RHS H6 UK planting calendar
When to plant basil in RHS H6 (UK)
Sowing, planting, and harvest dates calibrated to H6's 120-day UK season (Upland Scotland, the Cairngorms foothills, the Pennines, high ground in the north of England and north Wales).
Marginal — needs polytunnel or wall protection
Key dates for basil at RHS H6
| Stage | When | Anchor |
|---|---|---|
| Indoor seed start | late April (28 April) | 6 weeks before last frost |
| Move under cover | mid-June (16 June) | 7 days after last frost (late May to early June) |
| First harvest (estimate) | mid-August (15 August) | ~60 days from transplant |
Dates are typical for the regions H6 describes (Upland Scotland, the Cairngorms foothills, the Pennines, high ground in the north of England and north 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 H6
H6 describes UK gardens with winter minima of minimum -20 to -15 °C. Last spring frost typically passes late May to early June; first autumn frost arrives early October, giving about 120 frost-free days. Basil are tender — they need soil above 10 °C to grow and stop setting fruit when night temperatures drop below 10 °C. At H6 that means transplant after the last spring frost (late May to early June) and pull plants when autumn cools (early October).
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 H6
- H6 gardens — Highland Scotland, Pennine uplands — have a short growing season, and basil need either a polytunnel or a south-facing wall to deliver a reliable crop.
- Wind exposure cuts as much as 2 °C off ambient temperature in upland gardens — site beds in the lee of a hedge or wall before worrying about frost dates.
- Choose short-season cultivars: tomatoes (Glacier, Sub-Arctic Plenty), peppers (King of the North), squash (Spirit, Sweet Mama). Long-season heirlooms will run out of summer before they finish.
- Cool wet ground keeps soil temperatures below 10 °C well into May — black plastic mulch or biodegradable paper mulch lifts soil temperature by 3-4 °C and brings sowing dates forward by two weeks.
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 transplant: ~60 days.
- Wait until soil reaches 10-12 °C before transplanting outdoors — cold UK soils stall tender crops for weeks.
Common mistakes — H6 × basil
- Choosing a long-season cultivar: H6's 120-day window is too short for big beefsteak tomatoes or long-season peppers — pick short-season cultivars (Glacier, Sub-Arctic Plenty, King of the North).
- Skipping season extension: direct outdoor sowing fails most years at H6/H7 — a polytunnel, cloches, or a fleece-covered raised bed turns a failure into a crop.
- Transplanting into cold soil: even in a polytunnel, UK soil sits below 10 °C until mid-May at this rating. Use black plastic mulch to warm the bed a fortnight ahead of transplanting.
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 H6describes. 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 H6 is roughly equivalent to USDA zone 6. Curated by the Growli editorial team.
Keep going
- How to grow basil — full guide
- RHS H6 — 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