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Onion Growing Tips

Onion Row1

onions in gardenOnions are a high yield crop. Twenty to 50 onions can grow in 1 to 1½ square feet of space. But onions are easily grown in odd spaces alongside both slower and faster growing vegetables.

Green onions can be ready in 20 to 30 days after planting. Dry bulb onions can take 100 to 175 days to reach maturity.

Types of Onions

Seeds, sets, and transplants. Onions can be grown from seeds, sets (young, small dormant bulbs grown the previous year), or transplants. Growing onions from seed can take as much as five months. You will find seed for many varieties or cultivars of onions. Sets are easier to plant than seeds or transplants. Sets mature in as little as two months and are less susceptible to disease. But, cultivar selection is limited for sets. (Avoid onion sets with bulbs larger than a dime–they are likely to bolt.) Transplants are small seedlings that look like scallions. Transplants require about two months to reach maturity.

Bulb or bunching onions. Select bulb or bunching onions depending upon your intended use. Bulb onions can range from the small pearl onions to very large Spanish types. Bulbs are white, yellow, or red at harvest. Bunching onions–also called scallions or green onions–are grown for their tender, green top stalks. They are harvested before bulbs fully form.


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Long or short day onions. Onions grow tops in cool weather and form bulbs in warm weather. Temperature and day length control the timing of bulbing. Long-day onions require long hours of daylight–14 to 16 hours per day–to reach maturity. Long-day onions grow best in northern latitudes. Short-day onions grow best in mild-winter southern latitudes. They grow through the fall and winter and form bulbs when daylight increases to 12 hours per day in early summer. (Onions will be slow to grow if temperatures linger in the 30°s and 40°sF.)

onion planting
Planting onion sets

Planting Onions

Garden site. Onions grow best in loose, well-drained sandy loam. Turn lots of well-aged compost and manure into the onion bed in advance of planting; turn the soil to at least 8 inches deep. Onions prefer a soi pH of 6.0 to 7.5.

Starting seeds. Sow onion seeds indoors 8 to 12 weeks before the last average frost date. Sow seeds in pots, flats, or trays. Thin seedlings to one inch apart when they are four inches tall. Sow seed outdoors two weeks before the last average frost date in spring or four weeks before the first expected frost in autumn. Avoid sowing onion seed directly in the garden until the soil temperature has reached 50°F. Outdoors sow onion ½ inch deep and 1 inch apart. Later, thin to four inches apart. Growing onions from seed will give you the widest choice of varieties.

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Setting out starts. Seedlings (starts) can be transplanted to the garden in early spring as soon as the soil can be worked–usually about 2 to 3 weeks before the last frost when the soil temperature is at least 40°F; the air temperature should be at least 45°F. Set bulb onion seedlings one to two inches deep–depending on the size of the bulb–and four to six inches apart. Set starts for scallions one inch apart. To encourage development of bulbs, soak them in compost tea for about 15 minutes before planting. About four weeks after planting, gently push back the soil atop bulbs; this will help them to grow larger. Onions grown from transplants mature more quickly than onions grown from seed.

Planting sets. Choose sets that have bulbs about ½ inch in diameter. (Larger sets may go to seed before producing decent-size bulbs. Sets with smaller bulbs may not grow well.) Plant bulbs with the pointy end up; the rounded end is the rooting end. Set bulbs ½ to one inch deep and four to six inches apart–depending on the size of the bulb at maturity. Onions sets are often labeled “red,” “white,” or “yellow”–you may not know the exact variety you are growing.

Onion row
Onions require constant moisture during bulb enlargement.

Caring for Onions

Food and water. Onions are heavy feeders. Feed onions with a rich fertilizer, such as fish emulsion, early in the season to develop large plants and bulbs. (Or you can use an organic fertilizer, 5-10-10.) Give a second feeding about a month after the first feeding or side-dress rows with a band of aged compost. Keep onions evenly watered early in the season. They require constant moisture during the bulb enlargement stage; dry conditions early on will cause bulbs to split. Give each plant about 1 inch of water each week (about 1.6 gallons). Transplants require more water than sets. At midsummer–or about a month before harvest after bulbs have formed and when the necks of the onions begin to soften, cut back on food and water and allow bulbs to mature in drier, less fertile soil.

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Weeding. Keep onion beds well-weeded. Onions are shallow rooted. Cultivate often and shallowly. Pull weeds by hand close to bulbs to avoid up-turning plants. Use a sharp hoe only to cut off weeds at soil level. Because onions leaves are thin and strappy they do not block the sun from the soil which, in turn, allows weed germination. Onion beds require more weeding than other vegetable beds.

Mulch. After the soil has warmed, place a 1- to 2-inch layer of mulch around onions to discourage weeds and conserve soil moisture. Use aged compost or chopped leaves around onions. Keep the mulch back from bulb tops once they start to develop. (To grow large onions, keep both mulch and soil pulled back from the top two-thirds of developing bulbs.)

onion harvest time
When bulbs are ripe, leaves will begin to yellow and fall over.

Onion Harvest and Storage

Harvest. New growth from the center will stop when bulbs start forming. When bulbs are ripe, leaves will begin to yellow and fall over. After about three quarters of tops have fallen over, use the back of a rake to horizontally bend over the remaining tops. The bent leaves will cause the plant to divert the rest of its energy to the bulbs and away from leafy growth. After the tops turn brown in a day or two, lift the bulbs with a garden fork on a sunny day, and leave them to dry in the sun. When bulb outer skins are dry and the tops withered in about a week, wipe off any soil, and cut away the tops. If the weather is damp, allow the onions to dry in an airy place. You can loop the leaves through the mesh of a fence or framed chicken wire or braid then into a garland to dry.

Storing. Keep onions in a cool, dry place to prevent rotting. Hang them in mesh bags or braids. Cured onion bulbs will store from one month to a year depending on the variety.

Also of interest:

How to Grow Onions

Bulb Onion Growing: Day Length and Temperature

How to Grow Onion Sets

Growing Bulb Onions: Pick the Right Variety for Your Garden

Onion Seed Starting Tips

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Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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