UK Plant Hardiness Zones
The RHS hardiness rating system tells you the minimum temperatures a plant can survive. It runs from H1a (tropical plants needing a heated greenhouse) to H7 (fully hardy plants that tolerate any UK winter). Understanding these ratings helps you choose plants that will thrive in your local conditions.
RHS Hardiness Ratings
| Rating | Min. Temp (°C) | Description | Example Plants |
|---|---|---|---|
| H1a | Above 15°C | Heated greenhouse — tropical | Orchids, bromeliads, tropical ferns |
| H1b | 10 to 15°C | Heated greenhouse — subtropical | Bougainvillea, many succulents |
| H1c | 5 to 10°C | Heated greenhouse — warm temperate | Pelargoniums, citrus, fuchsias (tender) |
| H2 | 1 to 5°C | Unheated greenhouse / mild winters only | Tomatoes, peppers, dahlias (tubers) |
| H3 | −5 to 1°C | Half hardy — mild areas only | Tree ferns, echiums, agapanthus |
| H4 | −10 to −5°C | Hardy through most of UK | Japanese maples, olive trees, penstemons |
| H5 | −15 to −10°C | Hardy through most UK winters | Hydrangeas, rosemary, most roses |
| H6 | −20 to −15°C | Hardy across all of UK | Lavender, apple trees, buddleia |
| H7 | Below −20°C | Fully hardy everywhere | Oak, birch, hawthorn, snowdrops |
UK Regional Guidance
The UK climate varies considerably from south to north and coast to inland. Here is a rough guide to what hardiness ratings you can grow outdoors in different areas:
| Region | Typical Min. Winter Temp | Safe Outdoor Rating |
|---|---|---|
| South West England (coastal) | −2 to 2°C | H3 and above |
| London / South East | −5 to −2°C | H4 and above |
| Midlands | −8 to −5°C | H4–H5 and above |
| Northern England | −10 to −6°C | H5 and above |
| Central Scotland | −12 to −8°C | H5–H6 and above |
| Scottish Highlands | −15 to −10°C | H6 and above |
Urban gardens benefit from the "heat island" effect and are often 2–3°C warmer than surrounding countryside. Sheltered, south-facing walls can allow you to grow plants rated one zone lower than your area normally supports.
Hardiness ratings are based on the RHS system introduced in 2012. Actual cold tolerance depends on many factors including wind exposure, soil drainage, and how established the plant is. New plantings are always more vulnerable than mature specimens.