Best tips for growing onions: How to plant onions 

Onions, allium cepa, are a kitchen staple among everyone! If you’ve ever dipped your toes into having a home garden you’ve likely added onions to your list of seed packets or plants to grab! In this post, we outline all the tips and tricks for the best results in your home garden for an onion bulb size you’ll be happy to harvest! With so many different types of onions, it’s important to understand a little bit about where you live, what your soil is like and the best time to be planting your onions for a successful crop!

3 Onion Planting Options: seeds, sets, and transplants- outline differences 

  • Onion seeds are exactly what they sound like. Teeny tiny seeds. It takes about 12 weeks from the time the seed is planted until it is an onion plant and ready to go outside, so you need some advance planning. In Iowa, we start our onion seeds the first week of February to be ready to plant them outside mid-April.
  • Onion sets look like tiny onions and are the most common type of onion sold in greenhouse stores. They are technically second-year onions, meaning they are more prone to bolting than an onion seed or transplant. If you’ve struggled to grow large onions, it’s likely because you are planting these sets since they are readily available at most local garden centers. 
  • Onion transplants/plants are great if you are not able to start your own seeds and manage them for 10-12 weeks. They look like green onions that you will plant them outdoors 4 to 6 weeks before your final frost in the spring.

Not sure when to start seeds or planting? We love Seedtime for our planting calendar and understanding our growing zone!

How to get seedlings

If you are not going to plant your own onion seeds, onion transplants are your best bet for a successful onion harvest. Here are our recommended sources for onion plants:

  1. Your local garden centers or local nursery should have onion plants in stock. If they don’t ask them to carry them! They are WAY more reliable than bulb onions. 
  2. A local market gardener likely will have extras from a bulk purchase or start their own from seed
  3. Ordering from Dixondale is our favorite. 
Onions growing in a greenhouse

Types of Onions: Choosing the best type

The day length of your particular area determines what onions will grow well in your area. In short, here’s a brief description of using your daylight hours and latitudes for onion growth. Onions also have a very, very long growing season, which is why they are planted before the final frost dates have passed. 

  • 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 onion varieties (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 onion varieties 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. 

How many days does it take onions to grow? 

From seeding, most onions take 100-120 days but refer to the seed packet for a more exact estimate. In Iowa, we start seeds in February, plant them outside in April, and harvest in late July. If you are buying transplants or sets, it’s 60-80 days. 

How do you plant onions? 

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.  

  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. 


Planting onions about 4 inches apart lends plenty of room to thin out small onions or green onions as you need some after the storage crop from the previous year is gone! By the bulbing phase, the goal is to have onions 6 to 8 inches apart. 

Hand planting onions

Planting Onion sets or starts 

  1. About 6 weeks before the last frost in your area, plant onion sets or starts/transplants about 1″ deep in the soil.
  2. Dig a shallow trench, drop plants or sets about 4-6 inches apart, and then backfill.
  3. Firm the onions into the soil, then water.
  4. 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. 
Onions growing in field

Scaling up your operation? As you can see in our photos, we don’t sow onion seeds or plant the plants by hand any more, we love this paper pot planter for efficient planting and seeding many of our crops! If you are past the stage of being a home gardener, it’s one investment that’s worthwhile for your small operation.

Where should onions be planted?

Onions require full sun, a lot of organic matter, and a long growing season. Plant them in a space that can be dedicated to onions from early spring all the way into late summer. Onions will make great companion plants for greens, spinach, carrots, tomatoes, and many other crops and repel pests but it is important that onion seedlings have full sun since this signals the process of the onion to produce a bulb once the length of daylight is appropriate for the onion. 

By building fertile soil and planting in loose soil in the early spring, there’s a large chance of having a successful harvest of onions in the summer days! 

Spring growing versus overwintering onions

Overwintering onions means you seed them in the late fall and then they come up in the spring. This allows you to beat the weeds a bit, as well as check something off the busy spring list. In Iowa, we do not have much luck overwintering onions. Here are some great resources from Johnny’s Seeds about overwintering onions. It’s a great way to get a jump start on the spring as they start growing in early April, but in Iowa, they seem more prone to bolting. If you are not picky about having an onion crop that stores well, give it a try to get a really big head start the following spring.

Close up on growing onions

What pests and diseases affect onions?

Thrips and onion maggots are the two biggest pests that affect an onion crop. Most pests can be prevented by planting your crops in a different area each year. It’s a good idea to never put crops in the same little space two years in a row. 

Downy mildew is the disease most onion growers experience. To prevent this, use a drip irrigation system and not an overhead watering. Downy mildew typically happens because the tops are getting too wet. Prevention of these plant diseases is easily done without the use of synthetic or organic chemicals in a home garden! 

It is important to rotate where your onions are planted each season to be able to provide healthy soil to produce strong plants and healthy soil. We always plant our onions where the potatoes were the previous year. 

If it is spring and you are local to us, feel free to reach out and see if we have any onion plants for sale! Typically, we start the seeds in February and have excess to sell by late March. 

See more blog posts for more garden tips from us at Pleasant Grove Homestead!

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