Climate and Hardiness Zones: Choosing Plants That Survive Your Winters
Climate matters. A plant might look perfect in the garden centre, but if it can't handle your winter cold (or summer heat), it won't survive long. Hardiness ratings help you choose plants that will actually thrive year after year in your local climate.
RHS hardiness ratings (UK and Europe)
The Royal Horticultural Society uses H1–H7 ratings based on minimum winter temperatures. Most of the UK falls in H4–H5, with sheltered urban areas sometimes H3 and Scottish highlands reaching H6–H7.
- H1a–H1c Heated greenhouse only. Tropical and subtropical plants.
- H2 Frost-free greenhouse or mild coastal spots. Tender plants.
- H3 Survives -5°C to 1°C. Needs winter protection inland.
- H4 Hardy to -10°C. Survives average UK winters outdoors.
- H5 Hardy to -15°C. Cold winter areas.
- H6 Hardy to -20°C. Very cold winters.
- H7 Hardy below -20°C. Survives the coldest UK conditions.
USDA hardiness zones (USA)
The USDA system divides North America into zones 1–13 based on average annual minimum temperature. Most US gardeners are in zones 4–9.
- Zones 1–3 Extreme cold (northern US, Alaska, Canada). Few ornamental plants survive.
- Zones 4–5 Cold winters (upper Midwest, New England). Many perennials and shrubs thrive.
- Zones 6–7 Moderate winters (Mid-Atlantic, Pacific Northwest). Wide plant selection.
- Zones 8–9 Mild winters (Southeast, California coast). Tender plants often survive.
- Zones 10–13 Frost-free (Florida, Hawaii, southern California). Tropical plants outdoors year-round.
How to use hardiness zones
- Look up your location's zone (search "[your town] hardiness zone" or use Garden Pal's location feature).
- Only buy plants rated for your zone or colder. A plant rated H5 (or USDA zone 5) will survive zone 5, 6, 7, and so on.
- Microclimates matter: south-facing walls, urban heat islands, and sheltered spots can be one zone warmer.
- Wet winters kill plants that survive colder but drier climates. Drainage matters as much as temperature.
Beyond cold hardiness
Hardiness zones focus on winter cold, but other climate factors affect plant survival:
- Summer heat Some plants (e.g., many primulas) suffer in hot summers, even if winters are fine.
- Humidity Mediterranean plants often rot in damp UK winters despite being cold-hardy.
- Wind exposure Coastal and hilltop gardens face salt spray and desiccation that zones don't capture.
Start with local successes
Look at what's thriving in neighbours' gardens and local parks. If established plants look healthy nearby, they'll likely work for you too.