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How to Grow Onions

How to Grow Onions

Grow onions Onions can be grown for their green immature stems or they can be grown for their mature bulbs.

Onions grown for their green stems are called bunching onions. They are also called green onions, spring onions, and scallions. The terms are often used interchangeably.

The stems of most onions that produce bulbs can be harvested early as green onions. But not all bunching onions will grow bulbs if allowed to grow to maturity. When you select an onion to grow in your garden, make sure you get the type of onion you want—bulbing or bunching.

Selecting Onions for Your Garden: Things to Know

Shape and size: Some onions produce very small, nearly insignificant bulbs; these are called bunching onions, green onions, and scallions. Some onions produce small bulbs round or spindle in shade; these are called picking onions. Some onions produce medium-size globe-shaped bulbs; these tend to be sharp-flavored storage onions. Some onions produce large, round, mild-flavored bulbs for fresh use; Spanish and Bermuda onions are large, mild, and sometimes sweet.

Color and flavor: Onion colors range from white or yellow to red or purple. Yellow onions are all-purpose onions; they have a balance of astringency and sweet in their flavor. White onions have a sharper more pungent flavor; they are also more tender and have a thinner, papery skin. Red onions have red flesh and purple skin; the flavor of a red onion is similar to a yellow onion but the layers of a red onion are less tender and less meaty than a yellow onion. Generally, the strongest-tasting onions are the best choice for storing into the winter; they have the toughest skins.

Spring onions growing in rows
Spring onions in growing in the vegetable garden.

Daylight needed: Bulb onion varieties differ according to the amount of daylight needed for bulb formation. Some varieties require 12 hours of light each day to form bulbs (called short-day); some require 13 to 16 hours of daylight to form bulbs (called long-day). If you live in the North where summer days are long, grow a long-day variety. If you live in the South, where daylight hours do not vary by much year-round, grow a short-day variety. Grow short-day varieties where winters are mild; where you can grow onions through autumn and winter. Grow-long day onions where winters are cold. Onions are not sensitive to mild frost in either spring or autumn. It may be worth noting, long-day onions tend to be round, and globe-shaped and short-day onions tend to be flatter in shape.

Seeds, seedlings, sets: Onions can be grown from seeds, seedlings, or sets (sets are small bulbs grown the previous year). Obviously, sets have a head start; they will mature more quickly. Seedlings will need more time to produce bulbs than sets. Seeds will need a long growing season to produce bulbs; sometimes they produce a bulb the second season they are in the ground. Sets are only available in spring. There will be a greater selection of varieties if you shop for seeds than in sets or seedlings. If you live in a short growing season region, choose sets to ensure you have enough time to grow bulbs.

Planting Onions

Where to Plant Onions:

  • Grow bulb onions in full sun. Green onions can be grown in a partially shady spot.
  • Onions prefer loose, well-worked, well-drained soil rich in organic matter.
  • Loosen the soil to 6 inches (15cm) deeps and remove all lumps, stones, and roots.
  • Add well-aged compost or a commercial organic planting mix to the planting bed before planting. Turn the soil under to 12 inches (30cm) deep.
  • Sandy loam is good soil for growing sets. Loam is good soil for seeds and seedlings. Heavy clay soil can impede the development of bulbs.
  • If the soil is not well-drained, grow onions in raised or mounded beds.
  • A soil pH of 6.0 to 6.5 is recommended for growing onions.

Onions growing in gardenOnion Planting Time:

  • Onions are temperature sensitive: they require cool weather to produce their tops (early stages of growth) and warm weather to produce their bulbs (late stages of growth). Onions grow best in air temperatures of 55° to 75°F (13-24°C) Temperatures greater than 85°F (29°C) can cause soft, gray, watery 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.
  • Green onions for autumn harvest: Plant green onions 4 to 6 weeks before the first expected fall frost for autumn harvest; late summer or early fall temperatures should not be greater than 75°F (24°C).
  • for growing green onions.
  • Green onions for winter and early spring harvest: In mild winter regions, plant green onions in autumn for winter and early spring harvest.

About temperature and day-length: Bulb Onion Growing: Day Length and Temperature.

Planting onion bulbs
Plant onion sets pointed side up.

