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 frost||Planting dates|
|Jan. 30||Jan. 1-15|
|Feb. 8||Jan. 1-15|
|Feb. 18||Jan. 1-15|
|Feb. 28||Jan. 1-Feb. 1|
|Mar. 10||Jan. 15-Feb. 15|
|Mar. 20||Feb. 10-Mar. 10|
|Mar. 30||Feb. 20-Mar. 15|
|Apr. 10||Mar. 1-Apr. 1|
|Apr. 20||Mar. 1-Apr. 1|
|Apr. 30||Mar. 15-Apr. 15|
|May 10||Apr. 1-May 1|
|May 20||Apr. 20-May15|
|May 30||May 1-30|
|June 10||May 10-June 10|
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.
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