How to Grow Onions

Onions (Allium cepa) are hardy biennials grown as annuals for their flavorful bulbs. They thrive in full sun, loose soil, and consistent moisture. This guide explains planting, germination, care, harvesting, and climate considerations suitable for all regions.

Where Should You Plant Onions?

  • Best location: Full sun; onions need strong light for leaf production and proper bulb formation.
  • Soil: Loose, well-drained, nutrient-rich soil; pH 6.0–7.0.
  • Moisture: Even moisture during early growth; reduce watering slightly as bulbs mature.

When Should You Plant Onions?

  • Start indoors: Begin seeds 8–10 weeks before transplanting if outdoor conditions are still cool.
  • Transplanting: Move outdoors once temperatures remain stable and frost risk has passed.
  • Direct sow: Plant when soil is warm enough to support steady germination and root development.

How Do You Plant Onions?

  • Depth: Sow seeds ½ inch deep; plant sets so the tips sit just above the soil surface.
  • Spacing: Space bulbs 4–6 inches apart; scallions can be spaced 1 inch apart.
  • Quantity: Multiple plants ensure a steady harvest and allow selection of the best bulbs.

Which Plants Grow Well With Onions?

  • Companions: Carrots, beets, lettuce, cabbage, and herbs.
  • Avoid: Peas and beans, which may experience growth inhibition near onions.
  • Best pairing: Plants that do not compete heavily for nutrients and help deter pests.

How Do You Water and Feed Onions?

  • Watering: Keep soil evenly moist during early growth; reduce watering once bulbs begin swelling.
  • Feeding: Apply nitrogen-rich fertilizer early, then switch to balanced feeding during bulb formation.

How Do You Care for Onions?

  • Support: Onions do not require support; maintain loose soil around bulbs for proper expansion.
  • Weeding: Keep beds weed-free to reduce competition and maintain soil warmth.

Can You Grow Onions in Containers?

  • Container size: Minimum 8–10 inches deep; larger containers support better root development.
  • Winter growing: Not typical; onions prefer cool-season starts but require frost-free conditions to bulb.

What Pests or Diseases Affect Onions?

  • Pests: Thrips, onion maggots, and aphids; monitor plants and use organic controls as needed.
  • Diseases: Downy mildew, neck rot, and fungal issues; ensure good airflow and avoid overwatering.

How Do You Harvest Onions?

  • Timing: Harvest when tops fall over naturally and bulbs develop papery skins.
  • Leaves: Not harvested; foliage is grown to support bulb development.
  • Seeds: Collect seeds from mature flower heads; dry thoroughly before storing.