How to Plant Onions:

  • Onions can be grown from seeds, seedling transplants, and sets.
  • Planting seeds: Seeds can be started indoors 4 to 6 weeks before you plan to set seedlings out or you can direct sow seed in the garden when the soil temperature is at least 40°F (4.4°C). Sow seed ¼ to ½ inches (12mm) deep. The seed will germinate in 7 to 10 days at 70°F (21°C), longer in cooler soil. Thin seedlings from 1 to 2 inches (2.5-5cm) apart in rows 12 to 18 inches (30-45cm) apart; thin again for bulb onions from 4 to 6 inches (10-15cm) apart. The final size of the onion will depend on how much growing space it has.
  • Planting seedlings: Seedlings are onions that have begun growing. You can start seedlings indoors from seed or you can purchase onion seedlings at the garden center. Place transplants in the garden just slightly higher than the surrounding soil and they will settle into place. Space seedling transplants 2 to 3 inches (5-7cm) apart in rows 12 to 18 inches (30-45cm) apart. Thin again to 4 to 6 inches (10-15cm) or more apart allowing for bulb development.
  • Planting 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 (19mm) in diameter may go to seed before developing bulbs (these are best grown as green onions). Plant sets 1 to 2 inches (2.5-5cm) deep. Plant sets pointed side up. Space sets 2 to 3 inches (5-7cm) apart in rows 12 to 18 inches (30-45cm) apart. Thin to 4 to 6 inches (10-15cm) or more apart allowing for bulb development. The final size of the onion will depend on how much growing space it has.

Companion Plants for Onions:

  • Grow onions with beets, lettuce, strawberries, summer savory, and 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 (15cm); grow 8 to 10 green onions in a container 8 inches across.
  • Grow bulb onions in containers 8 to 10 inches (20-25cm) deep.

Caring for Onions

Watering and Feeding Onions:

  • Keep the soil evenly moist until plants begin to mature. Dry soil can cause onions to split; wet soil can cause bulbs to rot.
  • Transplants need more water than sets.
  • To conserve soil moisture, lay down 8 to 10 inches (20-25cm) of mulch when the soil warms in early summer.
  • Soil can be allowed to dry when leaves start to get yellow and brown and to droop over.
  • Side dress onion plants with nitrogen-rich alfalfa meal. You can also plant beans nearby; bean roots set nitrogen in the soil.
Onions growing in garden
Thin plants early to give bulbs room to mature to the desired size.

Maintaining Onions:

  • 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. Use the thinnings as green onions.
  • Bend but do not break stalks 2 to 3 weeks before harvest to hasten bulb development.
  • Cut back on watering close to harvest to keep the necks from rotting.
  • Heavily mulch onions that you plan to over-winter and harvest the second season.
  • Avoid planting onions near sweet potatoes; both can be attacked by wireworms.

Onion Pests:

  • Onion can be attacked by thrips and onion maggots.
  • Thrips can be sprayed away with a stream of water.
  • Place a 3 to 4 inch (7-10cm) square of plastic around the plants to discourage maggot flies from laying their eggs near plants.
  • Cover plants with a floating row cover to keep adult onion maggot flies from laying eggs on young plants.

Onion Diseases:

  • Onions are susceptible to Fusarium bulb rot, smut, onion leaf blight, onion smudge, and downy mildew, especially in commercial onion growing districts.
  • Plant disease-resistant varieties and keep the garden clean of debris.
  • Avoid planting onions where onions or garlic have grown the year before.
  • Remove and dispose of diseased plants immediately.
  • Be sure to select varieties that are suited to the day length in your area.
  • Warm the soil with clear plastic before planting, this will help prevent smut which appears as blisters on the skin of leaves and bulbs.
  • Parasitic nematodes applied to the soil can help control cutworms and onion maggots.
  • Pre-soak seed in compost tea for an hour to prevent damping off and other fungal diseases.
Onions at harvest
Bulbing onions are mature when the tops turn yellow and start to fall over.

Harvesting and Storing Onions

Harvesting Onions:

  • 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 (2.5-5cm) in diameter; split them off from the outside of the bunch.
  • Bulbing onions are mature when the tops turn yellow and start to fall over. To speed maturation, bend over the tops with your foot or the back of a rake.
  • Lift dry onion bulbs when they are 3 to 5 inches (7-12cm) in diameter after the leaves have dried. Use a spading fork to help loosen the soil and lift bulbs.
  • Be careful not to bruise bulbs when you lift them; bruised bulbs can rot in storage.
  • Cure onions after harvest by leaving them outdoors to dry for a few days. Spread them out on wire mesh set above the ground. Cure them out of direct sunlight to prevent sunscald.
  • 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 weather 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. Wrap them in plastic wrap.
  • 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. When the skins are dry and papery, cut the tops off about one inch above the bulb then store the bulbs in mesh bags.
  • Do not refrigerate mature onions.

