Growing Onions from Sowing to Harvest

Onions thrive when planted or sown in a free-draining soil, in a sunny spot. Be sure to clear the area of weeds to allow your onions to grow in the very best conditions. We also recommend using plant fertiliser on your soil with plenty of organic matter to benefit your onions’ root system!

Direct sowings can commence in spring as soon as the soil is workable and has warmed up a little. Rake the soil level then mark out seed drills about 1/2 inch (1cm) deep and 1ft (30cm) apart. Sow the seeds very thinly, cover back over then water along the rows to settle them in. Thin the seedlings in stages until they’re about 2in (5cm) apart for lots of smaller onions or 4in (10cm) apart for fewer but bigger bulbs.

Covering early sowings or transplants with row cover or garden fleece helps to speed things along at the start of the season, and may help reduce the tendency to bolt (or flower), which makes bulbs too tough to eat.

Some especially hardy varieties of onion may also be sown in late summer to sit through winter and give an extra early crop in spring or early summer.

How to Sow Onion Seeds

For the earliest start, sow onion seeds into plug trays or pots of potting mix to transplant later as seedlings. This avoids the need for thinning out, encourages a more economical use of seeds and, given the protection of a greenhouse or cold frame, means sowing can start at least a month sooner in late winter.

Fill trays with seed-starting or general-purpose potting mix, pressing it down into the cells for a solid fill. Sow a pinch of 4-8 seeds per cell, then cover with more potting mix to a depth of 1/4 - 1/2 an inch (1cm). Water with a fine spray.

Transplant the resulting seedlings while they’re still quite small to avoid disturbing the delicate roots. Make holes into the prepared ground, planting each clump of seedlings about 4in (10cm) apart before firming in and watering.

Wondering how to plant onion sets? In many regions, you may be able to buy onion transplants for immediate planting. Another alternative is to plant shallot onion sets. Sets are part-grown onions that are super-easy to grow and save time sowing. On the downside, they don’t store as well as onions grown from seed or transplants, and they carry a higher risk of bolting. There are, however, heat-treated varieties available that are more resistant to bolting. Nevertheless, sets are clear winners when it comes to convenience.

Plant sets in mid-spring into prepared, weed-free ground that's warmed up a little. Leave just the tips poking up from the ground and space them 2-4in (5-10cm) apart, depending on the final size of bulb you’re after. Some sets may also be planted in early autumn, to give a harvest up to two months earlier next summer.

Onions transplanted from module trays may be left as they are or thinned out once they’ve grown on to give bigger bulbs. You can enjoy the thinnings as green (or spring) onions.

As shallow-rooted plants, onions must be kept watered in dry weather. Keep on top of weeds too, hoeing carefully between rows, then hand-weeding within the rows so as not to damage the roots.

The best way to tell when harvest time is approaching is when the leaves of your onions are bent down towards the ground. Bulbs will continue to swell over the next few weeks before colouring up nicely in time for harvest.

What are you waiting for? Your growing journey starts right here at Mr Fothergill’s! Shop our onion seeds and onion sets today and let us help get your onion garden going.

If you have any further questions about how to grow onions, how to sow onion seeds or have any curiosities about any of our other products, don’t hesitate to contact us or take a deeper dive into our gardening blog!