RHS H3 UK planting calendar
When to plant garlic in RHS H3 (UK)
Sowing, planting, and harvest dates calibrated to H3's 230-day UK season (Coastal Cornwall, south Devon, the south coast of England, mild parts of Pembrokeshire and west Wales).
Key dates for garlic at RHS H3
| Stage | When | Anchor |
|---|---|---|
| Plant cloves outdoors | early October — mid-October (about 35 days before the first autumn frost) | 35 days before first autumn frost (mid-November) |
| First harvest (estimate) | mid-June of the following year | ~240 days from autumn planting |
Dates are typical for the regions H3 describes (Coastal Cornwall, south Devon, the south coast of England, mild parts of Pembrokeshire and west Wales). UK frost pockets, urban heat, and coastal moderation can shift the planting window by 1-2 weeks within the same rating band. Always cross-check against your local Met Office station for current conditions.
Why this timing works at H3
H3 describes UK gardens with winter minima of minimum -5 to 1 °C. Last spring frost typically passes late March to early April; first autumn frost arrives mid-November, giving about 230 frost-free days. Garlic need a cold period to bulb properly. Plant cloves in autumn so roots establish before the ground freezes; they overwinter dormant and break growth in spring. H3 delivers limited chill — softneck varieties are the right pick.
Garlic is the unusual one — plant cloves in autumn (4-6 weeks before the first hard fall frost) so they put down roots before winter, then break dormancy in spring and bulb up over the long days of early summer. Cold-winter zones grow hardneck varieties; mild-winter zones do better with softneck.
UK-specific tips for H3
- Coastal Cornwall and south Devon gardens can plant garlic a fortnight earlier than the rest of the UK — late April for tender crops is realistic on a sheltered south wall.
- Wet UK summers drive blight pressure on tomatoes and potatoes — choose blight-resistant cultivars (Crimson Crush, Lizzano, Sarpo Mira) and remove lower foliage to improve airflow.
- Slugs and snails thrive in H3 mildness — overnight checks in May and June are worth the effort.
Quick-grow guide
- Sun: Full sun — 6+ hours direct.
- Soil temperature for germination: Soil 10-15 °C (50-60 °F) at planting.
- Spacing: 4-6 inches (10-15 cm).
- Days to harvest from autumn planting: ~240 days.
- Mulch with 5-7 cm of straw or shredded leaves once temperatures drop in November.
Common mistakes — H3 × garlic
- Choosing hardneck varieties: H3 winters don't deliver enough chill hours for hardneck garlic — softneck (Solent Wight, Picardy Wight) is the right pick for mild coastal UK.
- Planting too early in autumn — warm soil in October triggers green growth before winter, weakening the bulb.
Source and methodology
RHS hardiness rating thresholds from the official RHS reference. Typical frost-date averages from Met Office regional climate summaries for the geographies H3describes. Crop timing offsets calibrated against UK extension references (RHS sowing calendar, Garden Organic, James Wong's UK growing tables) and cross-checked against US Cooperative Extension Service publications. For American readers cross-referencing, RHS H3 is roughly equivalent to USDA zone 9. Curated by the Growli editorial team.
Keep going
- How to grow garlic — full guide
- RHS H3 — typical regions and what else to plant
- All RHS hardiness ratings (H1a-H7)
- USDA hardiness zones — for cross-reference with US sources
- Fungus gnats in UK houseplants — guide