More harvest tips: How to Harvest and Store Onions.

Torpedo onions
Torpedo onions

Onion Varieties to Grow

  • 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’; S’weet 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.

About Onions

  • The onion is a hardy cool-season biennial usually grown as annuals.
  • The onion has narrow hollow leaves and a base which enlarges to form a bulb.
  • The bulb can be white, yellow, or red.
  • The onion flower stalk is taller than the leaves and will be topped by 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.
  • Botanical name: Allium cepa
  • Origin: Southwest Asia

More tips: Onion Planting.

Grow 80 tasty vegetables: THE KITCHEN GARDEN GROWERS’ GUIDE

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38 Comments

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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?

  6. 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.

  7. My onions are already in the seed bed where i planted seeds closed to each other and i don’t know how it’s gonna be when the time of transplant reaches. Guide me pliz because it is now one month since i planted

    • When your onion seedlings are 4 to 5 inches tall transplant them to their permanent growing bed. Be careful as you take the plants from the seed starting bed–try not to damage the roots. Space the plants 4 to 5 inches apart if you are growing them for bulbs. If you are growing green onions they can be as little as 2 inches apart.

  8. I have bunching onion bulblets that are about 4-6 in a cluster. Do I break these apart to start them growing? And when is the best time to do the planting in soil for a fall harvest in the south?

    • If an onion bolts and flowers you can save the seed to plant next season. To save seed let the flower dry out on the plant then snip the flower head and separate out the seed; dry seed may simply drop on to your clean surface. Place the seed in a paper envelope and keep it in a cool spot until planting time next season. Alernatively you can snip the flower from the plant before it is dry and allow it to dry indoors. Keep in mind that only seed from open-pollinated plants will grow true next season. Seed from hybrid plants seldom grows true to its parent.

  9. I have been searching online for information about planting onions. Anything I found gave very little information. This site is heaven sent. You have more information about onions than anything I have found. I learned a lot. Thank you.

    • If you are growing onions to form bulbs, thin the plants from 5 to 6 inches apart. Let them grow on until the tops begin to turn yellow and fall over in 3 to 4 months. You can then harvest the bulbs. If you are growing the onions for their green tops; you can begin clipping the tops at any time for adding to salads and cooking.

  10. Hi Steve: I’m currently considering what I can do with some green onion bottoms (I don’t want to do the water in a jar thing), which is why I’m here on your onion page. I can’t wait to plant them all! Thanks for the support, Steve. Keep up the good work!

    • The bottoms of green onions with a few roots attached can be rooted in water by keeping the roots pointed down. You can also “plant” the roots in just moist potting soil with the cut portion of the onion just above the soil level. New green shoots should appear in a few days if the crown is viable. Leeks also can be re-grown this way as can lettuce and other vegetables where some root is available for re-planting.

    • Short-day onions are suited for growing at latitudes of 25–35°–USDA Zones 7 and warmer. They will initiate bulb formation at 10–12 hours of daylight. Above attitude 35 plant long-day onions. If you are unsure, plant day-neutral onions. Stuttgart Germany is in Zone 7b. Stuttgart Arkansas is in USDA Zone 7b as well.

  11. I’d like to plant some onions in containers in between my raised beds to assist with pest control. Will they still repel pests if they are in containers?
    Thank you.

  12. Is it possible for me to grow sets for next springs planting, from seed I have right now? I live in zone 6. How would I do this? Thanks Steve.

    • Yes; plant the seed; when seedlings come up thin to 1 to 2 inches apart; lift the small bulbs before the first frost; store them in a cool place until you are ready to plant.

  13. I braid my onions but there are times when the stem of the onion is to thick. Is there a way of preventing this? Does bending them over a bit earlier help?

    • Planting biodegradable containers (and the plants in them) can be tricky. Cut slits in the sides of the paper pots and also in the bottom; this will allow roots to escape more quickly. Be sure that on part of the paper pot is exposed to the air; if the rim or lip of the pot is above the soil line, moisture can wick away from the roots through the paper; this can leave the roots dry.

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