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When to Plant Onion Seeds

Onion seedlings ready for transplanting

Onions are easy to grow from seed, but you must start early. In the Northern Hemisphere, sow onion seeds indoors in January and February, outdoors in April or May.

Onion bulb formation and daylight

Onion bulb formation is influenced by the length of daylight and the hours of dark each day. So-called long-day onions form bulbs when daylight grows longer in spring than the darkness of night. Short-day onions form bulbs when the hours of night and day are nearly equal or when nighttime hours are longer than daylight hours.

Plant long-day onion varieties in northern regions and short-day onion varieties in southern regions. Daylight in northern regions grows longer than nights as summer approaches, and daylight grows shorter in southern regions as summer approaches.

Garden centers commonly carry the appropriate onion varieties for the gardeners in their region.

Onion planting time and harvest

Onions started from seed indoors in late winter should form bulbs for summer harvest. Onions started from seed in the garden in mid to late spring can be grown as summer-bunching onions or fall-storage onions.

Onion seeds germinate best at 65℉ to 80℉ (18℃ to 27℃), but young plants should be grown in cooler temperatures, near 60℉ (16℃) and not warmer than 70℉ (21℃) by day and 50℉ (10℃) by night.

Onion planting dates for summer harvest

(These dates are for the Northern Hemisphere)

Average date of the last frostPlanting dates
Jan. 30Jan. 1-15
Feb. 8Jan. 1-15
Feb. 18Jan. 1-15
Feb. 28Jan. 1-Feb. 1
Mar. 10Jan. 15-Feb. 15
Mar. 20Feb. 10-Mar. 10
Mar. 30Feb. 20-Mar. 15
Apr. 10Mar. 1-Apr. 1
Apr. 20Mar. 1-Apr. 1
Apr. 30Mar. 15-Apr. 15
May 10Apr. 1-May 1
May 20Apr. 20-May15
May 30May 1-30
June 10May 10-June 10
Onions in garden
Onion seedlings in the garden; plastic sheeting warms the soil and keeps down weeds.

Onion planting details

  • Sowing depth: ½ inch (1.2 cm)
  • Space between plant after thinning: 3-4 inches (7.6-10cm)
  • Days to sprout: 10; start seed indoors 4 to 6 weeks before setting transplants in the garden
  • Days to harvest: 95-130
  • Storage period: 12-32 weeks
  • Seeds per 100 row feet: ½ ounce (14 grams)
  • Yield per 100 row feet: 100 pounds (45 kilos)
  • Suggested varieties: Check at a garden center or call the cooperative extension for varieties suited to your location.
  • Note: Control weeds whil plants are spindly and defenseless.

Related articles of interest:

How to Grow Onions

Growing Onion Bulbs: Pick the Right Variety for Your Garden

Planting Onions

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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