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.
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.
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 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 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.
Onion 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° Temperatures greater than 85°F 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 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.
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° Sow seed ¼ to ½ inches deep. The seed will germinate in 7 to 10 days at 70°F, longer in cooler soil. Thin seedlings from 1 to 2 inches apart in rows 12 to 18 inches apart; thin again for bulb onions from 4 to 6 inches 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 apart in rows 12 to 18 inches apart. Thin again to 4 to 6 inches 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 in diameter may go to seed before developing bulbs (these are best grown as green onions). Plant sets 1 to 2 inches deep. Plant sets pointed side up. Space sets 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. 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; grow 8 to 10 green onions in a container 8 inches across.
- Grow bulb onions in containers 8 to 10 inches 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 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.
- 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 can be attacked by thrips and onion 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.
- Cover plants with a floating row cover to keep adult onion maggot flies from laying eggs on young plants.
- 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.
Harvesting and Storing 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 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 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 no refrigerate mature onions.
More harvest tips: How to Harvest and Store 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.
- 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.