Project Guide

How to Grow Onions

How to Grow Onions
Onions sprouting in soil.

Onions are a cool-season crop. They can be planted as soon as the soil is workable – as early as four weeks before the last frost in your area. Onions are sensitive to light and temperature. In order to grow large onion bulbs, they need to get enough hours of daylight. Onions need full sun and at least 13 to 16 hours of light daily during bulb formation. In order to get a harvest during the summer, the onions must be planted as soon as your soil is tillable in the spring.

Onion Seed and Planting Sets
A pile of onion bulb sets.

Onions are available to plant from seed or in sets. Onion sets are dormant onion bulbs that are 1/2 to 1-inch in diameter and available for purchase from garden centers in late winter or early spring. The most popular varieties of sweet onions, yellow onions, red onions and more should be available at your local Garden Center. Other special varieties are available online. 

Onion seeds are available year-round and are less expensive than sets. However, seeds take longer to grow. Since onions already take a long time to mature, you can start them indoors. An onion’s average growth rate is 100 to 175 days till maturity. Whether in sets or seeds, here are a few varieties to consider:

  • Texas Super Sweet, Vidalia Onions, Yellow Granex and Southern Belle Red: Generally grown in the South and planted in the fall for harvest in spring. These have short daylight needs: 10 to 12 hours daily. 
  • Candy, Red Candy Apple and Super Star: Grown in the Midwest. These varieties have medium daylight needs: 12 to 14 hours daily.
  • Northern Candy, Copra, and Red Candy Apple: Grown in the Northeast. These have long daylight needs: 14 to 16 hours daily. 

It is possible to grow onion tops from a sprouted store-bought onion. You will not get new onions from it, but you can eat the sprouted greens. For best results, use fresh onion sets, onion plants or seeds. 

How to Plant Onions
A person planting onion sets.
  • If starting onions from seed, start indoors six weeks before last frost date.
  • When the outdoor temperature does not dip below 20 degrees Fahrenheit (-6 degrees Celsius), you can get the ground ready for planting.
  • Onions need full sun. Select a location where your onions won’t be shaded by other plants.
  • Onions need well-drained, loose and nitrogen-rich soil. Till or turn your garden soil as soon as it is workable in the spring. 
  • Onions are heavy feeders; they need constant nourishment in order to form large bulbs. Add compost and fertilizer to the soil before planting onion seedlings or sets. 
  • Plant onion sets or seedlings about 3/4-inch deep and spaced four to five inches apart in rows 12 to 18 inches apart.
  • Mulch between the rows of onions to help retain moisture and stifle weeds.
How to Care for Onions
Ripe onions in the ground.

Once you get the onions in the ground, they need weekly care until they are established.

  • Fertilize every few weeks. 
  • When the onions began to push the soil and you see the bulb surfacing, discontinue fertilizing. Do not pack the soil back around the onions. The bulb needs to emerge above the soil.
  • Onions need one inch of water per week (including rain water). 
How to Harvest Onions
Ripe onions curing on the ground.

Now that the wait is almost over, it’s time to harvest and cure the onions. But how do you know that your onions are mature? Typically, onions are harvested in late summer, before cool weather. There are a few signs that your onions are nearly harvest ready:

  • Some of your onions will flower. When an onion sends up a flower, this means they have stopped growing. Harvest these onions and use them immediately, since they will not store well.
  • As onions mature, the tops will turn yellow and begin to fall over. You can speed up the ripening process by bending the tops or stomping on them.
  • Lightly loosen the soil around the bulbs to expedite drying.
  • When tops are brown, pull the onions. If the weather forecast predicts dry weather, leave the pulled onions on the garden bed to cure. Handle them with care. They are not fully cured yet and the slightest bump will make it easy for rot to set in.
  • Once the onions have dried for a few days, the outer skins will be papery. Clip the roots and cut the tops back to 1 inch.
  • The onions need to dry for several weeks before you store them in a root cellar or pantry. Spread them out on a screen off the ground in a cool dry area until fully cured.
  • Store onions at 40 to 50 degrees Fahrenheit in a breathable mesh bag.
How to Grow Onions as Companion Plants
Onions and lettuce growing in a vegetable garden

Onions make a fine companion plant. Use a hedge of onions to protect spring peas from pesky animals. Save a little space on the outside of your spring peas planting area for an edging of onions planted 4 to 5 inches apart.

Some ideas for companion planting with onions: 

  • Use onions to mark varieties of slow-growing carrots, radishes or beets. Plant a band of onions between each variety.
  • Plant onions and lettuce in alternate rows, so the lettuce grows into a “green mulch” that shades out weeds. Harvesting the lettuce when it is young will give the onions the room they need to grow into big bulbs in late summer.
  • Use onions as a vertical element in mixed containers, or plant them in small groups behind pansies, calendulas or other annual flowers. Their neat, upright form makes them an edible ornamental.

Tip: All types of onions can be used as scallions when harvested young. If you love fresh green onions, grow them from sets planted an inch apart. To stretch your scallion season, hold back some sets for midsummer planting.

Whether you need the right tools, seeds or garden soil, The Home Depot delivers online orders when and where you need them.