A Complete Guide to growing Onions in the UK

Getting Started: Seed vs Sets

Seeds: Sow indoors from mid-winter to early spring, about 1.3 cm (0.5 in) deep in peat-free seed compost using modular trays—heat and light help germination. Harden off before planting out.
Onion sets (starter bulbs): Quick and easy. Plant outdoors early to mid-spring - or autumn for overwintering varieties - for a head start. Just plant so the tip shows above the soil.

Sowing outdoors from seed: Early to mid-spring once soil warms; thin seedlings to 5 cm and later to 10 cm apart.
Sets: Plant early to mid-spring for a summer harvest. Overwintering sets go in autumn, ready for June-July harvests.
Spring onions: Sow from July, or late summer/early autumn for overwintering, to extend your season.

Location: Choose full sun and well-drained soil. Heavy or waterlogged soils can make autumn-planted sets rot.
Soil prep: Work in garden compost or organic matter beforehand. If soil is poor, use a nitrogen-rich organic fertiliser in late winter.

From seed (indoors): One seed per module (or a few for smaller bulbs), then plant 5–10 cm apart, rows 25–30 cm apart. Wider spacing (up to 15 cm) yields larger bulbs.
From seed (outdoors): Sow 1.3 cm deep, thin progressively to 5 cm then 10 cm.
From sets: Plant 2 cm deep, tip just showing, spaced 5-15 cm apart, rows 20-30 cm apart; adjust spacing for size.
Containers: Use a container at least 45 cm wide and deep; use peat-free soil-based compost.

Watering: Water during dry spells, especially early on. Stop once bulbs swell to improve storage life. Avoid wetting foliage to reduce fungal diseases.
Feeding: Add compost before planting. Use organic, nitrogen-rich fertiliser if needed.
Weeding: Keep onion beds weed-free - onion plants have little foliage to suppress competition. Remove weeds gently.
Remove flower stems: If flower stalks appear, cut them to encourage bulb development and improve storage.

Timing:
- Spring-planted sets or seed-raised: Harvest July-October.
- Autumn-planted sets: Harvest June-July.
Harvest cues: Bulbs are ready when foliage falls over; cure in dry weather, then store in a cool, well-ventilated place.

Common issues: onion fly, white rot, downy mildew, rust, neck rot, bolting. Use crop rotation and sanitation to reduce risks.
White rot persistence: Can survive in soil for decades - problematic on allotments.

Some gardeners enrich compost with blood, fish & bone or chicken pellets and use liquid feed during growth, especially in planters.
For late-season sowing, quick-growing spring onions are a popular fallback to get something in the ground.