When to plant kohlrabi in RHS H4 (UK)

Key dates for kohlrabi at RHS H4

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. Kohlrabi 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.

Kohlrabi is a fast-maturing brassica that tolerates light frost down to about -4 °C but turns woody and pithy when daytime temperatures exceed 27 °C; transplant or direct-sow 3–4 weeks before the last spring frost for a summer harvest, or time a second sowing so plants mature in the cool days of early autumn. Harvest promptly when the swollen stem reaches 5–7 cm (2–3 inches) in diameter — larger bulbs become fibrous. Succession-sow every 3 weeks to extend the harvest window.

UK-specific tips for H4

• H4 is the typical southern English garden — kohlrabi 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: 6-9 inches (15-23 cm).
• Days to harvest from transplant: ~55 days.
• Sow in cool conditions — UK springs and autumns are ideal. Provide some afternoon shade if a heatwave arrives in July.

Common mistakes — H4 × kohlrabi

• Sowing too late: kohlrabi 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 kohlrabi — 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 kohlrabi in RHS H2
• When to plant kohlrabi in RHS H3
• When to plant kohlrabi in RHS H5
• When to plant kohlrabi in RHS H6

Other crops at H4

• When to plant tomatoes in H4
• When to plant peppers in H4
• When to plant basil in H4
• When to plant garlic in H4
• When to plant lettuce in H4
• When to plant bush beans in H4