RHS H4 UK planting calendar
When to plant radishes 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 radishes at RHS H4
| Stage | When | Anchor |
|---|---|---|
| Direct sow outdoors | early April (4 April) | 21 days before last frost (late April) |
| First harvest (estimate) | early May (2 May) | ~28 days from sow |
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. Radishes prefer the cool damp weather UK springs and autumns deliver — they bolt and turn bitter once daytime temperatures climb above 24 °C, which makes the British shoulder seasons their happiest time.
Radishes are the fastest root crop — spring types mature in 22-30 days from direct sowing, making them ideal row-markers alongside slower crops. Sow 2-4 weeks before the last spring frost as soon as soil can be worked; they bolt and become pithy and peppery hot if left too long in warming soil. Succession-sow every 7-10 days for a continuous harvest; daikon and winter types sown in late summer take 50-70 days and tolerate heavier frost.
UK-specific tips for H4
- H4 is the typical southern English garden — radishes 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: 7-29 °C (45-85 °F).
- Spacing: 2-3 inches (5-8 cm).
- Days to harvest from direct sow: ~28 days.
- Sow in cool conditions — UK springs and autumns are ideal. Provide some afternoon shade if a heatwave arrives in July.
Common mistakes — H4 × radishes
- Sowing too late: radishes bolt in heat — UK Junes can spike to 28 °C, so the productive window is March-May and again August-October at rating H4.
- Ignoring slug pressure: damp UK springs are slug heaven. Protect rows with copper tape or wool pellets, or sow into modules and transplant when seedlings have toughened up.
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 radishes — 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 radishes in RHS H2
- When to plant radishes in RHS H3
- When to plant radishes in RHS H5
- When to plant radishes in RHS H6