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Onion

Allium cepa

VegetablesFrost HardyToxic

Essential kitchen garden staple grown from seed, sets, or transplants. Their strong scent helps repel many common pests.

Overview

SunFull Sun
WaterLowCalculate watering amount
Height30–50 cm
Spread10 cm
Spacing10 cm
Row Spacing25 cm
Germination7–14 days
To Harvest90–120 days

Companion Planting

Toxicity

SeverityModerately toxic
Toxic toDogs, Cats, plantDetail.toxicTarget.horses

Contains N-propyl disulfide and thiosulfates which cause oxidative damage to red blood cells (Heinz body anemia). Cats are especially sensitive. All parts of the plant are toxic to pets whether raw, cooked, or dried. Safe for humans in normal culinary amounts.

Planting Tips

LocationFull sun (at least 6 hours of direct sunlight per day). Sheltered from strong winds.
SoilWell-drained, fertile, loamy soil with a pH of 6.0–7.0. Enrich with well-rotted compost before planting. Avoid heavy clay or waterlogged soils.
Sowing Depth1 cm
Planting Depth3 cm
WateringWater regularly with about 2.5 cm per week during the growing season. Keep soil evenly moist but not waterlogged. Stop watering once the tops begin to fall over to allow bulbs to cure.

1 cm of water per m² equals 10 liters. Use the Watering Calculator for exact amounts.

FertilizingApply a balanced fertilizer at planting and side-dress with a nitrogen-rich fertilizer every 3–4 weeks during the growing phase. Stop fertilizing once bulbs begin to swell.

Care Tip: Keep the bed weed-free as onions compete poorly with weeds. Do not hill soil around bulbs — they should sit on the surface as they mature. Bend tops over when they start yellowing to speed ripening, then harvest and cure in a dry, airy place for 2–3 weeks.

Common Pests

Onion Thrips (Thrips tabaci)

Symptoms: Silvery-white streaks or patches on leaves, distorted leaf growth, and stunted plants. Heavy infestations cause leaves to wither and turn brown.

Treatment: Spray with insecticidal soap or neem oil. In severe cases, use spinosad-based insecticides. Remove and destroy heavily infested plants.

Prevention: Use reflective mulch to deter thrips. Rotate crops annually. Avoid planting near grain fields. Encourage natural predators such as ladybugs and lacewings.

Onion Fly / Onion Maggot (Delia antiqua)

Symptoms: Wilting and yellowing of young plants. Larvae tunnel into the base of the bulb causing rot and a mushy texture. Seedlings may collapse and die.

Treatment: Remove and destroy affected plants immediately. There is no effective chemical treatment once larvae are inside the bulb.

Prevention: Cover beds with fine insect-proof mesh or fleece from sowing onward. Practice crop rotation (3–4 year cycle). Avoid thinning onions as the scent attracts egg-laying flies. Firm soil around plant bases.

Allium Leaf Miner (Phytomyza gymnostoma)

Symptoms: Rows of small white puncture marks on leaves. Larvae mine inside leaves and move into the bulb, causing brown streaks and secondary bacterial or fungal rot.

Treatment: Remove and destroy infested plants. No approved pesticide for home gardens is reliably effective once larvae are inside the tissue.

Prevention: Cover crops with insect-proof netting during the two flight periods (March–April and October–November). Rotate allium planting sites. Inspect transplants before planting.

Common Diseases

Downy Mildew (Peronospora destructor)

Symptoms: Pale green to yellowish patches on leaves that become covered with a grey-violet fuzzy mold in humid conditions. Leaves collapse from the tip downward. Bulbs soften in storage.

Treatment: Remove and destroy infected foliage immediately. Apply copper-based fungicides preventatively in wet weather. Improve air circulation around plants.

Prevention: Ensure wide spacing between plants for good airflow. Avoid overhead watering. Practice a 3–4 year crop rotation. Choose resistant varieties where available.

White Rot (Stromatinia cepivora)

Symptoms: Yellowing and wilting of leaves starting from the tips. A white fluffy fungal growth appears at the base of the bulb with small black sclerotia (hard round bodies). Roots rot away and plants can be pulled out easily.

Treatment: No effective chemical cure exists for home gardeners. Remove and destroy all infected plants and surrounding soil. Do not compost infected material.

Prevention: Practice long crop rotation (8+ years for heavily infected sites — the sclerotia persist in soil). Avoid moving soil from infected areas. Use clean sets and transplants. There are no fully resistant varieties.

Botrytis Leaf Blight (Botrytis squamosa)

Symptoms: Small white to pale-yellow oval spots with a green halo on leaves. In wet conditions the spots enlarge, leaves die back from the tips, and a grey mold may appear. Can progress to neck rot (Botrytis allii) in storage, with a soft, brown decay at the neck of the bulb.

Treatment: Remove and destroy affected foliage. Apply fungicides containing mancozeb or chlorothalonil at the first sign of symptoms in wet seasons.

Prevention: Ensure good air circulation with adequate spacing. Avoid overhead irrigation. Cure bulbs thoroughly before storage. Store in cool, dry, well-ventilated conditions. Rotate crops on a 3-year cycle.