Onion Growing Masterclass: My Guide to Bigger, Better Bulbs

11 March 2025, written by Benedict Vanheems

Onions are a must-grow staple of any vegetable garden – and believe me when I say you can’t match the taste of homegrown bulbs! – but growing good-sized onions requires knowing a few tricks of the trade, or you’ll end up with disappointing results.

I’ll be revealing what I’ve learnt over the years for bumper results, including whether you should start with seeds or sets, how to work out what type of onions will meet your needs best, and my secrets behind growing the biggest, best bulbs…

Starting Onions

There’s some debate on the best way to start onions: from seeds, sets or purpose-sold seedlings or transplants. I prefer seed because it’s cheaper and there’s less chance of the plants bolting, or flowering prematurely, which usually means a sub-standard bulb. The bulbs also tend to grow bigger from seed.

Sowing Onion Seeds

To sow seeds, fill a plug tray with screened, all-purpose potting mix. To hold the seeds make little depressions in the surface of the potting mix with your fingertips.

The easiest way to get onion seeds started is to multisow them, which just means sowing a little pinch of seeds into each plug – around five to eight – which can then be grown on together, all the way from seedlings to mature plants. Sow little pinches into each of the depressions, then lightly cover them with a little more potting mix.

The earlier you can sow onions, the better. Late winter is ideal, but you can sow up until early spring. I germinate mine indoors, at room temperature, then move them out into the greenhouse once they’ve come up. Of course, if it’s warm enough where you are you could just keep them in a greenhouse or cold frame or other sheltered spot from the off.

Once your seedlings are a few inches (6-7cm) tall, it’s time to get them planted.

Planting Onion Sets

But if you want to save a bit of effort and get faster results, sets could be the answer. Sets are immature onions dug up last season then kept in cold storage ready for planting in spring to pick up where they left off. However, there is one downside: onions are biennials, so they naturally want to bolt, or flower, in their second year to complete their life cycle. This makes sets more prone to bolting because they’re technically about to embark on that second year’s growth.

But most sets won’t bolt, and there are a few things we can do to reduce the risk of that happening. First, choose special heat-treated sets, and don’t plant them too early while it’s still very cold.

Spring is the perfect time to get them planted, but exactly when will depend on your climate. In my mild, temperate climate (Zone 8), I typically aim to get them in around the spring equinox.

Planting onions in plugs — Plant sets into plugs if your soil is still too cold and wet at planting time

Onions prefer a loose, well-drained soil. I prepare mine ahead of planting by adding a layer of either compost or well-rotted manure about an inch (3cm) deep. Onions like soil that’s fairly rich in nitrogen for plenty of leafy growth that will in turn put plants in the best position to swell bigger, better bulbs. Adding a sprinkling of a nitrogen-rich organic fertilizer like chicken manure pellets at this stage can really help things along.

To plant sets, simply nuzzle them down into the soil so that just the very tips stand proud of the soil surface. You can see which end is up by the pointy tips of the old foliage; the flatter side is where the roots will spring from. Space the sets about 6in (15cm) apart in both directions, or if you want the very biggest bulbs, then go a little wider apart still – up to 10in (25cm) between sets.

If your soil’s still cold and wet at planting time, you can plant sets into plug trays. Simply twist one set into each plug of potting mix. These will quickly root and can be grown on under cover until conditions improve outside, by which time they should have produced a reasonable amount of top growth and be champing at the bit to race away!

Types of Onion

The best bit about growing your own onions is, of course, the fantastic choice of varieties you can grow. From sweet, white onions to red onions to punchy yellow bulbs. With so much choice, deciding what to grow is a bit of a head scratcher, but you could do worse than pick one of these award-winning onions:

ALL-AMERICA SELECTIONS WINNERS

Sierra Blanca F1
Super Star F1

RHS AWARD OF GARDEN MERIT WINNERS

Autumn Gold Improved
Centurion
Fasto
Hercules
Hybound
Hylander
Jetset
Red Baron
Redspark
Rumba
Setton
Sturon
Stuttgarter

Large red onions — If big bulbs are your goal, choose a reliable variety and space them widely

Another thing to consider is your latitude – how close or far from the equator you are – which will determine whether you’re best growing a short, intermediate or long day type of onion. Short day onions are suitable for lower latitudes with shorter summer days. These onions will start to form their bulbs once daylength reaches around 10 to 12 hours. Long day onions on the other hand won’t start bulbing up until days reach 14 to 16 hours, so matching variety to daylength is important if you want to guarantee decent bulbs.

