Plant hardiness Ratings
Hardiness ratings introduced in 2012 as outlined on the Royal Horticultural Society (RHS) web site.
Rating: H1a - Tropical
- Temperature range: Warmer than 15ºC (>59ºF)
- Category: Heated glasshouse - tropical
Plants with a H1a hardiness rating need to be grown as a house plant or under glass all year round.
Rating: H1b - Sub-tropical
- Temperature range: 10ºC to 15ºC (50ºF to 59ºF)
- Category: Heated glasshouse - sub-tropical
Plants with a H1b hardiness rating can be grown outdoors in summer in sunny and sheltered locations but generally performs best as a house plant or under glass all year round.
Rating: H1c - Warm Temperate
- Temperature range: 5ºC to 10ºC (41ºF to 50ºF)
- Category: Heated glasshouse - warm temperate
Plants with a H1c hardiness rating can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth.
Rating: H2 - Tender
- Temperature range: 1ºC to 5ºC (34ºF to 41ºF)
- Category: Tender - cool or frost-free glasshouse
Plants with a H2 hardiness rating are tolerant of low temperatures but will not survive being frozen. Except in frost-free inner-city areas or coastal extremities requires glasshouse conditions in winter, but can be grown outdoors once risk of frost is over.
Rating: H3 - Half-hardy
- Temperature range: -5ºC to 1ºC (23ºF to 34ºF)
- Category: Half-hardy - unheated glasshouse / mild winter
Plants with a H3 hardiness rating are hardy in coastal / mild areas except in hard winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Can survive with artificial winter protection.
Rating: H4 - Semi-Hardy
- Temperature range: -10ºC to -5ºC (14ºF to 23ºF)
- Category: Hardy - average winter
Plants with a H4 hardiness rating are hardy through most of the UK apart from inland valleys, at altitude and central / northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Plants in pots are more vulnerable.
Rating: H5 - Hardy
- Temperature range: -15ºC to -10ºC (5ºF to 14ºF)
- Category: Hardy - cold winter
Plants with a H5 hardiness rating are hardy through most of the UK even in severe winters. May not withstand open or exposed sites or central / northerly locations. Many evergreens suffer foliage damage and plants in pots will be at increased risk.
Rating: H6 - Very Hardy
- Temperature range: -20ºC to -15ºC (-4ºF to 5ºF)
- Category: Hardy - very cold winter
Plants with a H6 hardiness rating are hardy throughout the UK and northern Europe. Many plants grown in containers will be damaged unless given protection.
Rating: H7 - Extremely Hardy
- Temperature range: colder than -20ºC (-4ºF)
- Category: Very hardy
Plants with a H7 hardiness rating are hardy in the severest European continental climates including exposed upland locations in the UK.