How to Grow Onions

Filed under: How to Grow, Tagged as: how to grow, onions

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.

How to prepare and cook onions: click here.

What is the difference between green onions, spring onions, and scallions? Click here.

Description. 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.

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.

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.

How to plant. 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.

Spacing. 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. 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.

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.

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.

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.

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.

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.

Storing and preserving. 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.

Common name. Onion

Botanical name. Allium cepa

Origin. Southwest Asia

Posted by Stephen Albert on January 24, 2009 under: How to Grow , Tagged as: how to grow, onions
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Questions about growing onions. I left some onions that were planted spring of last year, because the tops died out before I started to harvest, so I overlooked their locations. This year, these leftovers ae now very large and have huge round stalks with a blossom ball at the tops. I am unsure whether I should cut these flower stalks off or bend them over to halt them at this point.

Posted by: Ken | June 24, 2009 8:23 AM | Reply

Hello Ken: Your onions have bolted--sent up flowers stalks. As with garlic, once an onion flowers the bulb will not grow larger but smaller. The plant is now putting its stored energy into flowering and reproduction. With flowering, you will find a green flower stalk beginning to grow and emerge from the center of the onion bulb; this will make storage of the onion impossible; the stalk will rot in storage. It is best to lift and use the onions that have flowered as soon as possible.

Posted by: Stephen Albert replied to comment from Ken | June 24, 2009 10:30 AM | Reply

Will walla wallas withstand fall frost?

Posted by: Alan | September 9, 2009 7:02 PM | Reply

Your Walla Walla onions will be ready for harvest when the stalks fall over. The stalks may completely fall or you can harvest bulb onions as early as when the stalks are a quarter to a third falling over. As long as the bulb flesh is protected from frost or freezing air temperatures, onions can remain in the ground. You might want to protect the bulbs by mounding up soil or adding a layer of mulch. Onions can remain in the ground as long as the soil does not freeze and the bulbs are not directly exposed to freezing temperatures. If you live where the weather will turn wet and cold and the soil will become soaked, the best strategy will be to lift the bulbs and cure them--air dry for at least 10 days. This will prevent rot-causing organizims from entering the bulb through the neck.

Posted by: Stephen Albert replied to comment from Alan | September 10, 2009 9:48 AM | Reply

I planted one sweet potato this spring, when should i dig it up and will it withstand fall frost? Thank you

Posted by: Alan | September 11, 2009 8:50 PM | Reply

Sweet potato tubers should be fully developed 100 to 140 days after planting or when vines begin to wither and leaves turn yellow. If there is a frost, harvest your sweet potatoes immediately. Tubers are damaged by freezing or cold soils, so dig up sweet potatoes early rather than late--before the first frost. Better yet take up your tubers before the soil temperature goes below 50F. Remember sweet potatoes are native to Tropical America and the Caribbean where frost is unknown.

Posted by: Stephen Albert replied to comment from Alan | September 14, 2009 3:54 PM | Reply

hi, ive started some onion seeds off and they have been growing really well but the first two leaves have died back. is there a problem or is this normal?

Posted by: martin cole | October 20, 2009 2:23 PM | Reply

What you are describing--I believe--are the cotyledons, the embryonic seed leaves which are not true leaves. Cotyledons-- "leaves" -- are formed before a seed ever germinates and become the frist parts of the plant to break the ground and emerge into the world shortly after germination. A plant's true leaves form after the seedling emerges into the world. Shortly after the cotyledons fall away. It seems as though the plant has lost its "first" leaves, but it has not; the cotyledons have done their work and die back. This is normal, and if what you have described are the cotyledons, these true leaves will now go on to do the work of photosynthesis and get your onions growing.

Posted by: Stephen Albert replied to comment from martin cole | October 20, 2009 11:00 PM | Reply

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