When to plant onions

Most home gardeners grow onions from sets: small, part-grown bulbs sold in nets. Plant them from March to mid-April, once the soil has dried enough to work. That window holds for Germany, Austria and Switzerland as well as the UK.

Seed takes longer but gives you far more varieties to choose from. Sow outdoors in March or early April, or start seed indoors from January to March at around 59°F and plant the young onions out in April, six to eight weeks later.

For an early crop, plant winter-hardy sets from late September to mid-October, or sow winter onion seed in the second half of August. They root before winter and are ready from late May to July, a good month ahead of spring-planted onions. The trade-off: they keep only until early winter, while spring-planted onions last into the next spring.

Pick a sunny spot with loose, free-draining soil. Onions are light to moderate feeders: a bucket of compost per square metre dug in before planting is usually all they need, and they do well after a crop that was fed generously the year before.

Buy heat-treated sets if you can. The sets are kept above 77°F for three to four weeks before sale, which greatly reduces the risk of them running to flower later.

Push each set into the loosened soil so that only the tip shows. Space them 4 in apart in rows 10–12 in apart; closer gives smaller bulbs, wider gives bigger ones. A 120 × 80 cm raised bed takes three rows of twelve, 36 sets, or four rows and 48 sets if you close the rows up to 8 in.

Sow seed 0.4–0.8 in deep in rows the same distance apart. Thin the seedlings first to about 2 in, then to 4 in; the thinnings are good to eat as spring onions.

Onion leaves are thin and cast almost no shade, so weeds soon take over. Hoe or hand-weed regularly; in onion beds, weeding matters more than anything else you do.

Onions need little water. Water only in long dry spells, mostly in June and July, and stop once the bulbs have swollen in midsummer so they ripen and store well.

If a plant sends up a flower stem, snap it off as soon as you see it. The bulb stays edible but won't keep, so use those first. A cold snap in spring is the usual trigger.

Onions from sets are usually ready from early July, seed-sown ones from mid-August; in the UK the main harvest runs from July into September. The sign is the same for all of them: the leaves yellow, the neck softens and the tops topple over.

Lift them once a third to two thirds of the tops have fallen over, on a dry day. Loosen the bulbs with a fork rather than tugging on the leaves.

Leave the leaves on and let the bulbs dry for about two to three weeks, spread out in the sun or, better, under a roof where rain can't reach them. They are ready when the outer skins rustle and the necks are dry and tight.

Store onions cool, dry and airy: in nets, in shallow crates or plaited by their dry leaves. Spring-planted onions keep well into the next spring; winter onions are best eaten by early winter.

Sets are the quick, forgiving option. They get going fast, which makes weeding easier and gives an earlier crop, and they cope with a later start. On the downside they come in fewer varieties, are more likely to bolt, and can bring onion downy mildew into the garden with them.

Seed is cheaper and opens up the whole range of varieties, including old regional ones, but needs more time and care. Onion seed loses its ability to germinate quickly, often noticeably after a single year, so buy fresh seed each season.

Whichever you choose, start early. Onions switch from growing leaves to forming a bulb as the days lengthen towards midsummer, so the more leaf a plant has by then, the bigger the bulb.

Onion fly maggots tunnel into the bulbs; young plants yellow, wilt and can die. It is most damaging in June and July. Onions from sets usually outgrow the first generation better than seedlings do, and fine insect mesh gives extra protection.

Downy mildew shows as pale, oval patches on the leaves that later grow a grey-violet fuzz, above all in wet summers. The fungus survives in the soil and can also arrive on sets. Give the plants room so the leaves dry quickly, clear away diseased leaves at harvest, and don't grow onions on the same spot two years running.

White rot yellows the leaves and covers the base of the bulb with white fungus and tiny black resting bodies, which stay alive in the soil for at least 15 years. Never plant alliums where it has appeared.

Onions share the onion family with leeks, garlic, shallots and chives. German advisory services recommend leaving four to five years before any of them grows in the same bed again.

Onions and carrots are the classic mixed-planting pair, said to confuse each other's pests. The RHS notes that the onion scent has been claimed to put off carrot fly, and a mixed row does no harm, but don't rely on it as protection.

Mixed-planting tables also advise keeping onions away from beans and peas. We found no good evidence for that; it is tradition. The rule that does have a reason: keep onions away from other onion-family crops, because they share pests and diseases.