratings describe a plant's cold tolerance, from H1a to H7, rather than mapping geographic areas as American systems do.

Example plants

Typical H1a plants include many tropical foliage houseplants, such as calatheas, anthuriums and some palms, alongside warmth-loving orchids including phalaenopsis. Tender tropical climbers, indoor gingers and certain bromeliads may also fit this rating. Individual species and cultivars vary, so check the plant label or ask the RHS or a local garden centre before relying on a particular minimum temperature.

Protecting these plants

Keep H1a plants indoors or under heated glass throughout winter, away from cold draughts and chilled window panes. Maintain steady warmth, adequate light and careful ventilation, especially in humid rooms. Do not put them outdoors until summer warmth is settled, if at all. Bring any summer displays back inside well before autumn nights cool, and inspect plants for damage before moving them indoors.

RHS hardiness ratings describe a plant's cold tolerance, not a geographic zone. Local conditions vary; the RHS or a local garden centre can advise. Page updated 18 Sep 2026.