How To Plant And Grow Onions

Onions have been grown and bred for thousands of years, leading to the great diversity of plants available today, from sweet to pungent flavors, bulbing to bunching types, and regional varieties with their own distinctive flavors and uses. Onions are easy to grow and can be planted from seed, transplants, or small bulbs called “sets.” As they mature, onion plants form bulbs in response to changing temperatures and day length. As such, local climate and latitude dictate which varieties will produce bulbs in each region, as well as when and how to plant onions.

Plant Attributes

Common Name	Onion
Botanical Name	Allium cepa
Family	Amaryllidaceae
Plant Type	Biennial, Bulb
Mature Size	12-18 in. tall, 6-12 in. wide
Sun Exposure	Full Sun
Soil Type	Loamy, Well-Drained
Soil pH	Neutral Soil pH
Bloom Time	Summer, Fall
Flower Color	White, Purple
Hardiness Zones	4-10 (USDA)
Native Area	Central Asia
Toxicity	Toxic to Dogs, Cats, and Horses

Onion Care

Onions are a cool-season crop that can be grown throughout the United States and southern Canada, as far north as USDA hardiness zone 3. They are planted in the fall or spring, depending on location. Onions are a biennial plant, producing foliage in the first year of growth followed by flowers and seeds the second year. The bulb that we eat is a storage organ, where onion plants store energy that helps the plant overwinter. In the garden setting, onions are grown as an annual crop and the bulbs harvested before they can flower and set seed.

Light

Onions need full sun for highest yields, at least 6 hours a day. 

Soil

They require consistently moist, well-drained soil with a pH between 6.0 and 7.0. Loamy soils high in organic matter are best though onions tolerate sandy and clay soils. Sandy soils require more fertilization and irrigation to maintain even moisture. Clay soils commonly produce hotter onions. Both can be amended with well composted organic matter to improve tilth.

Water

Onion plants have a shallow root system and require even moisture for growth and development. Plants require one inch of rainfall or irrigation per week, ideally delivered through drip irrigation or furrowing. Young, establishing plants and those planted in sandy soils require watering more than once per week. Stop watering when the tops of the plants begin to fall over, a signal the bulbs are ready to harvest.

Temperature And Humidity

Onions are a cool-season crop and tolerate moderate freezes, but when cold snaps arrive, cover beds if temperatures will reach 20°F or below. Though plants tolerate cold, seedlings will grow slowly if soil and air temperatures are too cold. Avoid setting plants out too early. 

Fertilizer 

Onions are heavy feeders and require a good supply of nitrogen, however too much fertilizer can be problematic. Prior to planting, work a balanced (10-10-10) fertilizer into the soil along the intended planting line. Two to three weeks after planting, side dress with a nitrogen fertilizer alongside the row of onions, about six inches away from the plants. Use a fertilizer with ammonium sulfate (21-0-0) in alkaline soils or calcium nitrate (15.5-0-0) in acidic soils, applying at a rate of 1 cup per 20 feet of row. Repeat this application every 2 to 3 weeks until bulbs begin to form, making sure to water the onions after each application.

Types of Onions

Bulb initiation in onions is triggered by changes in day length. The different types of onions are classified according to when they bulb relative to the amount of daylight they receive. It is critical to select an onion variety that matches your region. A wide variety of onions are available within each day-length group, including those with red, yellow, and white skins. 

Short Day Onions

Short day onions begin producing bulbs when daylengths reach 10-12 hours. These varieties are best suited to locations between latitudes of 25 to 35 degrees and are ideal for southern gardens, ensuring a crop before the heat of summer arrives. In northern regions, they bulb too early to produce a good-sized bulb. Short day onions typically have a mild, sweet flavor, and are best eaten fresh. 

Intermediate Day (or Day Neutral) Onions

This group includes varieties that begin the bulbing process when daylengths reach 12-14 hours. Ideal in latitudes 32 to 42 degrees, the range of these onions overlaps that of short- and long-day onions. They perform well through much of the United States, except for the far south. Intermediate day onion varieties tend to be very sweet.

Long Day Onions

Long day onions do not begin bulb formation until day lengths reach 14-16 hours, making them ideal for northern latitudes between 37 and 47 degrees. This group has excellent storage qualities, and many have a strong, pungent flavor. 

Additional Onion Types

Several unique groups of onion have been bred over the centuries, taking on characteristics quite different from regular bulbing onions. Shallots and potato onions belong to the Aggregatum group, (Allium cepa var. aggregatum) and produce clumps of smaller bulbs rather than one large bulb. Most shallot varieties are considered “long day.” Chives, walking onions, leeks, and Welsh onions belong to different species of Allium. 

