Onion Plant Profile

onions growing

The Spruce / K. Dave

The common onion (Allium cepa) is a biennial bulb that is closely related to garlic (Allium satvium), shallots (A. ascalonicum) and chives (A. schoenoprasum). Onions have hollow, tubular blue-green leaves that emerge from a bulb that is actually a modified leaf structure with many layers. A shallow network of roots extend from the bottom of the bulb, and the onion bulb itself may push partially above ground as the plant matures.

Onions have a reputation for being difficult to grow, but with a little practice, most gardeners can do it successfully. They can be planted as seeds, as transplants (small seedling onions that have just sprouted), or as "sets" (small onion bulbs that are about to begin their second, final year of growth). It is more common to plant onions as sets, which have a good success rate and will develop into full-sized onions after several months. Since they are in their second year of growth, onions planted from sets may send up flower stalks near the end of the growing season.

In colder climates, onion sets are usually planted in the spring when the weather is still cool but not frigid—above 28 degrees Fahrenheit. In warmer climates, onion sets are often planted in the fall, where they will remain dormant through the winter and begin growth in the spring. It takes about 3 1/2 months for the sets to mature into full-sized onions.

If growing from seeds, onion seeds are usually planted indoors at least six weeks before outdoor planting time. Onion seedling transplants need outdoor temperatures above 50 degrees Fahrenheit before they can be moved into the garden.

Botanical Name Allium cepa
Common Name Onion
Plant Type Biennial bulb, usually grown as an annual
Size 12 to 18 in. tall; 6- to 12-in. spread
Sun Exposure Full sun
Soil Type Rich, well-drained
Soil pH Slightly acidic to neutral (6.0 to 7.0 )
Hardiness Zones 5-10 (USDA); grown as an annual everywhere
Native Area Uncertain; perhaps central Asia
Toxicity All parts are toxic to dogs and cats
planting onion bulbs

The Spruce / K. Dave

harvested onions

The Spruce / K. Dave

onions poking out of the ground

The Spruce / K. Dave

onions ready for harvest

The Spruce / K. Dave

How to Plant Onions

The best onion sets will be about the size of a marble. Larger bulbs have a tendency to bolt (set flowers) too early. If planting onions from sets, place them about 1 to 2 inches deep, spaced 2 to 6 inches apart. Rows should be spaced 12 to 18 inches apart.

Seedling transplants should be spaced 4 to 5 inches apart, also in rows spaced 12- to 18-inches apart. Seedling transplants tend to produce larger onions, since sets are still in dormancy, while transplants are already primed to grow vigorously.

Onion Care

Light

Onions need full sun—at least six hours per day—in order to properly grow. With onions, the more sunlight the better.

Soil

Proper soil is the key element to growing onions successfully. The soil needs to be extremely well-drained—even sandy—but also needs to have a good deal of well-decomposed organic material in it. Onions prefer a slightly acidic to neutral pH—6.0 to 7.0.

Water

Onions need regular water to support the swelling of the bulbs. Give them 1 inch of water per week, but don't overwater or allow the bulbs to sit in soggy soil, since this can lead to bulb rot.

Temperature and Humidity

One reason onions are considered somewhat hard to grow is that they are cool-season vegetables but also take a fairly long time to mature (90 days or more). Further, they do not really begin growing well until outdoor temps have reached an average of 50 degrees or so. This is why onions are generally planted from sets rather than seeds—seeds simply don't have enough ideal growing time to fully mature.

Optimal growing conditions for onion foliage is 68 to 77 degrees Fahrenheit. This will lead to rapid, full growth of the edible bulbs.

Fertilizer

Onions are fairly heavy feeders. Fertilize them every few weeks with a high-nitrogen fertilizer to support leaf growth, which will produce big bulbs (onion bulbs are actually modified leaf structures). Withhold feeding at the point where the onion bulbs begin to push the soil away.

Are Onions Toxic?

Onions contain a compound known as N-propyl disulfide, which causes a breakdown of red blood cells in dogs and cats. A pet that develops a taste for onion leaves or bulbs may suffer serious anemia.

Onion Varieties

There are three types of onions you can choose from. Onion sets or transplants purchased at a local garden center will usually be appropriate for your climate, but when buying mail-order seeds, make sure to choose the right variety based on your climate:

  • Short-day onions will begin forming bulbs when there are 10 to 12 hours of daylight each day. They work well in southern regions where summer daylight is comparatively short. Some common short-day onions include 'Southern Belle', 'White Bermuda', 'Granex', and 'Cipollini'.
  • Long-day onions begin forming bulbs when there are 14 to 16 hours of daylight per day. They are good for northern climates where the summer days are relatively long. Some recommended long-day onions include 'Walla Walla', 'Ring Master', 'Red Zeppelin', 'Yellow Sweet Spanish', 'Italian Red Torpedo', and 'Redwing'.
  • Day-neutral onions begin to form bulbs when they experience 12 to 14 hours of daylight each day. They are good for gardeners in the central U.S. Good varieties include 'Red Amposta', 'Early Yellow Globe', 'Cabernet', and 'Superstar'.

Harvesting Onions

The time required for the bulbs to mature depends on the variety and whether they were started from seed or sets. But you can harvest onions at any stage—even seedlings thinned from a row can be used as green onions.

Onion bulbs are fully mature when about half the top leaves have collapsed and when the bulbs’ skins have a papery feel. Bulbs allowed to remain in the ground until 50 percent or more of the green tops have collapsed will store longer.

Once you see that half the leaves have collapsed, very gently coax the remaining leaves down, without breaking them off the bulb. Then allow the bulbs to sit in the ground and cure for a couple of days before you lift them. You’ll have better luck digging up the onion bulbs, rather than pulling them. You don’t have to dig deep—just enough to loosen the remaining roots. Shake and brush off any loose soil and let the bulbs finish curing in a warm, dry place with good air circulation. Leave the leaves on. You can use fresh onions at any time now.

For storing onions, wait until the outside onion skins dry and the neck—the point where the leaves meet the bulbs—starts to shrivel. Then you can store them in a cool, dry location, such as your basement. Onions keep longer in cool temperatures (under 40 degrees Fahrenheit) but should not be allowed to freeze. Store onions in mesh type bags or by braiding the tops together and hanging; just make sure they are getting good air circulation

Common Pests and Diseases

  • Rot: In damp soils, you may encounter neck or stem rot or bulb rot. Avoid rot by making sure there is good soil drainage and air circulation.
  • Splitting: Bulbs will split or double if the soil is allowed to remain dry while the bulbs are forming.
  • Thrips: These small, yellowish-brown flying insects feed on leaves and can cause twisting and curling. Repeated attacks cause the plant to stop growing, so bulbs don’t mature. Plant resistant varieties and don’t plant onion near grain crops. Neem and insecticidal soaps provide temporary control.
  • Onion root maggots: These larvae hatch from eggs laid by brown flies near the base of onion plants. The hatching maggots burrow into the stems, feeding on the plants below the soil and eventually killing the onions. Rotate plants yearly to avoid infestation. Covering new seedlings will prevent eggs from being laid. Diatomaceous earth is also effective.

Growing Onions from Seed

If planting onions from seeds, plant them indoors in trays filled with seed-starter mix at least six weeks, and as much as 12 weeks, before outdoor planting time. Place the tray under artificial grow lights for 10 to 12 hours each day. Keep the potting mix damp but not soggy. When outdoor temperatures are routinely above 50 degrees Fahrenheit, transplant the seedlings into the garden.