ONION TYPE	LATITUDE	BULBING STARTS AT	NOTES
Short Day	25-35º	10-12 hours daylength	Plant early for bigger bulbs
Intermediate Day	32-42º	12-14 hours daylength	Sweetest tasting
Long Day	37º+	14-16 hours daylength	Best for storing

I live at a latitude of 52º, which means that I have enough daylight to plant long day onions, which typically start to bulb up on or around the summer solstice. Long day onions also tend to include varieties that store best – but each type has its advantages.

Planting Onion Seedlings

Onion seedlings can be planted at any point from this size up so long as the soil is workable and not overly wet. I’ve reserved a beautifully bright spot for my onions, because more sun means bigger bulbs.

Multisown seedlings need no thinning, so they can be planted exactly as they are. Each cluster of seedlings needs to be planted a little further apart than single plants to give them enough room to swell and grow. Plant them in a block about 10in (25cm) apart in both directions. Prepared the soil in the same way as for sets, with plenty of compost and a scattering of chicken manure pellets.

If you’re buying ready-to-plant transplants or starts, then they will arrive as individual seedlings, so these will need to be planted at the same spacings as sets, so at least 6in (15cm) apart, or use wider spacings to grow larger bulbs.

With your seedlings planted, give them a good drink. If Winter is still dragging on where you are, it’s a good idea to cover these early plantings with row cover fabric to help them transition, at least for the first couple of weeks, until it’s a bit warmer.

Watering onions — Keeping onions well-watered will help swell bigger bulbs

Watering and Feeding Onions

With your onions planted, it’s now a waiting game. Plants will put on plenty of leafy growth but won’t start swelling those bulbs until days are long enough.

Because onions have thin, strappy leaves they’re easily overpowered by weeds, so it’s essential to keep the soil around plants well-weeded. Another advantage of keeping the ground weed-free is improved airflow between plants. This, together with proper spacing – leaving enough space between plants or clusters or plants – should help to reduce the risk of disease. These are shallow-rooted plants, so be careful if hoeing or (perhaps safer) get in among plants and hand-weed.

Onions love to sunbathe but they also need moist soil, especially when the time comes to swell those bulbs, so water regularly in dry weather. Plentiful moisture means bigger bulbs, it’s as simple as that!

Onions mulched with grass clippings — Mulch the soil to help retain moisture

If you have it, drip irrigation can help to keep the soil moist while the leaves remain dry, which helps to minimize potential disease problems. I like to add a layer of mulch over the soil surface to help shade the soil and slow evaporation, which also reduces how often I have to water. Thin sprinklings of grass clippings are perfect for this.

If your soil isn’t especially rich there’s no harm in adding a relatively fast-acting nitrogen-rich organic fertilizer a month or two after planting – something like bloodmeal would be perfect for this, sprinkled around your onions then tickled into the soil with a hand fork.

When to Harvest Onions

With your seedlings or sets planted at the right time, kept watered in dry weather, and given the nutrition they need, you’re 99% of the way to a cracking crop of delicious bulbs – nice one!

If some plants do start to bolt, they won’t store well, and the bulbs are unlikely to swell further. Lift and use them as soon as possible. In fact, you can start enjoying any of your onions as soon as they reach a useable size.

But if you want to store onions for several months, wait til the necks of the onions go soft. This will cause the leaves to become top-heavy and flop over. Once half to two-thirds of them have flopped over, it’s time to dig them up. For me, this usually happens about a month after the summer solstice.

Ben Vanheems with a net sack of onions — Hang up cured onions in net sacks for plenty of airflow

Lift your onions on a warm, sunny day then move them to somewhere airy, but protected from the rain – racks are ideal. Allow the the bulbs to dry out for two to three weeks. This ‘curing’ process will create a papery outer skin that gives a tight seal, protecting the inner layers of the bulb so they keep for longer.

I love having an onion haul to dip into, and with that in mind aim to grow at least one variety renowned for storing well – well into the following spring.

BEST ONIONS FOR STORING