Other variations come from how the onions are produced. Pearl onions, for example, are short-day onion varieties grown under long-day summer conditions in northern latitudes. They are commonly planted at very high densities and bulb shortly after emergence to produce the small, specialty onions. Green onions (sometimes called scallions) are simply onions harvested at an immature stage. Harvesting yellow, white, or red onions for “green use” is a useful way to thin onion rows and allow adequate space for bulb development in the remaining plants.  

How To Plant Onion Bulbs

Onions can be grown from seed, transplants, or small, immature bulbs called “sets.” Planting time varies depending on the type of start you are using as well as your local climate. In northern gardens, onions are started in the spring for a late summer or fall harvest. Southern gardeners can start onions either in fall or late winter for an early summer harvest. 

Growing Onions from Seed

The most economical method for growing onions is from seed. Seed also provides the greatest availability in terms of variety selection. However, growing onions from seed is also the most difficult, with uneven germination common. You can grow from seed and ensure an even crop by starting seeds indoors 10-12 weeks before the expected transplanting date. To sow seeds directly into the garden, sow seeds in early spring as soon the soil can be worked in northern gardens. In southern gardens, direct sow seed in fall or late winter. 

Growing Onions from Sets

Onions can also be grown from sets, which are immature bulbs started from seed the previous season. Growing from sets is very easy, but plants may be susceptible to bolting. Sets are widely available at garden and agriculture centers or can be ordered from seed catalogues. They are more expensive than seed and the variety selection is extremely limited. Purchase sets that are no more than an inch in diameter.

Growing Onions from Transplants

Onion transplants are seedling onions ordered from seed companies. They come in small bundles of dormant plants, each the thickness of a pencil. Onion transplants produce more reliably than sets and are less prone to bolting. They also tend to be available in a wider variety of cultivars. Both transplants and sets can be planted in the garden when soil temperatures reach 50˚F, about 4-6 weeks before the average last frost in spring. 

Planting Steps

1. Prepare the planting bed by adding fertilizer as described above. Amend soils as needed with well-composted organic matter. Planting in mounded rows helps improve drainage in heavy soils. 
2. Plant transplants or onion sets 1 to 1½ inches deep in rows 12 to 18 inches apart. Space transplants 4 inches apart within the rows. If direct seedling, sow seeds ½ to ¾ inches deep, with 8 to 12 seeds per foot of row. Thin emerging seedlings to 4 inches apart. The thinned onions can be eaten as scallions.
3. Water plantings thoroughly. Continue irrigation and weed regularly but gently as spring weeds emerge.

Harvesting Onions

As onion bulbs near harvest time, the tops of the plant begin to fall over. Harvest the onions when one half to three quarters of the tops have fallen over. Use a spading fork to dig beneath the onions and lift them from the soil. Harvested onions need to cure before storage. Set them in a warm, well-ventilated area for two to four weeks or until the outer layers of the bulb are dry. The neck of the bulb will also tighten. Once cured, you can trim the tops off. Store onions in a cool, dry place. Do not allow bulbs to freeze.

Common Pests & Plant Diseases

While onions are easy to grow, they can at times be affected by pests. In the home garden setting, diseases are seldom a problem. However, good cultural practices can help prevent plant diseases. Plant onions in different parts of the garden each year, a practice called crop rotation. Planting in a well-drained soil or on raised beds is also beneficial.

Thrips and onion maggots are the two most common insect pests. Thrips feed on foliage, causing damaged leaves to take on a silvery color, with specks visible on the leaves. Thrips overwinter in weedy areas and on onion bulbs not removed from the garden. Good sanitation—keeping the planting area free of debris—can help manage this pest.

Onion maggots feed on the roots and bulb, which can lead to bulbs rotting during storage. Crop rotation will help with maggots, as will good sanitation. Onion maggots are attracted to undecomposed organic matter such as manure and rotting foliage. Do not amend soils with fresh manures, use only well composted organic matter. 

Common Problems

The most common problem with onion production is bolting, or premature flowering. When an onion goes to flower, the energy that was being used to produce the bulb is diverted to reproduction and the onion bulb will stop forming. If this happens, harvest the onions and use them like green onions or scallions. 

Onions bolt for a variety of reasons, mostly driven by temperature fluctuations. A period of good growing weather followed by a prolonged cold snap can trick the plants into flowering. Likewise, a cool spring followed by high heat can induce bolting. Bolting can be limited by planting at the right time for your area. If using onion transplants, make sure they are no larger than a pencil. Onion sets should be an inch or less in diameter. 

Onions are a rewarding crop to grow. They take up little space and fit almost anywhere in the garden. You can even tuck them into your flower beds. When selecting varieties to grow, consult growers at your local farmer’s market. They will know the best varieties for your region.