How to Plant Onions & General Growing Tips

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Learning how to plant onions is one of the easiest gardening tasks you can do. Onions are easy to grow and the rewards are delicious. A successful crop isn’t too far off if you know when to plant, whether to start from seed or onion sets and how to prepare the soil. What starts as little green sprouts in the spring will be juicy, appetizing bulbs in just a few months. 

Onion Sets planted in the ground.

One of my biggest passions in life has been gardening. It's truly where I worship and I have this deep desire to teach other people the knowledge I've gleaned over the years. It's for this exact reason that I wrote my first book, The First Time Gardener as I want to hold the garden gate open for those who are just beginning.

There's so much to be learned in the doing, in fact, I always say failure can sometimes be our greatest teacher. But there are plenty of mistakes you can avoid by simply having a guide.

A pack of onion seeds on a table.

Is It Better to Grow Onions From Sets or Seed

Neither method is necessarily better. I would recommend starting onions from seed only if you have a greenhouse and some gardening experience under your belt. Growing from seed takes more time and is more difficult, but it is less expensive. 

I often grow my onions from seed because I have a greenhouse where I can start them. Later, I transplant them into my garden. 

If you want to grow from seed, my post on starting seeds in six easy steps is a must-read.

Many people prefer buying onion sets because it is so quick and convenient. Onion sets are little started onion plants. These can be planted directly into your garden in the spring.  

Multiple bunches of onion sets on the ground.

Onion Sets

Short Day Vs Long Day Onions

When buying onion sets, you will need to choose between short-day, long-day, or intermediate-day.

The type that you need is based on the region you grow in. Basically, different types of onions require different amounts of sunlight hours to trigger them to bulb up. Make sure you have the right kind for your region.

Planting the right kind of onion will ensure you get bulbs, which is really the whole point. You can easily find out which type you need on the Dixondale Onion Plant Daylength Guide

A woman planting a row of onions in the garden.

When Is the Best Time to Plant Onion Sets

Even though they are very hardy, onion sets and garlic need to get established with roots before a hard freeze. When I lived in zone 7b, I planted my onions in January.

The month you might plant will change according to which growing zone you live in. I discuss this in my post on garden planning basics, it will help you determine which growing zone you live in and has lots of other great gardening information.

Make sure your soil is workable and not frozen. If there is a possibility of a freeze after you plant, cover them with straw mulch for protection.

It was late January when I planted my crop on our new farm in South Carolina. The weather was perfect, and then we had a huge cold snap. Although my onions didn't die, come spring all they wanted to do was go to seed.

There isn't necessarily anything wrong with this, but once an onion tries to go to seed, they're no longer good as a long-term storage onion. I think the reason the onions struggled was that they were acclimated to warmer temperatures and then had the drastic drop, so it sent them into a bit of shock.

I've had cold temperatures in Arkansas where my onions did just fine, even colder temps than we experienced this winter in SC, so I do think it has something to do with acclimating the onions to the colder temps ahead of time.

If you know of a hard freeze coming, or a drastic drop in temperature, hold off on planting your onions until it has passed.

Garden beds being prepared with compost.

How to Prepare the Soil for Planting Onions

Successfully growing plants really comes down to the soil quality, so don’t skip this important step. If you need to learn how to build your own compost, read this post on the Berkeley 18-day method of building compost.

Our compost piles weren't quite ready when I was ready to plant my onions, so we used purchased compost to amend the soil.

Choose Your Garden Bed

For spring onions, it is a great idea to choose the garden area in the fall and start building up the soil quality. Make sure that the garden is in an area that gets full sun.

Cover the garden bed with cardboard or a silage tarp. A layer of cardboard creates a barrier to kill the plants that are currently growing there, creates a sterile seedbed, and doesn’t allow new seeds to take root. The cardboard will also break down, adding nutrients and organic matter to the soil.

Here are easy instructions on how to prepare a garden bed with cardboard. If you used a silage tarp, wait until spring planting to remove it.

Loosen the Soil

When you are ready to plant, rake out rows in the prepared garden area. Rake down any high or uneven places in the garden too. 

Add Compost

Put down a layer of compost one to three inches deep over the rows. The compost will add microbes and nutrients that wash down into the soil as you water.

A hand planting an onion set into the soil.

How to Plant Onion Sets

Now that you’ve done all the hard work of soil prep, here comes the fun part.

Onion sets are very easy to plant. Simply push the onion set into the loose soil up to the neck, barely putting the onion bulb under the soil. The pointed or sprouted end should be sticking up out of the soil. Plant the onion sets about 4 to 5 inches apart. 

Mature onions growing in a raised garden bed.

More Tips

Your spring-planted onion bulbs should be ready to harvest in 3 to 4 months. The onion tops will begin to turn yellow and some of the stalks will bend over as a signal that it's time to harvest.

Once you see a good amount of onion stalks that have fallen over, go ahead and bend over the rest and wait a few days before harvesting.   

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