When to plant sweet potatoes in RHS H3 (UK)

Key dates for sweet potatoes at RHS H3

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 potatoes 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 potatoes are extremely frost-tender and demand warm soil — do not transplant slips until soil temperature at 4-inch depth holds at 18 °C (65 °F) or above, typically 3 weeks after the last spring frost. Short-season zones (z5-6) should start slips indoors under lights 5-6 weeks early to ensure 100-120 frost-free days. Avoid zones 3-4 without a floating row cover season-extension strategy; in z9-11 slips can go out as early as late March.

UK-specific tips for H3

• Coastal Cornwall and south Devon gardens can plant sweet potatoes 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: Soil 18-24 °C (65-75 °F) for slip rooting.
• Spacing: 12-18 inches (30-45 cm).
• Days to harvest from transplant: ~105 days.
• Wait until soil reaches 10-12 °C before transplanting outdoors — cold UK soils stall tender crops for weeks.

Common mistakes — H3 × sweet potatoes

• 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 potatoes — 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 potatoes in RHS H2
• When to plant sweet potatoes in RHS H4
• When to plant sweet potatoes in RHS H5

Other crops at H3

• 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