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Planting and Growing Onions

Many ripe onion bulbs as background, closeup. yellow onions
Image Credit:
New Africa/Shutterstock
Botanical Name
Allium cepa
Grown as
Biennial
USDA Zones
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
10a
10b
Toxicity
Toxic to Dogs and Cats
Sun Exposure
Full Sun
Water Needs
Moderate
Height
1–3 feet
Spread
Under 1 foot
Soil pH
Neutral
Special Features
Container-Friendly
Deer-resistant
Rabbit-resistant
Resilience
Cold Hardy
Plant Type
Vegetables
Written By: Catherine Boeckmann Executive Digital Editor and Master Gardener

Sweet, sharp, red, white, or yellow, onions are one of the easiest (and most rewarding) veggies to grow and they’re a gardener’s “must have” if you want to make your own salsas, pickles, and other canned goodies. 

In this guide, we’ll peel back the layers of how to grow any sort of onion and bring in a bumper crop of bulbs big enough to bring tears of joy to your eyes!

Planting

When to Plant Onions

  • Onion sets (recommended): Plant sets directly in the ground in early spring as soon as the ground can be worked and the danger of hard freezes has passed. This is typically 2 to 4 weeks before your last spring frost date.
     

  • Seeds: Onion seeds can be started indoors 8 to 10 weeks before your last spring frost. However, some gardeners start them even earlier to get larger transplants. Either way, onion seedlings should be hardened off and transplanted outside in April in most areas, or 2 weeks before the last spring frost when the risk of extreme cold is over for the season.

    If you want to plant a late crop of onions too, sow seeds outdoors 4 to 6 weeks before frost is expected in your area. Seeds should lie dormant during the cool season and sprout in spring.

Where to Plant 

Onions need full sun and should not be planted in areas where they’ll be overshadowed by other veggies. The more energy onions can get from the sun, the larger their bulbs will grow! For best results, make sure the soil at your planting site is well-draining and loose with a slightly acidic to neutral pH, and blend some aged manure or compost into the ground before sowing onion seeds or sets.

How to Plant Onions 

  • Starting seeds (indoors): Fill a shallow tray to the top with seed starting mix, sow onion seeds 1/4 to 1/2-inch deep, lightly water, and then set the tray in a warm and brightly lit location. When the seedlings sprout, thin out the weakest seedlings using fingernail scissors, and harden off and transplant the remaining seedlings outdoors when the weather starts to warm.
  • Starting seeds (outdoors): Prep your planting spot by removing weeds and blending compost or aged manure into the earth. Sow the seeds 1/4 to 1/2-inch deep in rows spaced 12 to 18-inches apart, and thin out the weakest seedlings when they’re a few inches tall so the remaining plants are spaced 3 to 4-inches from each other. Keep in mind that outdoor sowing can lead to patchy planting efforts, so you may want to overseed if you go this route.
  • Planting onion sets (outdoors): Prep your soil as above, then plant onion sets — pointy ends up — 1 to 2-inches deep. Space the sets 3 to 4-inches apart in rows that are 12 to 18-inches from each other, and cover the space between planting rows with a layer of straw mulch to stifle weeds. Don’t cover onions as they grow with more soil as this can interfere with the bulbing process.

Spacing for Onions

Growing

Watering 

Provide onions with about 1-inch of water per per week to keep them growing strong. If you want sweeter onions, water more often. You may need to increase watering during hot spells to prevent bolting, while well-mulched plants may require less frequent watering.

Fertilizing 

Onions are heavy feeders and demand constant nourishment to produce big bulbs. At planting time, I like to blend compost into the soil before sowing onion seeds or sets. Then, I feed my plants every few weeks with a balanced, organic liquid or granular fertilizer following the instructions on the fertilizer packaging, and I only stop fertilizing when my onions begin to bulb and push out of the soil.

Pruning and Maintenance 

Onions don’t need pruning, but it’s wise to keep a lookout for pests and to apply mulch as needed to keep weeds down.

Seasonal Care

Some hardy varieties of onions can potentially be planted in fall, overwintered outdoors, and then harvested in spring. If this thought appeals to you, look for onion varieties that are suited to cold weather and protect your crop with cold frames, thick layers of mulch, or frost blankets. 

