Onions are the quiet workhorse of the vegetable garden: they store for months, they go into almost every meal, and a bed of them costs very little to grow. They are also one of the longest crops in the garden, taking most of the season from planting to harvest.
Onions can be grown from seed, sets or transplants, and sets — small, immature bulbs — are the easiest way to start.
Onions at a glance
| Growing factor | What onions need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
| Family | Amaryllidaceae |
Starting seeds
This crop can be started indoors or sown direct, and either route works well. Indoors, sow in trays, cells or small pots and expect germination in 7–14 days; direct-sown seed goes into a prepared bed once conditions are right. Starting indoors buys a few weeks of season and steadier germination, while direct sowing is simpler and avoids transplant shock.
Shorter-season gardens in the cooler zones gain the most from starting indoors, while warmer zones can usually direct-sow with confidence. Our seed-starting calculator works out indoor sowing dates from your frost dates. Zones 11–13 are frost-free, so sowing windows there are about heat and rain instead — check your local extension service.
Onions can be started indoors or sown direct, but most gardeners grow them from sets — small bulbs planted out in spring — which is simpler and more reliable than seed. If you sow seed, start it indoors for the longest season or sow direct into a fine seed bed, and thin seedlings to their proper spacing so each bulb has room to swell. Sets go in with the tip just showing above the soil.
Transplanting and planting out
Onion sets are planted out in spring once the soil can be worked, at their proper spacing in rows, with the tip of each set just above the surface. Keep the bed weed-free — onions have shallow roots and hate competition — and water during dry spells while the bulbs are swelling. Avoid earthing soil up over the bulbs, which need sun on their shoulders to ripen well.
Space plants 4–6 inches apart — crowding is one of the commonest causes of small harvests. Our plant-spacing calculator works out how many plants fit your bed.
Soil and feeding
Onions grow best in rich, well-drained soil with plenty of organic matter. Dig in compost before planting: it improves heavy soil, helps light soil hold moisture, and feeds the soil life that feeds your plants. Aim for a soil pH of 6.0–7.0 — a simple soil test will tell you where you stand, and garden lime or sulfur can nudge pH up or down over a season, so test before you amend rather than guessing.
Feed lightly and steadily rather than in one heavy dose: too much fast nitrogen late in the season pushes leafy growth at the expense of the part you eat. Our fertiliser calculator works out amounts for your bed size, and the soil-volume calculator tells you how much mix a new bed needs.
Watering
Water needs are moderate: keep the soil evenly moist, watering deeply when the top inch or two dries out. Deep, infrequent watering builds deeper roots than a daily sprinkle.
Water at the base of the plant, in the morning, so the leaves dry quickly and the water reaches the roots. Our watering calculator gives a starting point for your bed size, and the watering schedule generator can build a routine around it. A mulch layer cuts evaporation and keeps moisture even.
Sunlight and temperature
Onions need full sun — a spot that gets sun for most of the day. Our sunlight calculator can check your spot.
This crop is rated hardy for cold tolerance, so it stands up to frost and even light freezes, and it is at its best in the cool seasons. In Zones 11–13, which are frost-free, heat rather than cold is the limit — grow it in the cooler months, with afternoon shade in the hottest weather. Our hardiness-zone finder will confirm your zone, and the frost-date calculator gives you the frost dates to plan around.
Pests and diseases
The pests and diseases most often reported on onions are aphids, slugs, cutworms. Healthy, well-watered plants resist attack better than stressed ones, and rotating beds so the Amaryllidaceae family does not repeat in the same spot helps keep soil-borne problems down.
- Aphids — small sap-sucking insects on new growth; wash off with a strong jet of water and encourage ladybirds.
- Slugs — chew ragged holes in leaves at night; water in the morning so the surface dries by evening.
- Cutworms — caterpillars that chew through young stems at soil level; a cardboard collar around each stem protects it.
Inspect plants weekly, especially leaf undersides, and act early. Our pest-identifier helper can help identify what you are seeing.
Companion plants
Good companion plants for onions include:
- Marigolds — their roots discourage some soil pests
- Basil — attracts pollinators
- Onions — their scent is said to confuse pests
- Lettuce — shallow-rooted, fills gaps without competing
Our companion-planting finder can suggest more pairings for your bed plan.
Harvesting
Onions are ready when the tops flop over and start to yellow. Lift them on a dry day, leave them to dry in the sun for a week or so, then cure them in a warm, airy place until the necks are papery and tight. Only well-cured onions with dry necks store well; use any with thick, soft necks first. Braided or hung in a cool, dry place, cured onions keep for months.
Expect the crop to be ready about 90–120 days after sowing, though weather and variety shift that window by a week or two either way. Our harvest-date calculator works out your expected harvest date from your sowing date, and the vegetable-yield estimator can estimate how much a bed should produce.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Plants bolting to flower | a cold snap after planting, or stress | Bolting is triggered by cold weather after planting out, so avoid planting sets too early. |
| Small bulbs | crowding, weeds or dry soil while bulbs swell | Thin to the proper spacing, keep the bed weed-free and water during dry spells. |
| Thick necks that will not dry | too much nitrogen late in the season, or wet weather at harvest | Stop feeding once bulbs start to swell, and lift and cure onions promptly when the tops fall. |
Plants bolting to flower
Bolting is triggered by cold weather after planting out, so avoid planting sets too early.
Small bulbs
Thin to the proper spacing, keep the bed weed-free and water during dry spells.
Thick necks that will not dry
Stop feeding once bulbs start to swell, and lift and cure onions promptly when the tops fall.
Frequently asked questions
When should I plant onions?
Sow once the soil is workable in spring — this crop is rated hardy for cold tolerance, so it can go in early. For zone-specific timing, see zone 3 or zone 6, and check your local extension service for the dates that fit your area.
How long do onions take to grow?
Onions take 90–120 days from sowing to maturity, and seed germinates in 7–14 days. Weather and variety shift the harvest window by a week or two, so judge readiness by look and feel rather than the calendar alone.
How far apart should I plant onions?
Space plants 4–6 inches apart — that is the room a mature plant needs, and crowding means poor airflow and small harvests. Our plant-spacing calculator works out how many fit your bed.
Should I grow onions from seed or sets?
Sets are easier and more reliable: they are small bulbs planted in spring, they establish quickly and they are ready sooner. Seed gives you more varieties to choose from and usually stores better, but it needs a longer season and more care early on.