How to Grow Onions:
Growing Onions Organically

Learning how to grow onions is appropriate for home gardeners of all ages. They’re not difficult to grow and require little attention once planted, but they can try the patience of young gardeners because all the action seems to be taking place under the soil!

While they’ll probably grow without it, onions prefer full sun and moist soil with good drainage. Onions grown from seed take longer to mature and harvest than do onions grown from sets.

How to Grow Onions: Plant Snapshot (Allium cepa)

How to Grow Onions
  • Family: Alliaceae (Onion family)
  • Annual or Perennial? Annual: Ready to harvest 100 days after planting.
  • Recommended varieties: Newburg, Cortland, Copra, Yellow Sweet Spanish
  • Cold tolerance: Newburg, Cortland, Copra, Yellow Sweet Spanish
  • When planning vegetable crop rotation, group with crops from this family: Hardy - Will survive hard frosts and can be planted 2 to 3 weeks before the average date of the last 32 degree F (0 degree C) temperature in spring.
  • Companion plants (see Companion Planting Charts for more info):
    • Companions: Beets, Broccoli, Brussels Sprouts, Cabbage family, Carrot, Cauliflower, Celery, Chamomile, Collard, Cucumber, Dill, Kale, Lettuce, Pepper, Potato, Radish, Rose, Savory, Squash, Strawberry, Tomato
    • Avoid: Asparagus, Beans, Gladiolus, Peas
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How to Grow Onions: Planting the Seed

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Start onion seeds indoors about eight weeks before the last frost date. Sow seeds directly into flats, cover with plastic for a greenhouse effect and keep the soil continually moist. A heat mat for seedlings can help to speed up the germination rate.

Make sure the seeds and seedlings get plenty of even moisture – especially for the first few weeks.

Onion “sets” are tiny onion bulbs and are used by gardeners who want to get a head start on growing onions. Both onion seeds and onion sets may be planted outdoors as soon as weather permits and when the soil is at the proper temperature - usually around the month of March. When planting onion sets, the pointed top should face up and be even with the soil’s surface.

Harden off onion plants before planting outside by gradually exposing them to outdoor weather over the course of a week.

Snapshot: Planting Onions

  • Planting depth: For transplants and seeds, about ½ inch (1.3 cm); for sets, about 1 inch (2.54 cm)
  • Spacing in rows: about 3 inches (7.62 cm)
  • Germination soil temperature: 70 to 85 degrees F (21.1 to 29 degrees C)
  • Days to germination: 5 to 10
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How to Grow Onions: From Germination to Pre-Harvest

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Once seeds have germinated, place flats in a sunny window or beneath a grow light. You should thin onion seedlings to stand about two inches apart, and when they have reached a height of about five inches, cut them back to three inches to prevent leggy plants.

Transplant onion plants into your garden about four weeks before the last frost.

Keep garden weeds to a minimum either by pulling by hand or hoeing them very carefully from the onion bed. Onions are easily damaged, so don’t be too aggressive with your tools.

Give onions about an inch of water weekly, and make sure to allow the bed to dry out in between watering (this does NOT apply during the first few weeks after planting when onions need continual moisture). Well-composted soil shouldn’t require much fertilization, but use organic fertilizer for vegetables monthly if your soil's fertility is less than ideal.

Snapshot: Growing Onions

  • Preferred soil pH (see soil pH tester for more information): 6.0 to 7.0
  • Growing soil temperature: 50 to 95 degrees F (10 to 35 degrees C)
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How to Grow Onions: Harvesting Time

Onions may be harvested at any time, as they are edible at any stage of growth. However, if you want large onions, it is best to put off harvesting until the end of their growing season. A clue to when onions have stopped growing is the appearance of the onion plant's tops. When the tops lose color and fall over, the onions should be left in the ground for another ten days.

Choose a sunny day to pull onions from the ground, and try to let them dry in the sun for up to a week so that the root system at the base of each onion will die. Onions will then need to have their tops removed before they are ready to cure in a warm, well-ventilated area. Leave them there for two to three weeks so that they will be dry enough to store for an extended period.

Snapshot: Harvesting Onions

  • Time to harvest: About 20 weeks after planting
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How to Store Onions and/or Freezing Onions

First, some onions store better than others. If you are growing onions for storage, choose varieties that store well.

Once cured, onions should be stored in mesh bags in a dry, cool, well-ventilated area.

To prepare onions for freezing, peel and chop them or cut them into rings. Wrap onions in several layers of plastic wrap, then place them into a plastic freezer container for up to three months.

Onion seeds are viable for one year.

Snapshot: Storing Onions & Freezing Onions

  • Storage temperature: 32 to 55 degrees F (0 to 12.7 degrees C)
  • Humidity: 60 to 70% relative humidity
  • Storage life (unfrozen in above conditions): up to 1 month
  • Storage life (frozen): 3 months
  • Seed longevity: About 1 year
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How to Grow Onions: Pests & Diseases

The following pests and diseases have been known to affect the success of growing Onions. For more information about preventing and controlling them, see Organic Garden Pest Control.

Pests

  • Cutworms
  • Grasshoppers
  • Leafminers
  • Mites
  • Bulb mites
  • Wheat curl mite
  • Nematodes
  • Onion maggot
  • Snails/slugs
  • Thrips

Diseases

  • Bacterial soft rots
  • Basal rot
  • Black mold
  • Blue mold rot
  • Botrytis leafspot (Leaf blight)
  • Downy mildew
  • Neck rot (Botrytis bulb rot)
  • Pink root
  • Purple blotch
  • White rot
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