How To Grow Onions From Seeds – Beginner’s Guide

Onions are a staple crop in many diets. In this post we will cover how to grow your own onions from seed, what onion varieties will work best in your area and when it’s a good idea to start your onion plants! We will also discuss why you should grow them from seeds or buy plants instead of the popular onion sets found in greenhouses. 

Benefits of growing onions from seed

There are numerous benefits to growing your own onion seedlings. In our experience, since onion bulbs or sets are from second year onions, they tend to bolt and not grow as large as onions grown directly from seed or plant. however, it can be hard to find the types of onions you want as onion plants in the garden stores. 

Other benefits to starting your own onions from seed include: 

• you control the seeding date 
• gets your hands in the dirt early! 
• greater selection of varieties than grocery store or nursery-grown transplants 

Onion seed vs onion sets 

Onion sets are the small onion looking “bulbs” you find in greenhouse store that typically charge per pound. Onion seeds are tiny seeds. These seeds grow into a green onion, which creates a bulb and turns into a large onion. In comparison, onion sets are premature onions that were forced into dormancy to plant the next growing season. They look like a tiny onion and are what is more commonly found at nurseries, but planting onions from seed or even buying onion plants is a much more reliable harvest. 

When to start onions from seed 

Growing onions from seed is a long process because you are basically growing a green onion indoors, planting it outside and waiting for onion bulbs. Seeds should be started inside 10 weeks before the last frost date and then planted outdoors in the early spring. The entire process from seed to harvestable onion takes about 100 days. 

Selecting onion seeds

When you search for types of seed packets you will often find pelleted vs not pelleted seeds. Pelleted seeds have a coating on them to help you see and handle the small seeds. In our case, we use a seeding tool to help us and the pelleted seeds help make the onion seeds a consistent size. If you are on a home garden scale and sow seeds by hand, the pelleted seeds are not helpful. 

Onion seeds also have a very short shelf life. If your seeds are more than one year old, I would suggest seeding more than you think you need because the germination rate may be poor. If they all germinate, the good news is you can never have too many onions– or you can eat them as green onions. 

Choosing the best onions for your area: day long, short day, and day neutral onions 

Onions form a bulb based on the length of daylight at your latitude. While it is fun to grow many varieties of onions, it can be very frustrating to not get the best results when a variety is not made for your area. 

• 10-12 daylight hours for short-day onions (for Southern latitudes)
• 12-14 daylight hours for intermediate day varieties (in the middle)
• 14-16 daylight hours for long-day onions (for Northern latitudes)

Using the data above, you could plant onions at any time of year, but without the length of day being right for the area you are living in, they would not bulb unless the day length hours match. These types of onions being relative to how close you live to the equator is why Washington state is known for Walla Walla onions and similar sweet onions but they do not grow well in Missouri. In Iowa, we can experience luck with quite a few short day onions or long day onions, but getting healthy young plants off to a good start is critical to healthy large bulbs. 

 For help choosing the right variety for your area, this PDF from Johnny’s provides all the great options for onions in your area.

When to plant

The goal is to have your onion plant ready to go outside as soon as the soil is workable. In Iowa, this is mid-March. This makes for our onion seeds being sown in mid-February and planting outside in late March. Keep in mind you will not have good success trying to plant outside of the timeline for your area because the bulbs are formed based on the length of sunlight in your area. 

Starting onions from seed

One of the most important things to know is that onion seeds do not store well. If you are using seeds more than one-year-old, the germination rate will decrease significantly. Plan to always use new seed or overplant. Healthy onions also start from healthy soil, so be sure to use a fresh bag of seed-starting mix for the most nutrients.  

1. Using 4-inch pots or flat trays, fill with high-quality soil. We love these pots as they are deep enough to get nice healthy roots going before it’s time to plant. 
2. Broadcast seeds across the soil. They can be seeded pretty thick as they will be separated at planting time. 
3. Add 1/4 inch of soil to the top of the seeds
4. Use a light mist or spray bottle to keep the soil moist. It’s best if the temperature of the area is about 75 degrees Fahrenheit.
5. In about 7-10 days your onions should be sprouted.
6. Keep plenty of moisture in our onions AND give them a haircut about every 10 days to keep them 6 inches tall. This promotes a thicker stalk. You will need them started under a grow light or in a sunny window. 

Planting onion plants  

About 6 weeks before the last frost in your area, transplant the onion plants about 1″ deep in the soil. In our photos we are using a paperpot transplanter, but this can easily be done with a hoe or by hand. The onions transplant size are about pencil thickness when we plant them outdoors. 

1. Dig a shallow trench, drop plants about 4-6 inches apart, and then backfill.
2. Firm the onions into the soil, then water.
3. Mulch the bed right away to retain moisture and reduce weed competition. Onions are heavy feeders, so we’ve had good luck mulching with grass if it’s been a warm enough spring that we can mow around planting time. 

Harvesting Onions

Onions can be eaten at any stage in the growing process. We like to eat green onions as soon as possible, but once there’s a bulb formation you can use the fresh onion in cooking even though it is a smaller bulb. It will not store for months at this point but you can eat it! Wait until you see the onion tops all over to know the bulbs are done producing before you pull your onion to store for late winter. Keep as much of the paper on the onion as you can, and do not wash them. 

How many onions should I grow?

There are a few ways to decide how many onion transplants should be grown. With a new garden, we always suggest not growing more than you can manage. it’s better to manage a little space of garden better than a large unmanageable garden. Following that advice, I think 30-50 onion transplants would be a great option for a first garden. If you have some garden experience, think about how many onions you eat in an average week and multiply by 52 for the entire year’s crop! If you plan to grow some different varieties, think about how many of those you eat in an average week. For example, our family of six eats about 1 yellow Patterson onion per day, but only about 2 red onions per week. This leaves our family needing about 365 Patterson onions and 104 red onions. There are weeks of the year that onions will be growing that can be harvested and used, but there are also weeks you may use more than planned. I find this method to be a great predictor of how much the family needs. Keep track of these numbers from year to year so you can get an accurate count over a long time. 

More tips for growing onions

Learn more about all of our best tips and tricks for growing onions in your home garden with the link below!