Onion
Allium cepa
About Onion
Onion is the second most important vegetable crop globally after tomato, grown in virtually every country. It is essential to cuisines worldwide and is both consumed fresh and processed (dehydrated, pickled, frozen). India and China are the largest producers. Onion is one of the most profitable vegetable crops for smallholder farmers in Africa, providing high returns per unit area. The crop requires specific day-length conditions for bulbing — short-day varieties for tropical regions, long-day varieties for temperate regions. Storage onions can be kept for 3-6 months, providing year-round supply from seasonal harvests.
Growing Conditions
Full sun. Day length critical for bulb initiation — short-day varieties bulb at 11-12 hours, long-day at 14-16 hours.
350-550 mm per crop. Consistent moisture during bulb expansion. Reduce irrigation 2-3 weeks before harvest for curing.
Well-drained, fertile loam to silt loam. pH 6.0-7.0. Avoid heavy clay (causes poor bulb shape).
Optimal 15-25°C for growth. Bulb initiation requires specific temperature × day-length interaction. Bolting (premature flowering) triggered by cold exposure.
Low to moderate. High humidity promotes purple blotch, downy mildew, and storage rots.
Cool season in tropics — 90-150 days. Sow seed or transplant during cool dry season.
Step-by-Step Growing Tips
Select varieties appropriate for your latitude (short-day, intermediate, or long-day)
Transplant seedlings at 6-8 weeks old when pencil-thick
Space at 15 × 10 cm on flat beds or 4 rows per 1 m raised bed
Control thrips aggressively — they vector diseases and reduce bulb size
Stop irrigation when 50% of tops have fallen over (maturity indicator)
Cure bulbs in shade with good airflow for 2-3 weeks before storage
Disease Risks to Watch
All diseases →Onion purple blotch (Alternaria porri) causes purplish-brown zonate lesions on leaves that girdle and kill foliage, reducing bulb yield 40-60%. Thrips damage worsens the disease. Manage with mancozeb + chlorothalonil sprays every 7-10 days, thrips control, proper spacing for air flow, and avoiding overhead irrigation during bulb development.
Downy mildew produces angular yellow patches on upper leaf surfaces with grayish-purple fuzzy growth underneath. Caused by Peronospora or Plasmopara oomycetes, it thrives in cool wet weather. Manage with resistant varieties, preventive mancozeb or copper sprays, good air circulation, and avoiding overhead irrigation. It requires free water to infect — keep leaves dry.
Companion Planting
✅ Good Companions
❌ Avoid Nearby
Frequently Asked Questions
When is the best time to plant Onion?
Cool season in tropics — 90-150 days. Sow seed or transplant during cool dry season.
What soil does Onion need?
Well-drained, fertile loam to silt loam. pH 6.0-7.0. Avoid heavy clay (causes poor bulb shape).
How much water does Onion need?
350-550 mm per crop. Consistent moisture during bulb expansion. Reduce irrigation 2-3 weeks before harvest for curing.
What are the most common diseases affecting Onion?
The most common diseases are: onion purple blotch, downy mildew. Monitor regularly and practice crop rotation.
What plants grow well with Onion?
Good companions include: carrot, lettuce, tomato. Avoid planting near bean, pea.
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