When to plant onions in RHS H6 (UK)

Key dates for onions at RHS H6

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. Onions 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.

Onions are day-length sensitive: long-day varieties (zones 1–6) begin bulbing when days exceed 14 hours, short-day types (zones 7–10) bulb at 10–12 hours, and intermediate-day varieties span zones 5–6. Start seeds indoors 10–12 weeks before the last spring frost and transplant out 4–6 weeks before it — young onion seedlings tolerate frost down to about -6 °C once hardened off. In zones 8–10 a second planting from sets in autumn is common, overwintering for an early-summer harvest.

UK-specific tips for H6

• H6 gardens — Highland Scotland, Pennine uplands — have a short growing season, and onions 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: 10-35 °C (50-95 °F).
• Spacing: 4-6 inches (10-15 cm).
• Days to harvest from transplant: ~110 days.
• Sow in cool conditions — UK springs and autumns are ideal. Provide some afternoon shade if a heatwave arrives in July.

Common mistakes — H6 × onions

• Sowing too late: onions bolt in heat — UK Junes can spike to 28 °C, so the productive window is March-May and again August-October at rating H6.
• 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 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 onions — 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

Same crop, nearby ratings

• When to plant onions in RHS H4
• When to plant onions in RHS H5
• When to plant onions in RHS H7

Other crops at H6

• When to plant tomatoes in H6
• When to plant peppers in H6
• When to plant basil in H6
• When to plant garlic in H6
• When to plant lettuce in H6
• When to plant bush beans in H6