General
Onion (Allium cepa) is a biennial plant grown as an annual for its edible onion. Onion size is determined by the amount of leaf growth achieved before onion formation begins, which is controlled by day length rather than plant age. Early planting and uninterrupted growth are key to success. Varieties are adapted to specific day lengths (long-day, intermediate, short-day), making correct variety choice essential.
Light
Full sun is essential for onion development. Onions require long, bright days to trigger onion formation, and insufficient light results in weak growth and small onions. Choose the sunniest possible location, ideally with unobstructed light throughout the day.
Soil type
Loose, well-drained soil rich in organic matter is crucial. Onions form shallow roots and developing onions that struggle in compacted or heavy clay soils. Sandy or loamy soil amended with compost works best. Avoid waterlogged ground, which increases the risk of rot and disease.
Water requirement
Moderate and regular watering is needed, especially during early leaf growth and the start of onion formation. Inconsistent watering can lead to stress, bolting, or split onions. Reduce watering once onions are fully sized and the tops begin to fall to improve storage quality.
Nutrient requirement
Moderate feeder. Onions benefit from nitrogen early in the season to build strong leaf growth, as each leaf corresponds to one ring in the onion. Once onion formation begins, excess nitrogen should be avoided in favor of potassium to support firmness and storage.
Precultivation
Starting onions from seed indoors allows for a longer growing season and larger onions, especially in cooler climates. Sow seeds densely and grow seedlings cool and bright to prevent stretching. Trim leaf tips if necessary to keep plants sturdy. Transplant gently, as onions dislike root disturbance.
Planting
Plant onions outdoors as early as soil conditions allow; they tolerate light frost. Set transplants shallowly so the developing onion sits just at the soil surface. Adequate spacing ensures proper onion size and airflow, reducing disease risk.
Pruning
No pruning is required. Do not bend or break the tops intentionally, as this interrupts photosynthesis and reduces final onion size.
Maintenance
Keep beds weed-free, as onions compete poorly with weeds. Mulch lightly if needed, but avoid covering the onion. Water consistently during growth, then reduce as maturity approaches. Monitor for pests such as onion fly and thrips.
Harvest
Harvest when most tops have naturally fallen and begun to dry. Lift onions gently and cure them in a warm, dry, well-ventilated place for 2–3 weeks until necks are fully dry. Proper curing greatly improves storage life.
Common issues
Poor onion formation due to incorrect day-length variety, bolting from cold stress or interrupted growth, onion fly damage, thrips, and neck rot caused by late overwatering or poor curing.
Rotation schedule
Rotate onions and other alliums on a 3–4 year cycle to reduce pest buildup and soil-borne diseases.
Pollination
Insect-pollinated when allowed to flower in the second year. Pollination is not relevant for onion production, as onions are typically harvested in their first year.
Companion plants
Carrots, lettuce, beets, strawberries, chamomile, and other shallow-rooted or fast-growing crops. Onions can help deter some pests through their scent and fit well into mixed vegetable beds.
Incompatible plants
Peas and beans may perform poorly nearby, possibly due to root competition or differing nutrient needs. Avoid planting onions repeatedly in the same bed.
