RHS H4 UK planting calendar
When to plant garlic in RHS H4 (UK)
Sowing, planting, and harvest dates calibrated to H4's 180-day UK season (Most of southern England, the south Midlands, south Wales, and coastal areas across the rest of the UK).
Key dates for garlic at RHS H4
| Stage | When | Anchor |
|---|---|---|
| Plant cloves outdoors | mid-September — late September (about 35 days before the first autumn frost) | 35 days before first autumn frost (late October) |
| First harvest (estimate) | mid-May of the following year | ~240 days from autumn planting |
Dates are typical for the regions H4 describes (Most of southern England, the south Midlands, south Wales, and coastal areas across the rest of the UK). 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 H4
H4 describes UK gardens with winter minima of minimum -10 to -5 °C. Last spring frost typically passes late April; first autumn frost arrives late October, giving about 180 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. H4 delivers reliable chill — both hardneck and softneck work.
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 H4
- H4 is the typical southern English garden — garlic grown outdoors here perform similarly to USDA zone 8b/9a Americans seeing in their references.
- Wet July-August stretches drive blight and powdery mildew — water early in the day, mulch to reduce splash, and remove lower foliage as plants establish.
- Slug pressure peaks in May after the first warm wet spell — protect young transplants for the first three weeks with copper tape or wool pellets.
- Light intensity is lower than in equivalent USDA zones — UK summer days are long, but cloud cover means more total hours but lower instantaneous photon flux than a US zone 8 garden. Plants tend to be leafier and slower to ripen.
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 — H4 × garlic
- Treating UK climate like the US zone 8: although temperature minima match, UK summers are cooler, wetter, and cloudier. Add 1-2 weeks to days-to-harvest figures from US sources.
- Sowing into cold wet soil: UK spring soil holds water longer than equivalent US zones. Wait for soil to dry enough to crumble in your hand before sowing.
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 H4describes. 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 H4 is roughly equivalent to USDA zone 8. Curated by the Growli editorial team.
Keep going
- How to grow garlic — full guide
- RHS H4 — 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
Same crop, nearby ratings
- When to plant garlic in RHS H2
- When to plant garlic in RHS H3
- When to plant garlic in RHS H5
- When to plant garlic in RHS H6