RHS hardiness rating
H4 — Hardy (average UK winter)
minimum -10 to -5 °C · minimum 14-23 °F · 180-day growing season
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H4 sits at minimum -10 to -5 °C. In practical UK terms, H4 suits most of southern england, the south midlands, south wales, and coastal areas across the rest of the uk. For comparison with US references, H4 is roughly equivalent to USDA zone 8.
What grows at H4
Plants commonly recommended for H4 UK gardens:
- Buddleja davidii
- Lavandula angustifolia
- Rosa (most modern shrub and floribunda roses)
- Sarcococca (sweet box)
- Camellia japonica
- Choisya ternata
- Ceanothus (most cultivars)
- Hydrangea macrophylla
- Wisteria
- Magnolia (M. grandiflora, M. soulangeana)
- Vegetables — tomatoes, peppers, runner beans, courgettes outdoors with full summer
- Fruit — apples, pears, plums, cherries, figs (south-facing wall)
Climate notes for H4
H4 is the rating most southern English gardens design around. Tomatoes and peppers grow outdoors here without a greenhouse most years. Beware of frost pockets — a low-lying garden in a "H4 region" can sit a full band colder than the regional figure.
Frost dates and growing season
| Typical last spring frost | late April |
|---|---|
| Typical first autumn frost | late October |
| Growing season length | ~180 days |
| Temperature minimum (Celsius) | minimum -10 to -5 °C |
| Temperature minimum (Fahrenheit) | minimum 14-23 °F |
| Approx USDA equivalent | zone 8 |
Frost dates above are typical for the region the rating describes, not strict cutoffs. A south-facing walled garden in a H4 area can run a fortnight warmer; a frost pocket at the bottom of a valley can run a fortnight colder. Always cross-check against your local Met Office station before sowing tender crops.
When to plant — crop by crop in H4
Sowing, transplant, and harvest dates calibrated to a H4 winter:
- When to plant tomatoes in H4
- When to plant peppers in H4
- When to plant basil in H4
- When to plant garlic in H4
- When to plant lettuce in H4
- When to plant bush beans in H4
- When to plant cucumbers in H4
- When to plant summer squash in H4
- When to plant peas in H4
- When to plant carrots in H4
Source and methodology
Temperature thresholds from the RHS hardiness rating reference. Frost-date typicals are taken from Met Office regional climate summaries for the geographies each rating describes. Plant lists curated by the Growli editorial team from the RHS Find a Plant database and UK extension references. USDA-zone equivalents are approximate cross-references for readers comparing American catalogues.