How to Grow Onions

Updated on February 5, 2020
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Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

What are Onions?

Onions (Allium cepa) are a member of the allium family. They are related to leeks, garlic and chives. Onions are probably one of the oldest foods known to humankind so scientists are unsure where they originated.

There is evidence that they were a part of Bronze Age diets. The construction workers who built the Egyptian pyramids ate them. Ancient Greek athletes ate onions in the belief that it would improve their performance. Roman gladiators rubbed onions on sore muscles. During the Middle Ages when coins were scarce, people paid their rent in onions.

For those of us who like to cook, onions are part of the aromatics used in the bases of most soups and stews. Some brave souls, myself included, love to eat them raw in salads or on top of our burgers.

What is the Difference Between Long Day Onions and Short Day Onions?

The term “day length” refers to the number of hours of sunlight during a 24 hour period. Certain plants will only start growing and maturing when they receive a minimum amount of sunlight each day.

Onions come in three types. Long Day onions originated in Northern Europe where summer days are long. They will only start to form a bulb when they receive at least 14 hours of sunlight per day. Intermediate onions start producing bulbs with slightly less sunlight, 12 to 13 hours. They are commonly found in Southern Europe and Northern Africa. Short Day onions are a recent development. They only require 11 to 12 hours of sunlight to begin forming their bulbs.

Check the day length of each variety before you plant.

How to Grow Onions From Seed

Onions are easy to grow but can be tricky to get started. If you grow them from seeds, you have more varieties available to you. Two things that you should be aware of if you decide to grow your onions from seed: the seed must be fresh (or germination will be poor to non-existent) and it can take up to four months for your plants to mature and form bulbs. You will need to start your seeds indoors four to six weeks before your last frost.

How to Grow Onions From Starts

Most gardeners grow their onions from starts. Starts are seedlings that have been grown in greenhouses by local nurseries and mail-order catalogs. The advantages of using starts is that they mature sooner than plants you started from seed and they are less prone to disease. The downside is that only a few varieties are available as starts. Another drawback is that starts tend to bolt, or flower, prematurely.

How to Prepare Your Soil to Grow Onions

Onions need full sun and well-drained soil. If you have clay soil, add a little sand or compost to give texture and drainage to your soil. Raised beds or rows are an excellent way to grow healthy onions.

Onions are also heavy feeders, meaning they need a nutrient rich soil. It’s best to add your soil amendments in the previous fall so that the soil has a chance to absorb them.

How to Grow Onions

You can set out your seedlings, if growing from seed, or plant starts in the spring as soon as the soil starts to warm up. In mild winter areas, you can plant your onions in the fall for spring harvest. Onions are cool season plants at first. They need cooler temperatures to form their foliage. As the temperatures warm, they switch to forming their bulbs.

The easiest way to plant your starts is using a dibble to poke 2 inch holes in the soil in which to set your plants. I use my dibble to plant starts and small bulbs. It is marked to show how deep you are planting so that you always get the correct depth when you poke your hole.

Plan on 4 to 5 inches spacing between plants to allow the bulbs space to grow and 18 inches between rows.

At a minimum, you should be giving your onions 1 inch of water per week. Too little water, and the bulbs will dry out and split. Make sure you are watering right at the roots and not from overhead. Drip irrigation or a long handled watering wand is helpful. To help keep your soil moist, put down a 2 inch layer of mulch around your plants.

Mulch also has the advantage that it prevents weeds from sprouting. Onions do not compete well with weeds. Save yourself the backbreaking labor of weeding by hand or hoe and use mulch instead.

How to Harvest and Store Onions

As your onion bulbs grow, they will begin to push their way up out of the soil. Do not try to push them back in or cover them up. They are fine. Don’t harvest them yet. They aren’t ready.

If you notice that some of your plants are sending up flower stalks, cut them off. You don’t want your plants to waste energy in blooming and producing seed. You want them to concentrate on making bulbs.

Onions are ready to harvest when their foliage turns brown and falls over. Carefully dig them up and then lay them in a warm airy location to cure. Brush off as much dirt as you can but don’t remove the outer paper layers. As they dry, the roots will shrivel and the foliage close to the bulb will dry out. This seals the bulbs and prevents rot so that they can stored.

About 7 to 10 days after harvest, the roots and foliage should be completely dry and you cut off the roots and foliage or cut off the roots but leave the foliage so that you can braid your onions. Store your harvest in a cool, dry place. Properly cured and stored onions will keep anywhere from four months to one year depending on the type of onion.

© 2016 Caren White


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    • profile image

      Mwamba mathews 

      20 months ago

      I love farming

    • OldRoses profile imageAUTHOR

      Caren White 

      3 years ago

      Thanks, Flourish!

    • FlourishAnyway profile image


      3 years ago from USA

      Well written. Your expertise shines through.

    • OldRoses profile imageAUTHOR

      Caren White 

      3 years ago

      Thanks AudreyHowitt! I wrote it because I too am planning on planting onions in the spring. Thank you for reading and commenting.

    • AudreyHowitt profile image

      Audrey Howitt 

      3 years ago from California

      I was just thinking about planting onions this next go-round--so loved seeing this come up on my feed today


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