How to Grow Onions
Allium cepa
When to Start
Direct sow outdoors as soon as soil reaches 6°C — typically early to mid spring. Can also be started indoors 9 weeks earlier for a head start.
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Growth Stages
Approximate days from start
Care Essentials
- Balanced fertiliser at planting. Avoid high-nitrogen feeds after mid-summer.
- Rotate — don't grow alliums in the same spot for 3 years. Sets are easier than seed for beginners.
Companion Planting
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Growing Conditions
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- Sow indoors
- —
- Transplant out
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- First harvest
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Spacing & Planting
| Plant spacing | 8 cm |
| Row spacing | 25 cm |
| Sow depth | 1 cm |
| Mature height | 40 cm |
| Mature spread | 10 cm |
For bulb onions grown from sets or transplants. Proper spacing is critical for good bulb size.
Care Guide
- Feeding
- Balanced fertiliser at planting. Avoid high-nitrogen feeds after mid-summer.
- Watch for
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- Onion fly
- White rot
- Downy mildew
- Neck rot
Pests & Diseases
Pest Onion Fly
Identification: Yellowing and wilting foliage, soft mushy bulbs when lifted, small white maggots inside the base of the bulb
- Cover with fine insect mesh from planting
- Firm soil around sets to prevent egg-laying at the base
- Crop rotation on a 3-year cycle
- Avoid thinning (the smell attracts flies) — sow at final spacing
- No effective chemical treatments available to home gardeners
Pest Allium Leaf Miner
Identification: Lines of white dots on leaves from adult feeding punctures, brown pupae inside bulbs, distorted and rotting plants
- Cover with insect mesh during the two flight periods
- Crop rotation
- Remove and destroy affected plants promptly
- No effective chemical treatments available to home gardeners
Pest Onion Thrips
Identification: Silvery-white streaks on leaves, tiny elongated insects visible on leaf surfaces, stunted growth
- Encourage natural predators
- Keep plants well-watered during dry spells
- Remove plant debris
- Pyrethrum-based spray in severe cases
Disease Onion White Rot Sclerotium cepivorum
Symptoms: Yellowing and wilting foliage, white fluffy fungal growth around the base of the bulb with tiny black sclerotia
Treatment: No cure. Remove and destroy affected plants immediately. Do not compost.
Prevention: Strict crop rotation (sclerotia survive 15+ years). Never move soil from infected areas. Buy clean sets.
Disease Onion Downy Mildew Peronospora destructor
Symptoms: Pale, greyish-purple fuzzy growth on leaves, leaves yellow and collapse from the tips, bulbs fail to develop properly
Treatment: Remove affected foliage. Improve air circulation. Harvest early if infection is severe.
Prevention: Space plants well for airflow. Avoid overhead watering. Rotate crops. Choose resistant varieties.
Disease Neck Rot Botrytis allii
Symptoms: Soft, brown rot developing at the neck of stored bulbs, grey mould visible, bulbs collapse in storage
Treatment: Discard affected bulbs immediately to prevent spread in storage.
Prevention: Cure bulbs thoroughly before storing. Store in cool, dry, well-ventilated conditions. Buy treated or certified sets.
Growing Conditions
| Min. soil temperature | 6°C |
| Germination temperature | 10–25°C |
| Base growth temperature | 6°C |
| Min. daylength to start | 9h |
| Photoperiod sensitivity | long day |
Frequently Asked Questions
- How far apart should I plant Onions?
- Plant Onions 8cm apart with 25cm between rows.
- When can I harvest Onions?
- Onions is ready to harvest approximately 110 days after transplanting.
- Is Onions frost hardy?
- Onions is classified as Hardy.
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