Grow onions from seed, seedlings, or sets (small dry onion bulbs started the year before).
- Bulbing onions require 80 to 120 days to reach harvest.
- Green onions are harvested before they form bulbs, in 40 days or less.
- Spring onions form small, immature bulbs and are harvested in 40 to 60 days.
Onion Planting Calendar. Onions require cool weather to start growth and produce green leaves for green or spring onions and warm weather to produce mature bulbs. Onions seeds, seedlings, and sets are frost hardy.
● Spring planting. Plant onion seed, seedlings, and sets in early spring in northern and cold-winter regions for harvest the following autumn after they have formed bulbs. Sow seeds directly in the garden as early as six weeks before the average last frost date, or start seed earlier in late winter in a greenhouse or cold frame and transplant seedlings to the garden in early spring.
- 12-10 weeks before the last frost in spring: direct-sow scallions and onions in a plastic tunnel or cold frame, or sow indoors for transplanting into the garden 6 to 4 weeks before the last frost in spring.
- 6-4 weeks before the last frost in spring when the soil is workable: direct-sow onion and scallion seeds and sets in the garden; transplant onion seedlings to the garden.
● Autumn planting. Plant onion sets in late summer or early autumn in southern and warmer regions. Over winter these onions for lifting mid-summer or autumn the following year.
Place. Plant bulb onions in full sun. Green onions can be planted in partial shade.
Soil. Sow onions in rich, well-drained soil with plenty of garden compost. Before planting, work the soil to a depth of six inches removing stone and lumps. Acidic soil with a pH of less than 6.0 will result in a more pungent flavor. Add lime to the soil to make it less acidic. For green onions, add a handful of nitrogen-rich blood meal to each square yard or meter before planting.
Spacing Green Onions. For green or pulling onions, sow seed ¼ inch deep, 2 to 3 inches apart. Plant onion sets 1 to 2 inches deep, 3 to 4 inches apart.
Spacing Bulb Onions. For bulb onions, plant sets 6 to 8 inches apart. Space rows 12 to 18 inches apart. The final size of each onion bulb will depend on how much space it has to grow. Onions can be started from seed indoors ten weeks before the average last frost date.
Onions can be inter-planted between other vegetables such as tomatoes or cabbage.
Container Growing Onions. Onions can be grown in containers. Bulb onions should be planted 3 to 6 inches apart in a container at least 8 inches deep.
Water. Water onions until the bulbs swell. Let the soil dry out once bulbs have formed and the leaves begin to yellow and droop.
Suggested Onion Varieties. Choose an onion variety suited for your region. Check with a garden center or the cooperative extension for recommendations. Onions are sensitive to day length. Long-day onions are best suited for northern regions where summer days are longest. Short-day onions are suited for southern regions where summer days are not as long as those in the north. American and Spanish onions are long-day onions. Bermuda onions are short-day onions.
More tips on How to Grow Onions.