Sowing to Harvest: Growing Onions

Reader Contribution by Benedict Vanheems
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Photo by Getty Images/rootstocks

Growing Onions from Sowing to Harvest

Yellow, red, and white varieties of bulb onions are available to the home grower.You can find a list of varieties by double-clicking on the Onion icon in our Garden Planner. Add some onions to your plan then view the Plant List to check recommended sowing, planting and harvesting dates for your garden.

Planting Onion Seeds

Sow onion seeds into plug trays or pots in a greenhouse or cold frame for the earliest start, at least a month sooner than outdoors sowings. Sow four to eight seeds per plug tray cell.

Onions prefer a sunny, open spot in rich, well-drained soil. Grow them in raised beds or on mounds if your soil is heavy and tends to remain wet.

Transplant the clumps of seedlings while still quite small so as not to disturb the roots. Planted each clump about 4in apart.

Sow seeds direct into the soil once the soil has warmed up in spring. Mark out seed drills about half an inch deep and a foot apart. Plant the seeds thinly, cover back over then water. Thin the seedlings in stages until they’re about 2in apart for smaller bulbs, or 4in apart for fewer but bigger onions. You can enjoy the thinnings as green onions. Cover early sowings or transplants with row covers.

Some hardy varieties of onion can also be sown in late summer for an extra early crop in spring or early summer.

Planting Onion Sets

In many areas you can buy onion transplants for immediate planting. Alternatively, plant sets: part-grown onions which save time sowing. Unfortunately they don’t store as well as onions grown from seed or transplants, and they are more likely to bolt, (there are some heat-treated varieties available that are more resistant to bolting), but they are very convenient.

Plant sets 2-4 in apart in mid spring once the soil has warmed up a little, leaving the tips poking out of the soil. Some varieties can be planted in early fall for harvestable bulbs up to two months earlier next summer.

Onions must be kept well-watered in dry weather. Keep weeds in check by hoeing between the rows and hand weeding within the rows.

Harvesting and Storing Onions 

Harvest time is not far away when most of the leaves have bent down towards the ground. Allow bulbs to swell and color up for a few weeks before harvesting.When they’re ready, lever them out of the ground using a fork or trowel.  Onions for storing need a period of curing somewhere dry and well-ventilated first – an airy shed or a greenhouse is fine. In warm, dry climates you can simply leave the onions where they are on the soil surface. Space bulbs out for good airflow between them – you may like to use a rack for this. After about two weeks the skins should have toughened up, and they can then be stored in nets, tied into bundles or woven into onion strings. Onions will store until at least midwinter, and as long as spring.

Get More Tips with These Great Gardening Resources

Our popular Vegetable Garden Planner can help you map out your garden design, space crops, know when to plant which crops in your exact location, and much more.

Need crop-specific growing information? Browse our Crops at a Glance Guide for advice on planting and caring for dozens of garden crops.

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