How to Grow Onions: From Sets and Seed to Harvest

Onions are one of the most rewarding crops you can grow at home. They take up little space, store for months, and turn up in almost every savoury recipe you'll ever cook. Whether you're working with a backyard bed, a raised planter, or a few large containers on a balcony, onions are forgiving enough for first-time growers and satisfying enough to keep experienced gardeners coming back every season.

Sets vs. Seed: Which to Choose

There are two common ways to start onions, and the right choice depends on your patience and your climate.

Onion sets are small, immature bulbs that you plant directly into the soil. They're the easiest route for beginners: they establish quickly, tolerate cool soil, and are less fussy about timing. The trade-off is a smaller selection of varieties and a slightly higher chance of bolting (flowering early) if the weather swings.

Onion seed gives you a far wider range of varieties and, many growers argue, better long-term storage. The catch is time — onions from seed need to be started indoors 8–10 weeks before your last frost date, then hardened off and transplanted. If you want big, storable bulbs and don't mind the head start, seed is worth it.

A third option, transplants (young seedlings sold in bunches), splits the difference: more variety than sets, less waiting than seed.

Understanding Day-Length: The Step Most Beginners Miss

This is the single most important thing to get right, and it's where a lot of disappointing harvests come from. Onions bulb up in response to day length, and varieties are grouped accordingly:

Long-day onions need 14–16 hours of daylight to bulb. Best for northern/higher-latitude regions (roughly the northern US, Canada, northern Europe, USDA zones 6 and colder).

Short-day onions bulb at 10–12 hours of daylight. Best for southern/lower-latitude regions (southern US, USDA zones 7 and warmer, and similar climates worldwide).

Day-neutral (intermediate) onions bulb across a wide range and are a safe choice almost anywhere.

If you plant a long-day variety in a short-day climate, the plant will never get the day length it needs and you'll harvest tiny bulbs. When buying, check the label and match it to your region. If you're unsure, day-neutral varieties are the reliable middle ground.

Soil and Site

Onions want full sun — at least 6 hours of direct light a day — and loose, fertile, well-drained soil. Heavy or compacted ground produces small, misshapen bulbs. Before planting, work in 5–8 cm (2–3 in) of compost and aim for a soil pH of 6.0–7.0.

Onions are shallow-rooted and dislike competition, so a weed-free bed is non-negotiable. Raised beds and large containers (at least 25 cm / 10 in deep) work very well because they give you the loose soil onions crave.

Planting

Plant sets or transplants 10–15 cm (4–6 in) apart in rows 30 cm (12 in) apart. Push sets in just deep enough that the tip pokes above the surface — about 2.5 cm (1 in) deep. Plant too deep and the bulb struggles to swell.

Timing depends on your hemisphere and climate, not the calendar. As a rule, plant in early spring once the soil is workable (around 4–6 weeks before your last expected frost), or in autumn for overwintering varieties in mild-winter regions. Always check your local last-frost date rather than copying a fixed month — a grower in a cool maritime climate and one in a warm inland one may plant the same variety weeks apart.

Watering and Feeding

Onions need consistent moisture, especially while bulbs are forming. Aim for about 2.5 cm (1 in) of water per week, more in hot, dry spells. Irregular watering causes split or double bulbs.

Feed every 2–3 weeks with a balanced or slightly nitrogen-rich fertiliser through the leafy growth stage, then stop feeding nitrogen once bulbs begin to swell — late nitrogen pushes leafy growth at the expense of the bulb and hurts storage life.

Mulching lightly with straw or compost conserves moisture and keeps weeds down, but keep mulch away from the bulb's neck to prevent rot.

Common Problems

Bolting (flowering): Usually triggered by a cold snap after planting or stressed sets. Snap off flower stalks early and use those bulbs first — they won't store well.

Onion fly/maggots: Floating row cover at planting is the best defence.

Thrips and downy mildew: Improve airflow with proper spacing and avoid overhead watering late in the day.

Small bulbs: Almost always day-length mismatch, overcrowding, or weed competition.

Harvesting and Curing

Onions are ready when the green tops naturally yellow, soften at the neck, and flop over — typically 90–120 days after planting. Stop watering about a week before harvest. Lift the bulbs gently on a dry day.

To store onions for months, you must cure them: lay them in a single layer somewhere warm, dry, and airy, out of direct sun, for 2–3 weeks until the necks are completely dry and the skins are papery. Then trim the tops to about 2.5 cm (1 in), brush off loose soil (don't wash), and store in a cool, dark, well-ventilated spot. Properly cured storage onions can last 4–8 months.

Grow More Than You Need — and Share the Rest

A single bed of onions often yields far more than one household can use before they sprout. That's exactly the kind of surplus Locavori is built for: a swelling harvest is a chance to trade with a neighbour for the courgettes or tomatoes you didn't grow, and to keep good food out of the bin.

Onions are a confidence-building crop. Get the day-length match right, give them sun and steady water, and cure them properly — and you'll have a pantry staple you grew yourself.

Ready to grow, swap, and share your harvest with your neighbours? Join Locavori free →