How to Plant and Grow Onions

Small onion bulbs in a paper bag

A few years ago in late spring, when I made my weekly stop at the feed store, I peeked into the plastic basket with the handwritten sign that said "onion sets, buy one lb get one free, $1.50". 

There were some small bulbs left in the bottom, along with a lot of dirt and chaff. I felt sorry for the ones that were left, and I told the owner I would buy them all if he didn't charge me for the debris. 

He laughed, said that sounded like a good plan, and bagged them all up for me. I came home with over a pound of onion sets at a very, very deep discount.

That was the last year I planted onion sets though. The following year I planted onion transplants instead and had the most amazing harvest ever. So that's what I've been using ever since: onion plants rather than onion sets.

The difference between onion seeds, sets and plants

There are three ways to buy onions for planting in your own garden: seeds, sets, and plants

Sets are what you'll probably find at the store, wherever you shop. A big box store with a garden department, a hardware store that sells seeds and plants in spring, a farm and home store such as Tractor Supply Co, or a local feed store.

Onion sets (also called bulbs or bulblets) are simply onion plants that were started from seed, harvested early and then dried, to be planted in your garden at home. Sets can be any onion variety, just like onion seeds and onion plants come in many varieties. 

Sets are small, dry bulbs, usually prepackaged in a net bag.

They give you a head start vs growing onions from seeds.

Onion transplants, garden pots and tools on a yellow background

Onion plants (transplants), on the other hand, are grown from seed the same year that they are harvested and sold. Instead of being kept dormant over the winter, they are sold fresh. 

Plants are sold in bundles, and each little plant is about two- to four-inches long, with green tops and roots on the bottom end.

If your goal is to produce onions with large, firm bulbs, plants are a better choice than sets. 

Are there any advantages to growing onions from seed then? Of course there are. You have a much wider choice of varieties to choose from when you grow from seed. (Be sure you choose a variety with the correct day-length for where you live, as you'll see below.)

Onions are actually biennial, which means that it takes two years (two growing seasons) to grow from seed. During the first year, onions grow leaves and bulbs. During the second year, the onion plant grows a flower stalk that grows tall and flowers. The flowers then produce seeds, which completes the end of the onions' life cycle.

Onion bulbs that have bolted (gone to seed) don't keep as well. The flower stem is present inside the onion bulb and reduces the onion's storage life. Heat is what makes an onion bolt (and other vegetable plants too, such as lettuce, radish and more).

Which type of onions you should grow

The type of onion you choose to grow depends on where you live. There are short-day onion varieties, long-day onion varieties and intermediate-day varieties.

To figure out which type of onion would do best in your garden, draw an imaginary line across the United States from west to east. The line begins in the west, near San Francisco, and ends on the east coast somewhere near Washington DC.

It runs roughly along the bottom of New Mexico and Arizona, and across the top of Oklahoma, Tennessee and North Carolina.

If you live above this line, in the northern half of the country, you should grow long-day onions, and if you live in the southern half, you'd grow short-day onions. 

Just to make things interesting, there is also a narrow band that runs from west to east right along that imaginary line, and to the north and south of the line, across the middle of the US. If you live inside this band, you can also grow intermediate-day onion varieties. 

I consider myself fortunate to live in Oklahoma, where I can grow both short-day and intermediate-day onions!

Because onions will form bulbs according to the number of hours of sunlight, it's important to choose the right type of onions. Growing the wrong type will result in smaller bulbs and a smaller harvest.

How to plant onion seeds

Onion seeds should be started indoors about eight-to-ten weeks before your last frost date. Soak the seeds before planting. Sow seeds about a quarter-inch deep  

Transplant the seedlings into the garden in fertile soil with good drainage, just before your average last frost date.

How to plant onion sets

Soaking onion sets before planting - while not necessary - will help them sprout faster. 

Plant onion sets one inch deep and five to six inches apart in each direction in fertile, well-draining soil, about 4 to 6 weeks before your average last frost date.

You can plant onion sets slightly closer together if you plan to harvest some early as green onions, so that you are thinning the row as you harvest. The remaining onions will have enough room to form bulbs.

How to plant onion transplants

Plant your onion transplants four to six weeks before the average last frost date. Plant about one inch deep in well-draining fertile soil. If you plant your onions too deep it will affect their ability to form bulbs. 

Space them five or six inches apart. If you plan to harvest some onions early to use as green onions, you can plant them more closely. Harvest every other plant, leaving the rest at least five inches apart. They need this room to form bulbs.

onion transplants in a shipping box

Ideal growing conditions for onions

Raised garden beds are an ideal place to grow onions. Drainage is easy to regulate in raised beds. If you are growing onions in the ground, rake the soil into raised rows about four inches above ground level.

Onions' favorite soil is between 6.0 and 7.0 pH. Adding a generous amount of organic material to the soil will help the plants' small root system by improving the structure of the soil and enabling the soil to hold moisture, but not be soggy. 

To get your onions off to a good start, add a bucketful of compost or well-aged (not fresh) manure to the soil and mix it in well before planting.

Onions are heavy feeders. Fertilizer that's high in nitrogen (the first number in the fertilizer's description) is recommended every two or three weeks during the growing season. Blood meal and bone meal are good choices.

They are also thirsty plants, needing an inch of water per week. If your garden doesn't receive this much rain in a week, be sure to water your onions.

Onions need full sun, especially when they begin to form bulbs. Don't plant large, bushy vegetables such as tomatoes where they will shade the onions.

When the bulbs begin to swell in late spring, gently move the soil away from your onion plants with your hands so the bulbs are exposed. Leave only the bottom of the bulbs in the soil. Stop fertilizing. 

Grass and weeds compete with onions for both moisture and nutrients, both of which are important for a good onion crop. Keep the onion bed weed-free for best results.

Companion plants for onions

Onions are great companion plants for so many vegetables and herbs. They help deter pests from some, attract pests away from others, and improve the flavor of many other crops. Plant onions with one or more of these companion plants for an improved harvest.

  • Brassicas such as broccoli, cabbage, cauliflower and more
  • Lettuce and other greens, and Asian greens such as pak choi
  • Tomatoes, peppers, eggplant
  • Strawberries
  • Potatoes
  • Root crops such as carrots, parsnips, radishes and beets
  • Herbs such as chamomile and mint
  • Marigolds

However, don't plant onions near these vegetables, as they will stunt the onions' growth:

  • Asparagus
  • Peas
  • Pole beans and bush beans

Harvesting onions

How will you know when your onions are ripe and ready to harvest? How do you cure onions for storage? 

I've actually written another post devoted to this subject, so let's consider it Part Two. You'll find it here:  Harvesting and Curing Onions.

You can learn how to grow perennial Egyptian walking onions here. They're grown a little differently but taste just as delicious!

Whether you choose to start your onions from seed, sets or transplants, I hope you've been inspired to grow a delicious crop.

Onion transplants ready to plant

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