How to Grow Onions: A Complete Organic Growing Guide

When I was in high school, my cousins and I worked at one of the largest onion farms in Michigan. As you may know, onions are very susceptible to weed pressure, but very difficult to mechanically weed.

Many farms utilize laborers like me to hand-weed onion rows. It was there that I picked up tips and tricks for how to grow onions that grew big and beautiful!

Whether you’re growing long or short day onions, red or white onions, these tips can take your onion harvest to the next level. Learn all the inside info like different types of onions, how to start them from seed, how to plant onions, and more, so you can learn how to grow onions just like the pros! 

The A-peel of Onions

Onions are one of the most loved and rewarding crops to grow. But they are also mysterious because many people struggle to grow them.

Onions aren’t really that hard to grow when you learn the right techniques. That’s why I am continuing our complete growing guide series with this post.

Use this growing guide and others, such as how to grow carrots, how to grow garlic, how to grow potatoes, and how to grow cucumbers, to grow bigger!

Types of Onions

Before you plant onions, finding out what type grows best in your area is essential. Most onions are either long day, short day, or intermediate day varieties.

Long day onions grow very well in northern states where the days lengthen to 14 to 16 hours (above latitude 37). They are typically excellent for long-term storage.

Intermediate day onions will start bulbing when the day length is 12 to 14 hours (between latitudes 32-42). Short day onions grow best in southern states with a day length of 10 to 12 hours (below latitude 35).

Day neutral onions can be grown in most regions of the United States because they adapt to various day lengths. For the best results, choose onions that are the right type for your area!

Gardeners often ask whether starting onions from seeds or planting onion sets is better. I'll let you in on a secret that the pros know!

Onions are biennials that flower in their second year of growth. When onions grow in the second year, they focus most of their energy on flowering, which reduces the bulb size.

Many onion growers grow small onions, store them, and sell them as onion sets for the next season. If you grow from onion sets, you may need to cut the flower stalk off and use the onion much sooner.

Onions started from sets will always have a green center that will never fully cure! (That green center is the flower stalk.) All of this means that when you grow from onion sets, they will flower that year and, not only produce onions with smaller size bulbs than they could, but the onions will have reduced storage time!

Starting onions from seed is the way to go! It will give you the largest, healthiest onions grown and harvested within one season.

When to Plant Onions

Onions need all the time you can give them, so start your onions from seed early!

• I highly recommend using our guide to timing when to start seeds or Clyde's garden planner to map out the best time frame for starting your onions indoors. I typically start my onions indoors about 10 to 12 weeks before my last estimated frost date.

• Start hardening off seedlings about 5 days before you want to transplant them outside.

• Plant onion seedlings outdoors after the last expected frost and when the temperatures stay consistently above freezing.

As you can see in the video below, there is a proper way of planting onions so that they grow right on the soil surface. Follow these steps to properly plant out onions:

1. Sunlight - Onions require full sun. Choose a location for your onions that gets maximum sunlight.

2. Amend the Soil - A good, sandy loam soil is the ticket for large onions. Amend the soil with organic matter like worm castings and compost to create a perfect base. Depending on what soil type you start with (like heavy clay soil), you may also need to add some sand. Pro-Tip: Don’t skip amending your soil! Compaction in the soil will lead to a pinched onion, reducing its size. A bulb free to expand in the soil will grow the biggest.

3. Check Soil pH - Test your soil pH before planting onions. For the best flavor, slightly acidic soil (pH of 5.5-6.5) is recommended.

4. Fertilize - Fertilize with a nitrogen-rich fertilizer like Trifecta+ or blood meal. Pro-Tip: Onions love lots of nitrogen! Many people think that onions require lots of phosphorus because they are a root vegetable. However, you want to prioritize growth because each leaf the onion puts out is a ring on the onion. The more leaves, the bigger the onion!

5. Proper Plant Spacing - Use a dibbler, trowel, or even a stick to create a divot in the soil where you want to transplant each seedling. Give onions enough space (4-6 inches) to reach an impressive weight. For high-intensity planting, 3-4" on center will be the ideal amount apart for each plant. Using high-intensity spacing methods will also cut down on the amount of weeding you will have.

6. Plant Onion Seedlings - Pull the onion seedlings out of their containers and plant one in each hole. Pro-Tip: Never plant onions deeper than the white of the onion! If onion bulbs are buried too deep, they will yield smaller onions. All onions, including green onions, should be planted this way to keep them healthy and growing on top of the soil!

7. Water - Water your newly planted onions well and consistently in the future. Onions like lots of water! If onions go dry, they will die! Pro-Tip: You will hear people say that onions do not like lots of water, and it will cause root rot. Onions are prone to root rot if you do not have the right soil. If you have a well-draining sandy loam soil, watering them frequently won’t cause a problem!

Follow these tips on how to grow onions to allow them to flourish:

• Weeding - Make sure to weed around your onions as much as possible. Weeds steal nutrients and water from the onion and can transmit viruses and diseases to it.

• Watering - The goal is to maintain even soil moisture and provide plenty of water. The more water, the more plump the onion can get. The soil should never be allowed to get bone dry. Pro-Tip: Provide extra water towards the end of the season until the tops fall over for harvesting. At this point, they will be dripping with flavor and ready to store.

• Pruning - To grow large organic onions, prune the leaves of each plant before they get heavy enough to crimp and fall over. Regular pruning will help add energy to the growing process and extend the growing season. Pro-Tip: I usually prune my onions when I transplant them as seedlings and then one more time during the season, about mid-July.

The onion tops you prune off are completely edible and delicious! They are basically larger versions of green onions, so use them in the kitchen just like you would use green onions.

Harvesting Onions At The Right Time

Don’t harvest your onions too early. Leave them in the ground as long as possible.

An onion should be left in the ground till the tops fall over and become like paper. Even once the tops die back, the onion will continue to absorb water for up to 2 weeks.

Check out this blog post for more information on harvesting, curing, and storing onions. Enjoy your exceptionally flavorful, organically grown onions!