RHS hardiness rating

The system of hardiness ratings enables gardeners to understand the hardiness of plants and gives a guide to the best growing conditions

What is the hardiness rating?

Hardiness ratings are determined by the lowest temperature a plant is likely to withstand. These ratings provide an overview of plant hardiness using one of the nine categories, H1a to H7. This a general guide to growing conditions, and should be used as a guideline only, as many other factors could affect a plant’s overall hardiness.

Hardiness ratings are provided for all Award of Garden Merit (AGM) plants, and also for plants included in the online Find a plant search.

Temperature guide to hardiness ratings

Rating Temperature ranges ºC (ºF) Category Definition USDA
H1a warmer than 15 (>59) Heated glasshouse – tropical Needs to be grown as a house plant or under glass all year round. 13
H1b 10 to 15
(50 to 59)
Heated glasshouse – subtropical 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. 12
H1c 5 to 10
(41 to 50)
Heated glasshouse – warm temperate Can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth. 11
H2 1 to 5 (34 to 41) Tender – cool or frost-free glasshouse 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 10b
H3 -5 to 1
(23 to 34)
Half-hardy – unheated glasshouse / mild winter 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. 9b/10a
H4 -10 to -5
(14 to 23)
Hardy – average winter 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. 8b/9a
H5 -15 to -10
(5 to 14)
Hardy – cold winter 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. 7b/8a
H6 -20 to -15
(-4 to 5)
Hardy – very cold winter Hardy throughout the UK and northern Europe. Many plants grown in containers will be damaged unless given protection. 6b/7a
H7 colder than -20
(< -4)
Very hardy Hardy in the severest European continental climates including exposed upland locations in the UK. 6a-1

Additional information

  • This hardiness system was introduced by the RHS in 2012, and supersedes the H1–H4 ratings, which are not the direct equivalents of the current ratings
  • The temperature ranges are intended to be absolute minimum winter temperatures (°C), not the long-term average annual extreme minimum temperature used for the USDA zones

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