Growing in Different Climates

Onion varieties are categorized as “short-day,” “long-day,” or “day-neutral” plants depending on the day length that’s required to trigger bulb growth. It’s important to select the right varieties for your location!

  • Short-day onions grow best in the southern United States and begin to bulb when the sun stays up for about 10 to 12 hours per day. The earlier short-day onions are planted, the larger they’ll get. These onions can potentially be planted in the fall and overwintered outdoors in warm areas.
  • Intermediate, or day-neutral, onions are best suited to slightly cooler climates and begin to bulb when the day length is around 12 to 14 hours. 
  • Long-day onions are ideal for chilly northern areas and begin to bulb when they receive 14 to 16 hours of sun per day.

Harvesting

 

Harvested onions still covered in dirt in the garden . Photo by Rootstocks/Getty Images
Photo by Rootstocks/Getty Images

When to Harvest

Spring-planted onions are usually ready for harvesting by mid- to late summer. You’ll know it’s time to pull in your crop when the onion tops begin to yellow and fall over. When this occurs, you can speed up the ripening and harvesting process by bending the remaining leaves down to the soil.

How to Harvest 

Make sure to harvest onions in dry weather. Wet-harvested onions don’t cure well and are more likely to rot in storage!

To harvest, use a shovel or gardening fork to loosen the soil around onion bulbs and encourage drying. Just be careful that you don’t damage the bulbs while you work. Then, grasp the onion tops and pull them carefully from the ground to avoid bruising.

Tip: Any onions that send up flower stalks should be harvested ASAP. Flowering means the bulbs have stopped growing. These onions won’t store well and should be used right away.

Storage and Preservation

To keep onions fresh, cut the roots back and trim the tops to 1 or 2-inches in length. You can leave the tops in place if you’re planning to braid the onions later on.

This done, lay the trimmed onions on dry ground for a few days to cure if the weather permits. Or place them in a protected and well-ventilated spot that’s shielded from the rain — such as a garage or covered porch.

Once cured, hang onions in a mesh bag or nylon stocking, spread them up to two layers deep in a box, or braid and hang them in a cool, dry, well-ventilated area. A root cellar where temperatures stay between 40 to 60°F is ideal. Although many of us do it, onions don’t keep well in refrigerators as they often run too damp!

To make onions taste milder, soak them in milk or pour boiling water over the slices and let them stand for 20 minutes. Then, rinse with cold water.

Check out this video to learn how to harvest and store onions.

Pests and Problems

Onions contain sulfur-like compounds that give them their pungent scent and naturally repel many common garden pests. But while onions are naturally resistant to many plant problems, harvests can still be troubled by a few issues!

Common Problems 

  • Poor bulbing may occur if onions are grown too closely together or not provided with enough nutrients. Crops may also fail to bulb out if you choose the wrong onion variety for your growing location.
  • Yellowing leaves may indicate nutrient deficiencies or watering problems; however, onion leaves also naturally yellow as harvest time approaches.
  • Pests can still affect onions in some conditions, but they aren’t as likely to target these crops as other veggies.
  • Rot can turn onion bulbs mushy and destroy entire plantings if onions are grown in soggy spots.

Common Pests 

  • Thrips are tiny, sap-sucking insects that feed on many plants, including onions. You can prevent thrips by covering onion crops with floating row covers, while active infestations can be managed with weekly applications of an organic insecticidal soap spray.
  • Onion maggots are more likely to affect onions during extended periods of rain. Like thrips, these pests can be prevented by harvesting all onions at the end of the season and covering newly plantings with row covers.

Plant Diseases

Onions can succumb to white rot and other fungal issues if they’re grown in overcrowded or poorly draining conditions. You can avoid many of these problems by rotating crops around your garden, cleaning up old plant debris at the end of the season, and growing onions from seeds or sets purchased from reliable suppliers.

Types

Onions are divided into “short day,” “long day,” and “day-neutral” categories, and it’s important to select the right onion variety for your specific growing location. Local seed companies and plant nurseries usually offer onion varieties that are well-suited to local areas, but you may also want to look into these popular picks!

