When to plant radishes in RHS H7 (UK)

Key dates for radishes at RHS H7

Dates are typical for the regions H7 describes (High Scottish mountains, exposed glens, alpine gardens above 500 m. A tiny fraction of UK gardens experience true H7 conditions.). 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 H7

H7 describes UK gardens with winter minima of minimum below -20 °C. Last spring frost typically passes mid-June; first autumn frost arrives early September, giving about 90 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 H7

• H7 territory — high Scottish mountains, Cairngorm fringe, Caithness — gives a 90-day frost-free window. radishes are realistic only with full season-extension kit: polytunnel, fleece, and short-season cultivars.
• Wind chill is the dominant stress — even a healthy plant fails if it sits in a constant 30 mph gale. Walled or netted shelters change what is possible more than any temperature change.
• Soil warmth lags air temperature by 3-4 weeks at H7 elevations — black plastic mulch through April is the difference between a crop and a failure.
• Watch the autumn equinox carefully — once day length drops below 12 hours, most warm-season crops stop ripening regardless of temperature. Pick green and ripen indoors.

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 — H7 × 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 H7.
• 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 H7describes. 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 H7 is roughly equivalent to USDA zone 5. Curated by the Growli editorial team.

Keep going

• How to grow radishes — full guide
• RHS H7 — 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 H5
• When to plant radishes in RHS H6

Other crops at H7

• When to plant tomatoes in H7
• When to plant basil in H7
• When to plant garlic in H7
• When to plant lettuce in H7
• When to plant bush beans in H7
• When to plant cucumbers in H7