When to plant onions: seed, sets and climate

For a UK garden, the main choice is direct sowing in early to mid-spring or planting onion sets in spring. Seeds need a fine row and careful thinning; sets are small bulbs placed one by one. The RHS onion guide gives separate methods for each, and explains that lengthening days trigger bulb formation, so seedlings need to be growing strongly by late spring.

Decide between seed, sets and young plants

Direct sowing suits a gardener who wants a full row and can keep the bed weed-free while the seedlings establish. Sow outdoors from early to mid-spring, once the soil is starting to warm. The RHS also suggests sowing indoors from mid-winter to early spring in modules, then hardening off the plants before setting them outside.

Sets offer a simpler planting job. They are generally planted in early to mid-spring, with overwintering Japanese sets available for autumn planting. Rooted young plants are another option for a small number of onions. Check the packet: variety and intended season matter, particularly for sets bought for winter growing.

Sow seed while the soil is warming

Choose a sunny, free-draining site and weed it thoroughly. Loosen the surface so the seed has fine soil contact. Sow 1.3 cm deep in rows 20–30 cm apart. When seedlings appear, thin first to 5 cm, then later to 10 cm; closer spacing gives more bulbs but they will be smaller. Keep the seedbed lightly moist through emergence, without leaving it waterlogged.

For module sowing, the RHS gives the same depth, around 1.3 cm. Sow one seed per module, or three or four together if you want a cluster of smaller onions. Harden off indoor-grown plants in spring. The RHS says not to split a multi-seeded module into individual plants; plant the clump together, spacing clumps about 10 cm apart in rows 25–30 cm apart.

Plant sets for summer or an earlier crop

RHS guidance places ordinary onion sets in early to mid-spring, for harvest from July to September. Plant them about 2 cm deep with the pointed tip just above the soil. Leave 10–15 cm between sets and 20–30 cm between rows; wider spacing gives bulbs more room. Firm the soil and water in, then check that birds have not pecked sets out before they root.

Overwintering Japanese sets are a separate choice. The RHS says these can be planted in autumn for an earlier harvest, and they are less sensitive to cold than ordinary sets. Use only a variety sold for overwintering. An autumn date on a general onion calendar is not enough reason to plant a spring type early.

The French SNHF onion page gives broad spring planting for summer harvest and autumn planting for spring harvest, but does not set out a separate calendar for direct sowing, nursery sowing or each set type. Its plant and bulb spacing is separate from the RHS seed-row and set measurements above.

Keep notes on the method as well as the date

Onions have a limited root system, so prepare the bed with well-rotted compost and avoid fresh manure, as the RHS advises. Include onions in your rotation, too; the SNHF's three-to-five-year advice specifically names garlic, shallots and leeks, rather than giving a separate interval for onions. Maintain a weed-free row; young onions do not shade the ground quickly. Note the variety, whether you used seed or sets, spacing and date. Those details make it easier to compare the result next season than a date alone.

The vegetable sowing and planting calendar gives a wider seasonal view, and what to plant in October covers autumn jobs. For another crop grown from bulbs, see planting garlic in autumn.

References: RHS, Onions; SNHF, Oignon (in French).