How to Plant Onions: A Complete Guide for Success

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Onions are one of the most popular vegetables grown in home gardens. Onions are easy to grow if you give them the right conditions and pick varieties that grow well in your region. Understanding how to plant onions correctly, how to avoid common pests and diseases, and which varieties will do well in your area will lead to a bountiful onion harvest from your own garden.

Choose Your Onion Planting Method

Onions can be grown from seeds, sets, or plants. Each planting method has pros and cons that should be considered for your particular situation.

Seeds

Growing onions from seeds is the least expensive method. Although growing onions from seeds may save money, it is by far the most difficult method of the three options. Germination can be difficult, plant growth can be slow, and weeds can easily choke out the tiny seedlings.

In northern climates, onion seeds are usually started indoors about 6 weeks prior to the outdoor soil temperatures reaching 50 degrees Fahrenheit.

If you choose to plant from seeds directly in the ground, you can plant the seeds as soon as the ground can be worked in the spring. Plant seeds in rows 12 to 15 inches apart. Cover the seeds with 1/2 inch of soil. When the seedlings are 2-4 inches tall, thin them so they are spaced 2 to 3 inches apart. Onions grown from seed will take a full growing season to reach considerable size.

Sets

Onions sets are small onion bulbs that were grown the previous year, harvested, stored through the winter, and then distributed to garden centers and stores the following year. Usually, onion sets are sold simply as red, white, or yellow. Specific onions varieties are not available. Because the variety is not known, the flavor, use and storage properties can vary considerably.

Plants

Plants are onion transplants that are grown in southern areas of the United States in the winter. These young plants are pulled and bundled in bunches of 50 or 100 plants and then shipped to garden centers or growers in early spring. Specific onion varieties are available for purchase using this method.

I have had great production with onion plants here at the East Fork garden. I like to choose specific varieties that I know grow well in my local climate. Onion plants are my first choice because I can choose varieties that store well for extended use.

Long-Day, Intermediate-Day, and Short-Day Onions

Onions rely on day length to begin bulb formation. As soon as the day length reached a certain number of hours, onion plants stop using energy to form leaves and start using energy to grow larger bulbs. Each different type of onion has different bulb formation light requirements.

Long-Day Onions

• Bulb formation begins at 14-16 hours of daylight.
• Grown mostly at northern latitudes (Zone 6 and colder).
• Usually planted in late winter to early spring and mature in late summer.
• Long-day onions come in sweet, specialty, and storage varieties.

Long-Day Onion Varieties

• Patterson
• Red Carpet
• Rossa di Milano
• Walla Walla Sweet
• Powell
• Bridger
• Redwing
• Frontier

Intermediate-Day Onions

• Bulb formation begins at 12-14 hours of daylight.
• Best production in zones 5 and 6, but can be grown in any growing zone.
• Planted in early spring in northern climates, spring in central areas, or fall and winter in mild winter climates.
• Usually sweet onions.

Intermediate-Day Onion Varieties

• Calibra
• Cabernet
• Sierra Blanca
• Candy
• Monastrell
• Scout
• Expression

Short-Day Onions

• Bulb formation begins at 10-12 hours of daylight.
• Grown in mild winter climates (zones 7 and warmer).
• Usually planted in fall and mature in late spring.

Short-Day Onion Varieties

• White Castle
• Madalyn
• Red Rock
• Yellow Granex
• Gabriella

Site Selection for Growing Onions

Onions will grow best in an area with full sun. If you try to grow onions in the shade, the bulbs will not form correctly. Avoid planting in areas where you have grown garlic or onions in the last 2-3 years as diseases and pests can linger in the soil.

Soil Requirements for Onions

Onions love rich, loose, well drained, fertile soil with plenty of organic matter. Rocky or compacted soil may require some preparation before planting onions. If you are growing in heavy clay soil, make sure to add in some compost before planting, or your onion bulb growth may be hindered.

