RHS H1a UK planting calendar
When to plant basil in RHS H1a (UK)
Greenhouse-only growing notes for basil at H1a — UK winter heating and light requirements.
Heated greenhouse only — never outdoors in this rating band
Key dates for basil at RHS H1a
| Stage | When | Anchor |
|---|---|---|
| Sow under glass | late winter to early spring (under lights) (15 February) | Heated greenhouse — start when daylight reaches 10+ hours |
| First harvest (estimate) | ~60 days after sowing under glass | ~60 days from sowing |
Dates are typical for the regions H1a describes (Heated greenhouse and conservatory growing only — never reliably outdoors anywhere in 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 H1a
H1a describes UK gardens with winter minima of minimum above 15 °C. Outdoor cultivation is not viable at this rating anywhere in the UK — these plants need year-round greenhouse heating to survive a British winter. UK winter light levels are the secondary constraint: basil under glass slow to a near-stop from November through February without supplementary LED lighting.
Basil is one of the most cold-sensitive common herbs — it sulks below 10 °C and dies in light frost. Wait a full week after the last spring frost before moving transplants outside, or direct-sow two weeks after frost when soil hits 18 °C.
UK-specific tips for H1a
- Aim for night minimums above 15 °C year-round — a thermostatically controlled heater is essential for British winter glasshouse growing.
- UK winter light is the bigger constraint than warmth — short days and low sun angles slow growth from November through February. Supplementary LED lighting helps for actively-cropping plants like tomatoes and basil under glass.
- Ventilate aggressively on warm days — UK greenhouses overheat in May/June even when air temperatures are mild, and basil stop setting fruit above 32 °C.
- Damping off and Botrytis are the chronic UK glasshouse problems — keep air moving, water in the morning, and remove dead leaves promptly.
Quick-grow guide
- Sun: Full sun — 6+ hours direct.
- Soil temperature for germination: 20-25 °C (68-77 °F).
- Spacing: 8-12 inches (20-30 cm).
- Days to harvest from sowing under glass: ~60 days.
- Maintain night temperatures at or above minimum above 15 °C — UK winter cold is the main constraint for basil at this rating.
Common mistakes — H1a × basil
- Treating the greenhouse as frost-tolerant: basil at rating H1a need active heating from October through April. A "frost-free" unheated greenhouse drops well below the minimum above 15 °C minimum.
- Ignoring winter light levels — UK November to February daylight is too short for productive cropping without supplementary LED lighting. Most growers run as houseplants over winter and crop in summer.
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 H1adescribes. 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. Curated by the Growli editorial team.
Keep going
- How to grow basil — full guide
- RHS H1a — 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