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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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    • Thanks for reading Harvest to Table. All of the articles on onion growing can be found by going to the Topic Index and clicking on onions. The growing requirements for onions and tips will be found there.

  1. I have small flies that land on top of onion tops.. They turn yellow. Can’t find any information on how to get rid of them

    • The flies on your onions may be the adults of the onion maggot — a white maggots, 1/3 inch (8.5 mm) long that burrows into onion bulbs. These pests attack onions, shallots, garlic, and leeks. Yellow sticky traps will capture the flies. Tanglefoot — a product you can get at a garden center — is also a good organic substance that can be sprayed or spread on stems to catch the flies and anything else that lands on it. You can also sprinkle ground cayenne pepper, ginger, dill, or chili powder around plants to keep to female flies from laying eggs.

      • I covered my onions last night with miracle grow.I guess I’ll wash it down flat. I appreciate the education I just received. Thank you greatly.

        • Thanks for reading Harvest to Table. Feel free to be in touch on any growing questions that come up. Happy Gardening!

          • I peeled my onion sets b4 planting this year & results were good. Sets had white dots on some of the red onions. This yrs harvest had less bugs or rotted onions. Was removing outer skin Actuly the reason crop was better or just luck?

          • There are many factors that go into crop success. Peeling the outer skins of sets may well be one reason you had success. Removing the outer skin may allow onions to sprout more quickly. The white dots could have been mold; it’s always best to check sets before planting to be sure they are full and plump (not dry) and mold, mildew, and disease-free.

    • Beet, carrot, and onion seedlings need to be thinned by hand. Thinning the seedlings to 3 to 4 inches (7-10 cm) apart is the best way to get larger roots at harvest. You must thin by hand either nipping off the weakest seedlings with your fingernails or use a small scissors. This is a laborious and time consuming process. The alternative is buy these seeds on seed tape or ribbons; the seed is attached to a paper tape and pre-spaced for you by the seed company. A bit more expensive but less work for you.

  2. The tops of my onions are exposed. I was told to apply nitrogen but I’m concerned about damaging the onion. Should the bulbs be exposed?

    • As onion bulbs enlarge they often emerge from the soil. As long as the weather is dry but not too hot, this is not a problem. If the weather is very hot and dry, you can mound loose compost mulch up over the bulbs to shield them from sunscald. If the bulbs are formed and you are mid way or more to harvest, there is no reason to feed the plants nitrogen. If you do feed the plants nitrogen, use a mild formula such as compost tea or dilute fish emulsion–low nitrogen.

      • Thanks for your help. This is our first attempt at gardening. We have enjoyed our spinach, lettuce, cucumbers, peppers and squash so far this year. We plan on making pickles within the next two weeks.

  3. Thanks for the awesome tips, my first year for yellow onions and I’m scared I will not know what to do. Thanks for the information I feel more confident now.


    • If you have had not pest or disease problems in the current location and if you are constantly renewing the soil with the addition of aged compost then you can continue to plant in the same spot. Onions will deplete the soil of nutrients so it is important to continue to renew soil nutrients with the addition of compost or a commercial organic planting mix (add several inches to the planting bed each year). If disease or pests appear, it is best to rotate the onions to another planting area for at least 3 years.

    • You only need one seed to produce an onion bulb. However, most gardeners plant a few more seeds than they need when it comes to harvest; this allows for seed and plant failure over the course of the growing season. To produce an onion bulb you can plant a seed or a set–a set is a small onion bulb (it was grown last season–and gives the grower a head start on seed starting). You can find seeds or sets at a nearby garden center or nursery.

    • Pine straw–dry dropped pine needles–can be used as a mulch throughout the garden. Keep in mind that pine straw if allowed to decompose or if turned under in planting beds can make the planting bed more acidic. You can use pine straw around onions to keep weeds down and to slow soil moisture evaporation in hot weather. Like all mulches, don’t tuck the pine straw in too close to onion leaves–this is important to avoid leaf rot. Check your soil pH before you turn pine straw under.

  4. This is the best advice I’ve found about growing onions.
    I’ve grown a few vegetables and fruit but this is my first time growing onions.
    Thank you

  5. We have a pot of green onions that are doing quite well . My question is this – after they have grown tall a bulb like flower extends from the top . Are they edible ? Or is it just a seed pod ?

  6. do onions require copper? my onions rot from the bottom. and a big stem grows in the middle of onion why does it
    do that? thank you

    • Be sure you are growing the right onion variety for your regions–short day, long day, or day-neutral; also be sure to harvest at the number of days to maturity–don’t let the bulbs linger in the ground or new growth will be triggered. Copperas a micro-nutrient will vary from soil to soil. Look for a pdf available from the University of Minnesota Extension called Copper for Crop Production copper-for-crop-production.pdf

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