Onions love soil that is extremely well drained, even sandy. Good drainage is important so that your onions do not rot. Onions prefer a soil pH that is neutral or slightly acidic.

When to Plant Onions

In northern climates, it is best to plant onions outside in the late winter or early spring. As soon as your soil dries out enough to plant, you can plant onions. You do not need to wait until your last frost. Onions can withstand frost.

In southern climates, onions are planted in the fall and grown throughout the winter. As soon as temperatures cool in the fall, plant onions and grow them throughout the mild winter.

How to Plant Onions

Planting methods are slightly different depending on if you are using seeds, sets, or plants.

How to Plant Onion Seeds

Plant onion seeds directly in the ground in southern climates with long growing seasons or start them indoors 6 weeks before going outside in northern climates. Thin seedlings after they are 2-4 inches tall, to a 2-3 inch spacing if you are growing them for large bulbs. If you are growing them for green onions, you can leave them tightly spaced.

How to Plant Onion Sets

Plant onion sets in late winter or early spring as soon as the soil can be worked. Simply make a hole with your finger in the soil and drop the onion set into the hole with the pointy side up. Plant sets 1 inch deep in rows 12 to 15 inches apart. If you are growing for bulb production, space them 3 inches apart.

How to Plant Onion Plants

Plant onion plants in late winter or early spring. You can plant onions as soon as the soil can be worked. Plant healthy immature bulbs 1-2 inches deep (with the green tops up) in rows 12 to 15 inches apart. To produce large onions, place the plants 3 inches apart.

Common Onion Pests, Plant Diseases, and Weeds

Onion Rot Maggots: Root maggot larvae hatch from eggs laid by brown flies. They emerge near the base of the onion plants and burrow into the stems. Onion maggots feed on the plants and eventually kill them. The best defense against root maggots is crop rotation. If you rotate your onions to a different location each year, any larvae that hatch will not be able to find your onions.

Thrips: Thrips are small, yellow-brown flying insects that feed on onion leaves and can cause twisting and curling. Many onions varieties are resistant to thrips, so it is best to select one of these varieties.

Rot: If your onions are sitting in wet soil for too long, they can begin to rot. Good soil drainage and air circulation are the cure for rot.

Weeds: It is important to keep your onions weeded. Weed competition can hinder bulb formation and cause your onions to stay small. Mulching with grass cuttings or leaves will reduce weeds and hold in moisture.

Onion Watering Requirements

Onions need approximately 1 inch of water per week to form large bulbs. Keeping the soil consistently watered without too much or too little water will produce the best results.

Harvesting and Storage

Harvest onions at any stage depending on your desired use. Harvest green onions during thinning or harvest small or large bulbs for immediate use. If you plan to store onions as dry bulbs, it is a good idea to let them grow until their leaves begin to fall over and dry out.

Once onion leaves have fallen, and the bulbs have cured in the ground for 2-3 days, you can gently pull or dig up the onions. Brush off any loose soil and set the onions outside in a single layer in a warm, dry place until the leaves have dried and shriveled. This usually takes 5-7 days.

After the onions have been cured outside for 5-7 days, you can either cut the stems 2-3 inches above the onion bulb and place in storage, or you can braid the onion tops for hanging in storage. Check your onions in storage often and remove any that have started to rot or sprout.

If you choose a good storage variety, and store them in a cool, dry, dark place your onions will last for a long time. I have gone 6 months or more by storing onions in my unheated basement. Growing onions in your home garden is extremely rewarding! It is so nice to use home-grown onions instead of going to the grocery store. I hope using these tips gives you success in growing onions!

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Halli

Halli is a 4th generation gardener. Her great-grandmother had a beautiful garden and orchard and grew all the food to feed her large family.  Halli has been growing her own garden for over 20 years. Growing her own food, improving the soil, and feeding her family home-cooked meals are her passions.  Halli loves to connect with old, time-honored ways of cooking, gardening, and living.

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