Onion Growing Quick Tips

Onion Bianca di Maggio 2

Sowing: Sow onion seed in the garden after all danger of frost has passed in spring and the soil is at least 55°F (13°C)—warmer is better. In warm-winter regions, plant onions outdoors in the fall. Start onion seed indoors 8 weeks before transplanting seedlings to the garden. Sow seeds in a warm, well-lighted location—in a bright window or under grow lights. Sow seeds 1/4″ (6 mm) deep in seed starting mix. Firm lightly and keep mix just moist. Seedlings emerge in 7-14 days at 60-85°F (16-29°C).

Be sure to choose the correct variety for your day length. In the Northern Hemisphere, Southern gardeners should select Short Day varieties (10-12 hours of sunlight each day); Northern gardeners should select Long Day varieties (14-15 hours of sunlight each day); gardeners in between should select Intermediate Day varieties (13-14 hours of sunlight each day).

Growing: Seedlings can be transplanted into the garden when they are 3-4” (7.5-10 cm) tall. Move seedlings to a sheltered place outdoors to “harden off” 1 week before transplanting. Space plants 3-5″ (7.5-13 cm) apart for bulb onions, 2-3” (5-13 cm) apart for scallions and bunching onions. Grow onions in compost-rich, well-drained soil, in full sun. Give onions regular water keeping the soil evenly moist. Do not plant onion family crops in the same place 2 years in a row.

 Harvesting: Bulb onions are ready for harvest 80-120 days after seeding, 30-40 days from sets (small bulbs). Harvest green onions when they are 6-8” inches (15-20 cm) tall. Hasten the harvest of bulb onions by bending stalks to the ground (called “lodging”) about 100 days after sowing, when the tops have begun to turn yellow and fall over. A few days later lift bulbs and allow to dry (“cure”) in the garden covered or indoors in a warm, dry place with good air circulation for 1-2 weeks. Then cut the foliage about 1” (2.5 cm) above the bulb and store in a cool, dry location.

Read more detailed onion growing articles at 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. has more than 10 million visitors each year.


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