Long-day varieties:

  • ‘Yellow Sweet Spanish’: A long-day variety, these onions are prized for their large and rounded, yellowish-white bulbs.
  • ‘Ailsa Craig’: An heirloom plant with pale yellow skin and mild, sweet bulbs, ‘Alisa Craig’ onions are named after a Scottish island and grow up to 8-inches in diameter.

Short-day varieties:

  • ‘Stuttgarter’: Typically sold in sets, this early maturing variety produces yellow bulbs with a slightly flattened shape.
  • ‘Red Burgundy’: A good slicing onion, this variety has a mildly-favored, sweet white interior, but its shelf-life is relatively short.

Day-neutral or intermediate varieties:

  • ‘Candy’: Jumbo, golden bulbs with thick-flesh set ‘Candy’ onions apart from the rest. They also store well!
  • ‘Purplette’: A mini onion variety, ‘Purplette’ bulbs max out at around 1-1/2-inches across and have glossy, burgundy red flesh when raw, which turns a pretty pastel pink when cooked or pickled.

Wit and Wisdom

  • Onions were worshiped in ancient Egypt, where their concentric circles were seen as symbols of eternal life.
  • In the Middle Ages, onion juice was believed to cure baldness, snakebites, and headaches.
  • A generation or two ago, wounds were sometimes treated with a poultice of mashed onions applied as a paste.
  • A whole onion eaten at bedtime was once believed to cure colds by morning, while sliced onions were placed on the soles of people’s feet to draw out fever.
  • Early settlers made cough syrup by steeping raw onion slices in honey overnight.
  • A raw onion rubbed on a bee sting or insect bite will relieve the pain and itching.

Vegetables

About The Author

Catherine Boeckmann

Catherine Boeckmann

Executive Digital Editor and Master Gardener

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana.

Read more from Catherine Boeckmann

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Comments

<span>Annette Bachmeier</span> Sat, 07/27/2013 - 15:39
I did not know how to harvest my large red onions and I noticed they were very large and looked ready to pull. the leaves were still tall and green (2-3 feet). I pulled most of them and now read that I should have waited. What do you recommend I do?
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 07/29/2013 - 11:07

Onions are "mature" when the bulbs are big and the tops begin to turn yellow and fall over. Don't worry though; onions can be harvested at any stage. There is no rule.

 <span>Amor M. Gaokan</span> Fri, 07/26/2013 - 14:03
Have a good day,how many days or months before the onion will harvest?
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 07/29/2013 - 10:26

It depends on the variety and your climate, but the "days to maturity" are generally 100 to 120 days for mature bulbs. 

 <span>Sandy Bews</span> Thu, 07/25/2013 - 20:02
My onion sets grew wonderfully, nice big onions. Now I'm finding that the onions have split, not into two onions, but layers have split open and they aren't going to be usable! I don't know why??
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 07/26/2013 - 12:47

The usual reason for split bulbs is moisture stress, such as dry soil or waterlogged soil, especially when bulbs begin to form and enlarge. Even moisture is very important; Keep onions evenly moist, but not waterlogged, throughout the growing season.

 <span>Rajendiran</span> Mon, 07/22/2013 - 10:39
i am from india from tamil nadu. my onion plants leafs are became yellow and it couldn't able to continue its growth and became waste. the age of the plant is abt 40 days. how to make them to regrow...
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 07/22/2013 - 12:29

The Old Farmer's Almanac is a North American publication addressing needs of North American readers. We suggest you speak to gardening experts in India who understand your climate and conditions!

 <span>Mandalu9</span> Thu, 07/18/2013 - 12:14
My onions are extremely tall and have fallen over, but it started only 1 1/2 months after being planted. They're still green, they just can't hold themselves up and are about 2 ft tall. I feel like this is stunting bulb growth. What can I do without damaging or interrupting the other plants in my raised bed?
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 07/19/2013 - 10:05

The bulbs are probably small. You can eat them now or let them keep growing as long as the tops are green.  It will just take longer. You may be harvesting in August. Onions send out a leaf for each layer on the bulb. If some of the leaves fall over, it will send out more. No worries.