RHS H3 UK planting calendar
When to plant sweet corn 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 sweet corn at RHS H3
| Stage | When | Anchor |
|---|---|---|
| Direct sow outdoors | mid-April (19 April) | 10 days after last frost (late March to early April) |
| First harvest (estimate) | early July (3 July) | ~75 days from sow |
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. Sweet corn are tender — they need soil above 10 °C to grow and stop setting fruit when night temperatures drop below 10 °C. At H3 that means transplant after the last spring frost (late March to early April) and pull plants when autumn cools (mid-November).
Sweet corn is direct-sown only — it resents root disturbance and transplants very poorly. Sow in blocks of at least 4 rows (not single rows) for adequate wind pollination, 7-14 days after the last frost once soil temperature reaches 16 °C (60 °F). In short-season zones (5-6), warm-season su/se types reaching maturity in 70-75 days are most reliable; avoid extra-sweet sh2 varieties below 18 °C as germination fails. Succession-sow every 2-3 weeks for a continuous harvest, but do not mix different types within 400 metres to prevent cross-pollination that causes starchy kernels.
UK-specific tips for H3
- Coastal Cornwall and south Devon gardens can plant sweet corn 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 — 6-8 hours direct.
- Soil temperature for germination: 16-34 °C (60-95 °F).
- Spacing: 9-12 inches (23-30 cm).
- Days to harvest from direct sow: ~75 days.
- Wait until soil reaches 10-12 °C before transplanting outdoors — cold UK soils stall tender crops for weeks.
Common mistakes — H3 × sweet corn
- Treating UK climate like the US zone 9: 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 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 sweet corn — 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
Same crop, nearby ratings
- When to plant sweet corn in RHS H2
- When to plant sweet corn in RHS H4
- When to plant sweet corn in RHS H5