Garden Onion (Allium cepa)

How much light does a Garden Onion need?

Garden onion prefers full sun outdoors. All onion relatives should be grown in full sun for best results. All onions require full sun for the best growth, and full sun is the only outdoor light level on record for this plant. Plan for a plant about 1–1.5 ft (0.3–0.46 m) tall in that sun.

• Light: Full sun^1,2,3

How often to water a Garden Onion

Onions are shallow-rooted and require constant moisture for proper growth. Aim for soil that is moist but well-drained, fertile and loose. The preferred range runs from medium to wet. Medium is the lower end of that range and wet is the upper end. The recorded soil drainage for this plant is well-drained.

• Water: Medium to wet^2,4

Best soil for a Garden Onion

Garden onion grows best in a fertile, well-drained, loamy soil with plenty of organic matter. The ideal pH is between 6.2 and 6.5. A wider pH band of 6.0 to 7.0 is also named for well-drained soil that is high in organic matter. Drainage for this plant is recorded as well-drained. Moist but well-drained, fertile, loose soil is described for it as well.

• Soil drainage: Well drained^1,2
• Soil pH: pH 6.2–6.5¹

Fertilizing a Garden Onion

Garden onion is a heavy feeder. It has a high requirement for nutrients, supplied either by soil organic matter or by fertilizers. Soil that is fertile, with plenty of organic matter, is described as best for growing quality onions. Overall maintenance for the plant is rated medium.

• Fertilizer: Heavy feeder⁵

When to plant Garden Onion

Plant onions starting 6 weeks before your average last frost date. The same timing is recorded for direct sowing and for transplants. Onions and the related shallots, leeks and garlic all grow best during cool weather. Garden onion is classed as a cool-season crop.

• Direct sow: 6 weeks after last spring frost⁶
• Transplant: 6 weeks after last spring frost⁶

When and how to harvest Garden Onion

Harvest bulb onions when about three-fourths of the tops have fallen over. Days to maturity run from 85 to 120 for mature bulbs. The count starts from transplant. The falling tops are the stated sign for bulb onions. Mature plants stand 1–1.5 ft (0.3–0.46 m) tall and 0.5–1 ft (0.15–0.3 m) wide.

• Harvest: Harvest bulb onions when about three-fourths of the tops have fallen over.¹
• Days to maturity: 85–120 days from transplant⁵

How to propagate a Garden Onion

You can grow garden onion from seed, transplants or small bulbs called sets. The listed propagation methods are seed and offsets. The bulb makes several offsets during the growing season. Plant type is recorded as a bulb. Start planting about 6 weeks before your average last frost date. Seed, transplants and sets are the three starting forms named for onions.

• Propagation: Seed and offsets^2,7

Common problems

Thrips, onion maggot, bulb mites, cutworms, grasshoppers, nematodes, snails and slugs, wheat curl mite and allium leafminer are the pests listed for garden onion. The diseases listed are bacterial soft rot, basal rot, black mold, blue mold rot, botrytis leaf blight, downy mildew, neck rot, pink root, purple blotch and white rot.

• Common pests: thrips, onion maggot, bulb mites, cutworms, grasshoppers, nematodes, snails and slugs, wheat curl mite and allium leafminer⁸
• Common diseases: bacterial soft rot, basal rot, black mold, blue mold rot, botrytis leaf blight, downy mildew, neck rot, pink root, purple blotch and white rot⁸

Questions and answers

Is garden onion toxic to cats and dogs?

Garden onion is listed as toxic to both cats and dogs. The ASPCA lists it that way, and Pet Poison Helpline says onions, chives, garlic and leeks in the Allium family are poisonous to dogs and cats.

How much sun does garden onion need?

Garden onion needs full sun. All onions require full sun for the best growth, and full sun is the only outdoor light level listed for it. Its soil should be well-drained, and its water need runs from medium to wet.

When do you harvest garden onion?

Harvest bulb onions when about three-fourths of the tops have fallen over. Days to maturity run from 85 to 120 for mature bulbs, counted from transplant. Plant them starting 6 weeks before your average last frost date.