Plant onions sets (small bulblets) 3 to 4 months before the time you want to harvest mature bulbs; plant sets 3 to 4 weeks before you want to harvest immature green onions.
Start onion seeds indoors 4 to 6 weeks before the average last frost date in spring, and transplant them into the garden as soon as the soil can be worked.
About Onions. The onion is a hardy cool-season biennial usually grown as annual. The onion has narrow hollow leaves and a base which enlarges to form a bulb. The bulb can be white, yellow, or red. Onion flower stalk are taller than the leaves and topped clusters of white or lavender flowers. All varieties can be eaten young–within a few weeks of planting–as green onions. Spring onions, bunching onions, scallions and green onions are grown especially for their green tops. Bulb onions require 80 to 150 days to reach harvest. Bermuda and Spanish onions are milder than American onions. American and Spanish onions require more days to mature than Bermuda onions.
Onion Yield. Plant as many onions as necessary if green and mature onions are used frequently.
Site. Grow onions in full sun in loose, well-worked, well-drained soil. Loosen the soil to 6 inches deeps and remove all lumps, stones, and roots. Add well-aged compost to the planting bed before planting. Green onions can be grown in a partially shady spot.
Onion Planting Time. Onions are temperature sensitive: they require cool weather to produce their tops and warm weather to produce their bulbs. Plant onions sets (small bulblets) 3 to 4 months before the time you want to harvest mature bulbs; plant sets 3 to 4 weeks before you want to harvest green onions. Onion seeds are best started indoors: start seeds 4 to 6 weeks before the average last frost date in spring, and transplant them into the garden as soon as the soil can be worked. In mild-winter regions, plant onions in the fall or winter, depending on the variety.
Most onions are sensitive to day length. American and Spanish onions need long days to produce their bulbs, and Bermuda onions prefer short days.
About temperature and day-length: Bulb Onion Growing: Day Length and Temperature.
How to Plant Onions. Onions can be grown from seeds, transplants, and sets. Sets are small bulblets–about the size of a large pea–whose growth was interrupted before the bulbs developed. Bulblets larger than ¾ inch in diameter may go to seed before developing bulbs. These are best grown as green onions. Sets are planted just below the soil surface and are quick to begin growing. (But there are fewer onion varieties available as sets than transplants and seeds.) Transplants are onions that have begun growing. Place transplants in the garden just slightly higher than the surrounding soil and they will settle into place. Seeds are best started indoors where optimal temperature and warmth ensure germination.
More planting tips: Onion Seed Starting Tips.
Spacing Onions. Plant sets 1 to 2 inches deep. Place transplant bulbs just below the soil surface. Space sets and transplants 2 to 3 inches apart in rows 12 to 18 inches apart. Thin to 4 to 6 inches or more apart allowing for bulb development. Sow seed ¼ to ½ inches deep and ½ inch apart in rows 12 to 18 inches apart; thin seedlings to 1 to 2 inches apart. It will take two seasons to produce bulb onions from seed. The final size of the onion will depend on how much growing space it has.
Water and feeding. Keep soil evenly moist until plants begin to mature. Soil can be allowed to dry when leaves start to get yellow and brown and to droop over.
Companion plants. Beets, lettuce, strawberries, summer savory, tomatoes. Onions are easily inter-planted between larger crops such as cabbages or tomatoes.
Container Growing Onions. Green onions easily grow in containers 6 inches deep; 8 to 10 green onions can be grown in a container 8 inches across. Grow bulb onions in containers 8 to 10 inches deep.
Onion Care. Keep planting beds free of weeds to avoid competition for light, water, and nutrients. Thin plants early to give bulbs room to mature to the desired size. Bend but do not break stalks 2 to 3 weeks before harvest to hasten bulb development. Heavily mulch onions that you plan to over-winter and harvest the second season.
Onion Pests. Onion can be attacked by thrips and maggots. Thrips can be sprayed away with a stream of water. Place a 3 to 4 inch square of plastic around the plants to discourage maggot flies from laying their eggs near plants.
Onion Diseases. Onions are susceptible to bulb and root rots, smut, and downy mildew in commercial onion growing districts. Plant disease-resistant varieties and keep the garden clean of debris. Avoid planting onions where or onions or garlic have grown the year before. Remove and dispose of diseased plants immediately.
Onion problem solving: Onion Family Problems: Troubleshooting.
Onion Harvest. Snip onion leaves for flavoring throughout the season. Harvest green onions when bulbs are no larger than the diameter of the leaves. Bunching onions can be harvested when bulbs are 1 to 2 inches in diameter; split them off from the outside of the bunch. Lift dry onion bulbs when they are 3 to 5 inches in diameter after the leaves have dried. If you allow lifted onions to cure in the garden, be sure to lift them root and all from the soil or they may start growing again and become soft and watery. Cut tops away from stalks 1½ inches from the bulb if you do not plan to braid the stalks. Onions that come to maturity in cool tend to be sweet; onions that come to maturity in hot weather will be stronger flavored.
Storing and Preserving Onions. Green onions will keep in the refrigerator for up to one week. Mature bulbs should be allowed to air dry for about a week outside before being stored in a cold, dry place for up to 6 months. Do no refrigerate mature onions.
More harvest tips: How to Harvest and Store Onions.
Onion Varieties. Red: Benny’s Red (112 days); California Wonder Red (85 days); Giant Red Hamburger; Lucifer (106 days); Mars, Mercury; Red Baron; Red Burgermaster; Red Dutch; Red Globe; Red Mac; Redman (105 days); Rio Kyda Von; Southport Red Globe (120 days); Stockton; Wethersfield.
Yellow or White: Alisa Craig (110 days); Bingo (100 days); Blanco Duro (120 days); Buffalo (88 days); Burgos; Capable; Celebrity; Condor; Copper King (95 days); Copra (111 days); Duration (110 days); Early Yellow Globe (114 days); Eskimo (85 days); Fiesta; First Edition (105 days); Frontier; Gazette; Giant Zittau; Granex (110 days); Gringo; Headliner; Joint Venture; Kelsae Sweet Giant (110 days); Legacy (108 days); Lisbon White; New York Early (98 days); Norstar (85 days); Prince (106 days); Reliance (110 days); Riverside Sweet Spanish; Simcoe (110 days); Southport White Globe (110 days); Sweet Sandwich; Sweet Spanish Hybrid (110 days); Tarmagon; Texas Yello Grano, Valiant.
Sweet-Eating: Walla Walla (110-300 days), Yellow Sweet Spanish (110 days), Vidalia (110 days).
Green Onions-Scallions: Any of the above before bulbs are fully developed.
Common name. Onion
Botanical name. Allium cepa
Origin. Southwest Asia
More tips: Onion Planting.