RHS hardiness rating
H3 — Half-hardy (mild coastal areas)
minimum -5 to 1 °C · minimum 23-34 °F · 230-day growing season
What this rating means
The RHS rating system measures the lowest winter temperature a plant will reliably tolerate. H3 sits at minimum -5 to 1 °C. In practical UK terms, H3 suits coastal cornwall, south devon, the south coast of england, mild parts of pembrokeshire and west wales. For comparison with US references, H3 is roughly equivalent to USDA zone 9.
What grows at H3
Plants commonly recommended for H3 UK gardens:
- Phormium tenax (New Zealand flax)
- Cordyline australis (mature plants)
- Olearia
- Hebe (taller varieties)
- Salvia greggii / microphylla
- Penstemon (most cultivars)
- Trachelospermum jasminoides
- Pittosporum tenuifolium
- Melianthus major
- Agave americana (in coastal Cornwall only)
Climate notes for H3
H3 plants are the mainstay of seaside "exotic" UK gardens — Tresco, Tregrehan, Inverewe. They need a sheltered south-facing aspect, good winter drainage, and a mulch over the crown. A bad winter (think 2010 or 2018) wipes out a lot of H3 plantings in inland gardens, so use them only if you can accept the occasional loss.
Frost dates and growing season
| Typical last spring frost | late March to early April |
|---|---|
| Typical first autumn frost | mid-November |
| Growing season length | ~230 days |
| Temperature minimum (Celsius) | minimum -5 to 1 °C |
| Temperature minimum (Fahrenheit) | minimum 23-34 °F |
| Approx USDA equivalent | zone 9 |
Frost dates above are typical for the region the rating describes, not strict cutoffs. A south-facing walled garden in a H3 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 H3
Sowing, transplant, and harvest dates calibrated to a H3 winter:
- When to plant tomatoes in H3
- When to plant peppers in H3
- When to plant basil in H3
- When to plant garlic in H3
- When to plant lettuce in H3
- When to plant bush beans in H3
- When to plant cucumbers in H3
- When to plant summer squash in H3
- When to plant peas in H3
- When to plant carrots in H3
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.