Growing Onions

Planting, Growing, and Harvesting Onions

Onions
Pixabay

Onions are a cold-season crop, easy to grow because of their hardiness. Here’s how to grow an endless supply of onions in your garden!

Typically, onions are planted early in the spring and harvested in the fall after their tops begin to die back. In the southern U.S., some onion varieties can be planting in the fall.

Should I Grow Onions from Seed or from Sets?

We recommend using onion sets, which can be planted without worry of frost damage and have a higher success rate than planting from onion seeds or transplants. Onion sets are small onion bulbs that are sold specifically for gardening. Once planted, they develop into a full-size bulb after about 3-½ months.

Onion plants grow well in raised beds or raised rows at least 4 inches high. If you’d prefer to start your onions from seeds, check out our tips for growing onions from seed indoors.

Planting

When to Plant Onions

  • Generally speaking, plant onion sets outdoors when the weather is cool—not cold. Ideally, outdoor temperatures shouldn’t dip below 28°F (-2°C) after planting.
    • In regions with a frigid winter, plant onions as soon as the ground can be worked in the spring—usually late March or April.
    • In milder regions, onions can be planted in the late fall or winter. They will sit dormant during the cool season, but will be primed and ready to grow as soon as the longer, milder days of spring arrive. 
  • If planting from seeds, start them indoors about 6 weeks before you plan to transplant them to the garden. Onion seeds need temperatures of at least 50°F (10°C) to germinate properly.

Preparing the Planting Site

  • Select a location with full sun, where your onions won’t be shaded by other plants.
  • Soil needs to be well-drained, loose, and rich in nitrogen; compacted, rocky, or clay-heavy soil affects bulb development.
  • Add aged manure or compost to the soil in early spring, before planting. Onion plants are heavy feeders and need constant nourishment to produce big bulbs.
  • At planting time, mix in some nitrogen fertilizer.
  • Practice crop rotation with onions. In other words, don’t plant them in the same location year after year, as this can encourage the spread of diseases that affect the crop.

Onions in a row

How to Plant Onions

  • Because they mature much faster (and are less work overall), we recommend growing onions from onion sets (i.e., small onion bulbs) rather than from seeds. However, in mild regions with a long growing season, seeds are an option as well.
    • Tip: Choose onion sets that are around ¾-inch in diameter and no bigger. Larger ones may produce stiff necks and go to seed too quickly.
  • If planting from seed, bear in mind that onion seeds are short-lived, so start with fresh seeds each year. Start seeds indoors about 6 weeks before transplanting to the garden.
  • When planting onion sets, plant them between 2 and 6 inches apart, and don’t bury them more than 1 inch under the soil.
  • When planting transplants into the garden, space plants 4 to 5 inches apart in rows 12 to 18 inches apart.
  • Add straw mulch between rows of onions. This will help retain moisture and stifle weeds.
  • Find more tips for planting onions in different regions and soils.

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.

Check out this video to learn how to plant onions. 

Care

How to Care for Onions

  • Think of onions as a leaf crop, not a root crop.
  • Fertilize every few weeks with nitrogen to get big bulbs. Cease 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, onion plants do not need consistent watering if mulch is used. About one inch of water per week (including rain water) is sufficient. If you want sweeter onions, water more.
  • Onions will look healthy even if they are bone dry, so be sure to water during drought conditions.

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.
  • Onion Maggots: Cover your emerging onion crop with a fine mesh netting. Seal it by mounding soil around the edges. The onion maggot 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.

Harvest/Storage

How to Harvest Onions

  • Pull any onions that send up flower stalks; this means that the onions have stopped growing. These onions will not store well but can be used in recipes within a few days.
  • 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 them to speed the final ripening process.
  • Loosen the soil around the bulbs to encourage drying.
  • When tops are brown, pull the onions.
  • Be sure to harvest in late summer, before cool weather. Mature onions may spoil in fall weather.

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

How to Store Onions

  • Clip the roots and cut the tops back to 1 inch (but leave the tops on if you are planning to braid the onions).
  • Let the onions cure on dry ground for a few days, weather permitting. Always handle them very carefully—the slightest bruise will encourage rot to set in.
  • Allow onions to dry for several weeks before you store them in a root cellar or any other storage area. Spread them out on an open screen off the ground to dry.
  • Store at 40 to 50°F (4 to 10°C) in braids or with the stems removed in a mesh bag or nylon stocking.
  • Mature, dry-skinned bulbs like it cool and dry.
  • 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. Eat the sweet varieties first and save the more pungent onions for later.

Spacing for Onions

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

Recommended Varieties

Wit & Wisdom

  • Practice crop rotation with onions. Learn more about crop rotation.
  • 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.
  • 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’s skin very thin,
Mild winter coming in;
Onion’s skin thick and tough,
Coming winter cold and rough.

Recipes

Cooking Notes

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.

Growing Onions

Botanical Name Allium cepa
Plant Type Vegetable
Sun Exposure Full Sun
Soil Type Any, Loamy
Soil pH Neutral
Bloom Time
Flower Color
Hardiness Zones 3, 4, 5, 6, 7, 8, 9
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