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Growing Onions: Planting, Growing, and Harvesting Onions | The Old Farmer's Almanac

Onions

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Planting, Growing, and Harvesting Onions

The Editors
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Onions are surprisingly easy to grow! They are planted early in the spring and harvested from midsummer through the fall. Whether you start your onions from seed or from sets, there are some tricks of the trade that make the difference between a great crop and a disappointing one. See how to plant, grow, and harvest onions.

Should I Grow Onions from Seed or from Sets?

We prefer planting onion sets over starting them from seeds, simply because the sets establish quickly and are easier to plant.

  • Onion sets are tiny onions that mature in about 14 weeks. They can withstand light freezes and have a higher success rate than direct-sown seeds or transplants. The onion sets look like small bulbs and are sold at gardening stores; once they mature, they develop into a full-size bulb. Choose onion sets with bulbs that are 3/4 of an inch in diameter; larger ones tend to produce stiff necks and go to seed.
  • Of course, starting onions from seed is certainly doable, and may even be necessary in colder regions (Zone 5 and colder). Onions grown from seed require the soil to be at least 50°F to germinate, so these should be started indoors about 6 weeks before transplanting to the garden. If you’d prefer to try this method, check out our tips for growing onions from seed.

Practice crop rotation with onions. Don’t plant them in the same location year after year, as this can encourage the spread of diseases that affect the crop. Learn more about crop rotation.

Planting

Select a location with full sun, where your onions won’t be shaded by other plants. The more energy they can get from the sunlight, the larger their bulbs can grow. In the fall or early spring, mix aged manure or compost into the soil to improve texture. Ensure there are no rocks or debris. Soil needs to be well-draining and loose; compacted soil affects bulb development.

When to Plant Onions

  • In spring, plant onion sets outdoors as soon as the ground can be worked, usually in late March or April, when temperatures are no longer likely to dip below 28°F (-2°C).
  • In spring, start onion seeds indoors about 6 weeks before transplanting to the ground (once the soil is at least 50°F).
  • A fall-planted crop of onions needs at least 4 to 6 weeks of warm temperatures to become established in the ground. They will remain dormant during the cool season, As the temperatures and soil warm again in early spring, the onions come back to life.


Photo credit: YuriyS/GettyImages

How to Plant Onions

Onion plants are heavy feeders and require constant nourishment to produce big bulbs. At planting time, add nitrogen fertilizer. Many organic gardeners will add an inch of compost to the bottom of each row before planting. Or, dig a trench in the soil about 2 inches deep and 3 inches wide, and then fill the trench back in with about an inch of compost.

  • Bury onion sets 2 to 6 inches apart, gently pressing them into loose soil no more than 1 inch deep. (Use the closer spacing if you want to pull immature onions as scallions.)
  • Space transplants 4 to 5 inches apart and rows 12 to 18 inches apart.
  • Set the bulbs with the point end up. Again, don’t bury them more than 1 inch under the soil. It’s important that onions aren’t planted too deep, as this can affect bulb development.
  • Mulch with straw between rows to help retain moisture and stifle weeds.

Find more tips for planting onions in different regions and soils.

Spacing for Onions

Growing

We find it helpful to think of onions as a leaf crop (like lettuce or kale), rather than a root crop (like beets or carrots). Fostering healthy foliage growth ensures that the plants have enough energy to form large bulbs.

  • Ensure immature bulbs stay covered with light mulch to protect them, retain moisture, suppress weeds, and allow air circulation.
  • Do not cover emerging onions.
  • Fertilize every few weeks with nitrogen to get big bulbs. Stop fertilizing when the onions push the soil away and the bulbing process has started. Do not put the soil back around the onions; the bulb needs to emerge above the soil.
  • Generally, if light mulch is used, onion plants do not need consistent watering. About 1 inch of water per square foot per week, including rain water, is sufficient. If you want sweeter onions, water more. To deter bolting, water often during hot spells. 
  • To deter thrips, intercrop onions with tomatoes or carrots in closely alternating rows.

See our video demo to see how to plant and grow perfect onions! 

Harvesting

Pull any onions that send up flower stalks. This means that the bulbs have stopped growing. These onions will not store well, but can be used in recipes within a few days.

