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.