16 September 2024
| Rating | Temperature Range (C) | Category | Definition |
| H1a | More than 15 | Tropical – heated glass | Under glass all year |
| H1b | 10 to15 | Subtropical – heated glass | Can be grown outside in summer but performs better under glass |
| H1c | 5 to10 | Warm temperate – heated glass | Can be grown outside in summer |
| H2 | 1 to 5 | Cool or frost free | Will tolerate low temperatures, but does not survive if frozen. Requires glasshouse conditions. |
| H3 | -5 to 1 | Unheated glasshouse/mild | Hardy in coastal or mild areas except in hard winters or when at risk from sudden early frosts. Possibly hardy elsewhere with a sheltered wall or good microclimate. Will often survive with some artificial protection in winter. |
| H4 | -10 to -5 | Hardy – average winter | Hardy throughout most of UK except from inland valleys, at altitude and northerly locations. May suffer damage to foliage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may not survive during long wet winters in poorly drained or heavy soil. |
| H5 | -15 to -10 | Hardy – cold winter | Hardy in most places throughout UK, even in severe winters, but may not withstand open or exposed sites in northerly locations. Many evergreens will suffer foliage damage. |
| H6 | -20 to -15 | Hardy – very cold winter | Hardy in all of UK and northern Europe. |
| H7 | Less than -20 | Very hardy | Hardy in severest European
Climates including exposed upland locations in UK. |
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