RHS ratings run from H1a (greenhouse only) up to H7 (very hardy). Your local rating should be at or above the plant's rating to grow it outdoors.
The RHS bands
| Band | Temperature range | Description |
|---|---|---|
| H7 | down to −20 °C | Very hardy — survives prolonged cold below −20 °C |
| H6 | −20 °C to −15 °C | Hardy in a very cold winter |
| H5 | −15 °C to −10 °C | Hardy in a cold winter |
| H4 | −10 °C to −5 °C | Hardy in an average winter |
| H3 | −5 °C to 1 °C | Half-hardy — needs sheltered spot or unheated greenhouse |
| H2 | 1 °C to 5 °C | Tender — protect from frost |
| H1c | 5 °C to 10 °C | Warm temperate — outdoors only in mild areas |
| H1b | 10 °C to 15 °C | Subtropical — frost-free conditions |
| H1a | 15 °C and warmer | Heated greenhouse only |
How RHS compares to the other systems
Hardiness scales do not translate one-to-one, because each was built for a different climate and reads in a different direction. Compare the RHS scale with the other three:
- USDA hardiness zone (United States) – plant tolerates down to this zone.
- Canadian plant hardiness zone (Canada) – plant tolerates down to this zone.
- Australian (ANBG) zone (Australia) – plant tolerates down to this zone.
Example plants rated in the RHS system
- Agrimony Agrimonia eupatoria – H7
- Cherry — Sunburst Prunus avium "Sunburst" – H7
- Goji berry (Wolfberry) Lycium barbarum – H7
- Mountain mahogany Cercocarpus montanus – H7
- Sea buckthorn Hippophae rhamnoides – H7
- Witch hazel Hamamelis virginiana – H7
- Licorice Glycyrrhiza glabra – H6
- Eastern cyclamen Cyclamen coum – H5
- Caper bush Capparis spinosa – H4
- Red robin photinia Photinia × fraseri ‘Red Robin’ – H4
- Finger lime Citrus australasica – H3
- Taupata (Mirror plant) Coprosma repens – H3
- Oleander-leaf protea Protea neriifolia – H2
- Yellow mangosteen Garcinia xanthochymus – H1c