H1b - Tender, warm

Warm-climate plants requiring winter protection under glass, but often happy outside for the British summer once nights are reliably mild.

What it means

H1b plants need a warm, frost-free winter between 10 and 15 degrees C. They suit a warm greenhouse, conservatory or bright indoor space, then can be used outdoors for summer colour and structure. Even the mildest British coasts and cities are generally too cool for dependable year-round outdoor growth. This rating measures the plant's hardiness, not a geographic gardening zone.

Example plants

Plants often treated as H1b include mandevilla, many tropical cannas, colourful coleus, ornamental bananas and tender passionflowers. Some warm-climate palms, hibiscus and scented-leaved pelargoniums are also grown this way. They make effective patio or container plants in summer, but their label may specify a warmer or cooler minimum, particularly where named cultivars differ from the species.

Protecting these plants

Grow H1b plants in containers where possible, so they can move under cover before autumn cold arrives. Overwinter in bright, warm conditions above 10 degrees C and reduce watering without allowing compost to become dust-dry. Acclimatise gradually before placing them outside after late-spring cold spells. A sheltered, sunny summer position helps them grow strongly before their return 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.