  • Spring-planted onions tend to be ready for harvesting by mid-summer. 
  • When onions start to mature, the tops (foliage) become yellow and begin to fall over. At that point, bend the tops down or even stomp on the foliage to speed the final ripening process.
  • Loosen soil around the bulbs to encourage drying.
  • Harvest by late summer in dry weather. (Wet-harvested onions do not cure well and might rot in storage.)
  • When tops are brown, pull the onions. Handle them carefully, as the slightest bruise (now and in storage) will encourage rot. 
  • Cut the roots, Trim the tops back to 1 or 2 inches (but leave the tops on if you are planning to braid the onions together).

ONions. Photo by Rootstocks/Getty ImagesPhoto by Rootstocks/Getty Images

How to Store Onions

  • Set onions on dry ground for a few days to cure, weather permitting, or in a protected place such as a garage or barn. 
  • Once cured, hang onions in a mesh bag or nylon stocking; spread up to two layers deep in a box; or braid and hang them in a cool, dry, well-ventilated area. The ideal temperature range for storage is 40 to 60°F (4 to 15°C). Do not store in a refrigerator, as conditions will be too damp.
  • Check periodically for sprouting or rotting onions and remove them.
  • Don’t store onions with apples or pears, as the ethylene gas produced by the fruits will interrupt the onions’ dormancy. Onions may also spoil the flavor of these fruits (as well as potatoes).
  • A pungent onion will store longer than a sweet onion. Sweet onions have high water content and do not keep well. Eat the sweet varieties first and save the more pungent onions for later.

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

Wit and Wisdom

Can You Plant a Sprouted Onion? 

Yes, you can plant a sprouted onion, though you won’t get more onions from it. You will get lots of tasty green sprouts, however! Here’s how to do it:

  1. Fill a pot with potting soil and make a hole in the middle that is about the depth and width of the onion.
  2. Place the onion in the hole and cover with soil.
  3. Water and put the pot in a sunny spot.
  4. Harvest the green sprouts as needed for cooking.

If you get a sprout with a flower, wait until the flower goes to seed. Save the seeds for planting in the spring.

Onion Cures and Home Remedies

  • In the Middle Ages, it was believed that onion juice could cure baldness, snakebite, and headaches.
  • A generation or two ago, children were treated with a poultice of mashed onions applied as a paste to cover a wound.
  • A whole onion eaten at bedtime was prescribed to break a cold by morning, and sliced onions were placed on the soles of the feet to draw out fever.
  • Early settlers made a 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.

Onion Weather Folklore

Onion’s skin very thin,
Mild winter coming in;
Onion’s skin thick and tough,
Coming winter cold and rough.

Pests/Diseases
  • Thrips: To control thrips—tiny insects about as fat as a sewing needle—take a dark piece of paper into the garden and knock the onion tops against it; if thrips are present, you will spot their tan-colored bodies on the paper. A couple of treatments with insecticidal soap kills them. Follow the package directions. Spray the plants twice, three days apart, and the thrips should disappear. To prevent future infestations, consider using row covers.
  • Onion Maggots: Cover your emerging onion crop with a fine mesh netting or row covers. Seal it by mounding soil around the edges. The onion maggot fly likes to lay its eggs at the base of plants, so the netting should prevent that. You should also keep mulch away because the insects like decaying organic matter, and make sure you completely harvest your onions as the season progresses. Onion maggots are usually a problem in very rainy periods, so these precautions may be unnecessary if you have a dry season.
  • White Rot: This very serious disease starts with infected plant material. Avoid transplants grown in soil-based compost. Stick to starting from onion seed, onion sets and bulbs bought from inspected producers. With white rot, the foliage will yellow and wilt; below ground, white, fluffy fungal growth appears on the base of the bulb. Unfortunately, once rot sets in, there is no solution. Dig up the crop and dispose of it in the trash (do not compost). Avoid growing onions in the same location in future seasons, as it will remain infected for many years.
Cooking Notes

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

Chopping onions can sometimes look like a daunting task: There’s the skin, and the layers… Where to begin? Check out our tips for chopping onions in four easy steps. Onion skins actually have several health benefits, too, so don’t throw them out!

For more tips on using onions in the kitchen and cooking them correctly, click here.

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