Quick answer
Grow onions in full sun and well-drained soil from sets, seed or transplants. Keep their shallow root zone weed-free and evenly moist while leaves and bulbs are expanding, then stop routine watering as tops fall and bulbs mature. Lift in dry weather, cure until the neck and outer skins are completely dry, and store only sound storage varieties.
Variety selection depends on day length. Long-day, intermediate-day and short-day onions begin bulbing under different seasonal light periods. A type poorly matched to latitude may grow healthy leaves but form a disappointing bulb.
Sets, seeds and day length
Sets are small immature bulbs. They are simple to handle and shorten the time to harvest, although large sets can be more prone to bolting. Seed offers more cultivars and can produce well-shaped storage bulbs, but it needs a longer growing period. Transplants provide a middle route when indoor seed-starting space is limited.
The RHS onion guide describes both sets and seeds. Buy named cultivars and check whether they suit spring or autumn planting. Heat-treated sets are sold to reduce bolting risk in some conditions.
The University of Minnesota Extension explains that bulbing responds to day length. Long-day types are adapted to higher latitudes; short-day onions are used farther south. Local suppliers and extension services are the safest guide to the correct class.
Site and soil
Use an open position receiving at least six hours of direct sun. Onions need free drainage but also a soil structure that holds steady moisture near the surface. A pH around 6.0–7.0 is a practical target, confirmed by testing before amendment.
Incorporate mature compost before planting where organic matter is low. Avoid fresh manure and overfeeding. Too much late nitrogen keeps thick necks growing, delays maturity and can shorten storage life.
Onion roots compete poorly with weeds. Prepare a clean bed and weed shallowly so a hoe does not cut roots or bulbs. Mulch can suppress weeds, but a thick wet layer against bulb necks may hold damaging moisture late in the season.
Planting sets and sowing seed
Plant spring sets once the soil is workable. Place them about 10–15cm apart with the tip at or just above the surface, firming soil so birds cannot easily pull them out. Autumn sets require a suitable hardy cultivar and free-draining ground.
Direct-sow seed shallowly in early spring and thin to the final spacing. For larger bulbs, start seed under cover well ahead of transplanting. Harden seedlings and plant them without bending the roots sharply. Do not bury the neck deeply.
Closer spacing produces more, smaller bulbs; wider spacing allows fewer, larger ones. For containers, use a vessel at least 30cm wide and 15 litres in volume, with enough depth to maintain an even root zone. Space onions rather than filling every visible gap.
Watering and feeding
Onions are shallow rooted, so check moisture in the upper soil rather than assuming deeper dampness is available. Water thoroughly during dry weather while leaves and bulbs expand. Uneven moisture can reduce bulb size; constant saturation encourages root and neck diseases.
Stop routine irrigation when bulbs reach full size and tops begin falling naturally. This shift helps outer scales dry. Do not manually bend healthy green tops to force maturity: damaged neck tissue can become an entry point for decay.
Use soil-test recommendations for fertiliser. If nitrogen is needed, apply it early enough to support leaf growth, because each healthy leaf contributes to a ring in the bulb. Avoid concentrated late feeding.
Bolting, bulbing and crop care
Bolting is premature flower-stalk production, often triggered when a young plant experiences cold followed by longer days. Remove and use bolting onions promptly; the flower stem creates a hard centre and the bulb generally stores poorly.
Bulbs naturally rise partly above the soil as they expand. Do not earth them up like potatoes. Maintain airflow and avoid injuring the exposed scales while weeding.
Yellowing tips alone do not identify a disease. Examine whether symptoms begin on old or young leaves, whether there are defined lesions, and whether the bulb base is firm. Correct diagnosis matters because nutrient stress, onion fly, downy mildew and several rots require different responses.
Diagnosing common problems
| Symptom | Likely causes to investigate | First checks |
|---|---|---|
| Flower stalk forms early | Bolting after cold stress or unsuitable set size | Planting history, cultivar and temperature sequence |
| Bulbs remain small | Wrong day-length type, crowding, shade or drought | Cultivar class, spacing, sun and moisture |
| Leaves yellow and wilt suddenly | Root damage, onion fly or basal rot | Root plate, larvae and distribution in the row |
| Grey mould near neck in storage | Neck rot or incomplete curing | Neck thickness, harvest damage and storage airflow |
| Soft, wet bulb scales | Bacterial or fungal rot | Injury, smell, moisture and spread |
| Leaves show pale elongated patches | Downy mildew or weather injury | Leaf surface, humidity and progression |
Remove rotting bulbs promptly and do not store damaged onions beside sound ones. If a soil-borne disease is confirmed, lengthen rotation and avoid moving contaminated soil to clean beds.
Harvest, curing and storage
Harvest when roughly half the tops have fallen and are drying. Loosen bulbs with a fork rather than pulling hard enough to tear the neck. If weather is reliably dry, let them dry briefly on the surface; otherwise move them under cover with strong airflow.
Curing is complete when outer skins are papery and the neck is tight and dry. Trim tops only after curing, or braid suitable varieties. Store in shallow trays, nets or slatted containers in a cool, dry, ventilated place. Do not seal onions in plastic.
Sweet onions and bolted or damaged bulbs have shorter storage lives. Use those first. Check stored bulbs and remove any that soften, sprout or smell abnormal.
Sources and review basis
- How to grow onions — Royal Horticultural Society
- Growing onions in home gardens — University of Minnesota Extension
Day-length adaptation is regional. Use a locally recommended cultivar and treat general sowing dates as conditions-based starting points.