When to Plant Onion Sets: Autumn vs Spring

Onion sets — those little immature bulbs sold by the netful — are the easiest route to a proper onion harvest, and right now is one of the two moments in the year to plant them. Autumn-planted sets establish roots before winter, then sprint away in spring to give you bulbs weeks before your neighbours' spring-planted rows. Here is exactly when to plant, which varieties survive winter, and how to get them in the ground properly.

Sets vs seed: why sets win for most gardeners

An onion set is simply a small onion, grown from seed the previous season and halted early. Because it already has a season's head start, it is faster, more reliable in cold or heavy soils, and far less fiddly than raising onions from seed — the trade-off being a slightly higher tendency to bolt and a narrower choice of varieties. For a first or low-effort onion crop, sets are the answer.

Autumn planting: late September to late October

Overwintering sets should go in once the worst of the late-summer heat is over but early enough to root before real cold arrives — in most temperate gardens that means the last week of September through late October. Planted too early, the lush top growth gets battered by winter; too late, and the sets sit in cold, wet soil without rooting. Autumn sets bulb up as day length increases and are usually ready around June, up to six weeks ahead of spring-planted onions — perfect for filling the early-summer 'hungry gap'.

Only certain varieties are bred to overwinter. Reliable choices include 'Senshyu' (a Japanese overwintering onion, very cold-hardy), 'Radar' (mild, stores comparatively well, resists bolting), 'Shakespeare' (good keeper), and 'Electric' (a red for autumn planting).

Spring planting: March to April

Standard sets — 'Sturon', 'Stuttgarter Giant', 'Centurion', 'Red Baron' — go in from mid-March to mid-April, once the soil is workable and no longer freezing. They mature in August and generally store better than autumn-planted onions, which are best eaten within a couple of months of harvest. Many gardeners plant both: autumn sets for early bulbs, spring sets for the winter store.

How to plant onion sets, step by step

• Choose an open, sunny spot with free-draining soil — waterlogged winter soil is the number-one killer of autumn sets.
• Rake the bed to a fine tilth and firm it; dig in compost a few weeks ahead rather than fresh manure.
• Plant each set root-plate down, 10 cm apart, in rows about 30 cm apart.
• Push them in only far enough that the papery tip just shows at the surface — burying them deep rots them, and don't force sets into compacted ground; make a hole with a dibber instead.
• Water in once if the soil is dry, then let autumn rain do the rest.

Tip: Never plant onion sets where onions, garlic, leeks, or shallots grew in the last two years. Rotating beds is your best defence against white rot and downy mildew, which persist in soil for years.

Winter care

Autumn sets need almost nothing over winter: keep the bed weeded, firm back any sets lifted by frost, and replace the odd one that birds tug out (they love the papery tips — netting or fleece for the first fortnight helps until roots anchor them). Growth pauses in the coldest weeks and resumes in late winter. From early spring, weed regularly — onions hate competition — and water only in prolonged dry spells.

Bolting: the one thing to watch

A bolted onion throws up a flower stalk and stops fattening its bulb. Cold snaps after a warm spell, oversized sets, and too-early planting all trigger it. Buy heat-treated sets or bolt-resistant varieties like 'Radar', choose sets around marble size rather than the biggest in the net, and snap off any flower stalks the moment they appear — those bulbs won't store, so eat them first.

Wondering whether that yellowing leaf in the onion row is rot, rust, or just hungry soil? Snap it with PlantMe — the app identifies 500+ plant diseases from a photo and tells you whether it's treatable.