Ultimate Guide to Growing Onions

Onions are one of the two most well-known alliums – garlic being the other. Onions form bulbs of white, red, or yellow, depending on the variety.


Onion bulb formation depends on the number of daylight hours, or photoperiod, an onion plant is exposed to. Onions are classified into 4 basic groups, depending on the hours they require to form bulbs:

Short day varieties: 12-13 hours
Intermediate day varieties: 13.5 to 14 hours
Long day varieties: 14.5 to 15 hours
Very long day varieties: 16 hours or more

In reality, it’s the number of nighttime, or dark, hours that influence whether a particular variety of onion will form a bulb or not.

Gardeners in the northern regions of the United States will need to grow long-day varieties. Intermediate varieties will bulb in the central swath of the U.S., and short day varieties will bulb in southern regions. Very long day varieties will bulb only in places like Canada and Alaska where, during the peak of summer, the sun shines for nearly 24 hours.

Onions are classified as a ‘cool-season crop’ in gardening books. Onions prefer to begin growing in cool weather, and starting them in spring gives them the time they need to grow properly before bulb development. Bulbs will form more readily in warm weather, though less so at temperatures over 100 degrees. Onions are grown through the summer and harvested in the fall when the bulbs have fully developed.

In places with hot summers and mild winters, gardeners will likely be more successful growing onions from late fall through early spring. If you live in a place with hot summers, try growing a short day variety from late fall through the winter. Mulch them well.

Of course, you can grow any variety of onion anywhere, at almost any time of the year, knowing that you’ll always get onion greens and ‘scallions’. But if you want your onions to form bulbs, you’ll need to grow a variety suited to your area, and at the right time of year.

Should you grow from seed, or from sets? Sets are more expensive, per onion, but they will give you full-size onions by summer. The number of varieties available as onion sets are also limited. If you grow from seed, you’ll have a wider variety of onions available to choose from. You won’t be able to harvest full-size onions grown from seed until late fall, but these onions store better.

If you grow from seed in cold-weather areas, either sow the seeds indoors 4 to 6 weeks before the last frost date for your area, and then transplant the seedlings into the garden after all danger of frost has past. Or sow them directly in the garden as early as you can work the ground. This is more risky, since onions take some time to germinate and need consistent moisture. They must never dry out. When they germinate, the seedlings look like tiny blades of grass and may be missed, trampled on, or hoed out.

Onion seedlings, grown from seed.

It is important to get your onions as soon as possible so that onion can produce as many leaves as possible before bulb formation starts in response to the lengthening daylight. If the onion begins to bulb, but it doesn’t have as much support as it needs, it will only form a small bulb. If your onion produces lots of big, green onion leaves that can feed the growth of the bulb, you’re much more likely to get a nice big onion.

Onions should be planted in rows that are 12-16 inches apart. They should be thinned to 4 or 5 inches apart if you intend to allow the onions to bulb and grow large. It is best to thin the onions by late spring. If you wait too long, and choose to thin them out when they’re beginning to crowd each other, you will disturb the roots of those you’re hoping to form into full-size bulb onions.

onions growing too close together

A wee bit on the late side of getting the onions thinned out…

If the onion begins to flower the first growing season, bend the leaves so the flower hangs downward. This will prevent the flower from forming, which would take a great deal of energy away from the growing bulb. Don’t break the flower off completely, though, because onion leaves are hollow and water could collect inside the leaves and cause the onion plant to rot.

Onions are said to repel pests from other plants, but they do not make good companions for legumes. If you plant onions anywhere near beans and peas, the growth of both the beans and peas, and the onions, will be inhibited.

Onions are heavy feeders and need a good, rich soil to grow in. Many gardeners use ash to side-dress their onions, which fertilizes the soil and raises the pH.

Mulching your onions is an absolute must. Onions have shallow roots, and hoeing or even hand weeding are likely to disturb and even injure the roots. Mulching will help keep down weeds, and protect the shallow-growing onions as they form bulbs.

Onions also need consistent water to prevent the bulb from splitting. Mulch will help keep the moisture levels consistent.

The stem of the onions will natural bend over as harvest nears. The leaves will flop to the ground and begin to turn brown. If you want all your onions to finish maturing at the same time, knock over the leaves of the rest of the onions.

If you want to store your onions for several months, through the winter, the closer to autumn that they finish maturing, the more likely they are to store well. Onions grown from sets will probably harvest in mid to late summer, and are not likely to store well over the winter. Onions grown from seed will take longer to harvest, and will mature in the fall. These are the onions you should store for winter.

After pulling your onions, allow them to dry, or cure, in a warm, dry, well-ventilated area out of the sun for4-6 weeks. Then store them in a cool, dry place.

Many people are confused about the difference between shallots, scallions, onions, and green onions. Click here for an in-depth guide to the differences between these types of alliums.

Check out the growing guides for others in the allium family:

Scallions & Bunching onions
Elephant Garlic
Onion/Common Chives
Chinese/Garlic Chives

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