The RHS has devised a system of hardiness ratings to enable gardeners to assess the hardiness of garden plants
Hardiness ratings are provided for all AGM plants, and also for plants included in the RHS Plant Search.
Download the hardiness ratings (510kB pdf)
The hardiness rating serves as a general guide to growing conditions, and should be interpreted as follows:
| Rating |
Temperature ranges ºC (ºF) |
Category |
Definition |
| H1a |
warmer than 15 (>59) |
Heated glasshouse - tropical |
Needs to be grown as a house plant or under glass all year round. |
| 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. |
| 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. |
| 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 |
| 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. |
| 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. |
| 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. |
| 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. |
| H7 |
colder than -20 (< -4) |
Very hardy |
Hardy in the severest European continental climates including exposed upland locations in the UK. |
This hardiness system was introduced by the RHS in 2012, and supersedes the H1-H4 ratings which previously accompanied the award.
