How to Grow Onions From Seed - Why It's The Best Way to Grow Big Onions


Onions are the building block of so many delicious recipes! On every continent, you will find onions in a trio of vegetables that are used as the base in many savory dishes. These trios are referred to as soffritto in Italian cooking, mirepoix in French cuisine, and The Holy Trinity in other specialties. Without an onion, everything is out of balance, and your meals won’t have that exceptional flavor combination. And the good news is, growing onions from seed is easier than you think.

“It’s hard to imagine civilization without onions.”

Julia Child

yellow onions growing in field

Advantages of growing onions from seed:

1 - Onions are biannual, and sets sold in stores often go to flower and don’t produce a bulb.

2 - Starting onions from seed will give you fresh “onion sets,” not the dried-up ones that you see at gardening stores.

3 - Growing onions from seed is much cheaper.

4 - Another important reason for growing from seed is that it can keep you from infecting your garden with onion maggots, onion thrips, and white rot.

5 - Planning ahead and starting your seeds in the winter will give you the jump-start you need for a bigger bulb. And even if you didn’t get your seeds started earlier, you can still plant; you’ll just have a smaller bulb at harvest time. 

6 - It’s one of the first garden seeds you can start in the winter. Growing onions from seed will help fulfill your winter itch to get growing!

Onion seed planting tip: You don’t have to plant onions in individual pots/cells. You can separate them when they are starts.

Starting Onions From Seed

Once you start growing onions from seed, you won’t look back. Onions are simple to grow and if you’ve been itching to get something growing, start your onion seeds. 

  • Sprinkle onion seeds on top of moist seed starting mix. 
  • Cover with a light layer of soil or vermiculite of 1/4”.
  • Moisten the seeds with a spray bottle. 
  • Cover with a humidity dome. A covering will hold in the moisture. 
  • It will take 7 to 14 days for the seeds to start sprouting. (5 to 10 days for leeks)
  • The soil temperature can be between 55º to 70º F for germination. Check seed packets for specifics.
  • When seeds start sprouting, place them under grow lights. They will need at least 14 hours of light each day.
  • At this point, it is still important to keep the soil moist. Watering from below is the best method to keep the soil moist and avoid damaging the seedlings.

The trick to producing large onion bulbs is to give them a headstart. You can start onion seeds indoors 10 to 12 weeks before the last frost date. And since onion seedlings can tolerate a light frost, they can be set outside 4 weeks before the last frost date.

Transplanting Onions

When the onion seedlings are 5 to 6 inches tall, they can be transplanted. 

Start by carefully loosening the soil away from the roots of the seedlings. 

Then take each seedling and plant it 1” deep and 4” apart. 

Transplant onions into loose soil that is full of organic nutrients. The loose soil allows the bulb to grow easily. 

Be careful not to bury the seedlings too deep. While onions are considered a root vegetable, they have a fairly shallow root system and if planted too deeply, their bulb development will be stunted. 

You can put onion transplants outside about 3 to 4 weeks before the last frost date. 

Onion Growing Tips

The onion bulbs will eventually emerge from the soil. Don’t add more soil or cover them.

Onions prefer a less acidic soil pH of around 6 to 6.8. This range will also result in a better flavor.

Fertilize with a nitrogen-rich fertilizer until the bulbs start to emerge.

Weeds are a tough competitor for onions. Onions have shallow roots, so it's easy for the weed roots to take up the nutrients. 

Each leaf is a ring of the onion. So the larger the leaf, the larger the ring. If you want large onions, give them adequate space.

Long-day vs Short-day Onions

Bulbing onions are referred to as long-day or short-day. This refers to how much sunlight they need to form a bulb. 

Long-day onions need 14 to 16 hours of sunlight to form a bulb. 

Short-day bulbs need around 9 to 10 hours. 

Generally, the long-day onions do better north of the 36th parallel and the short-day ones will grow better in the south.

In climates with mild winters, short-day bulbs can be planted in the fall and harvested in spring.

yellow, red, and spring onions in basket

When to Plant Onions

Onions can be started indoors 12 weeks before the last frost date. Plant in spring in northern locations. In southern locations, onions can be planted in the fall as long as there are 4 to 6 weeks of warm temperatures.

Where to Plant Onions

Onions need full sun so that their leaves can soak up the light and develop big layers of onion. Plant onions where they won’t be shaded by other plants.

Fertilizing Onions

You can use compost in your onion garden, but make sure it is well decomposed so that it doesn’t encourage root eating bugs.

Onions need extra nitrogen. Apply an application of fertilizer when you first transplant. Then continue applications every 2 to 3 weeks until the onion neck starts to feel soft.

Giving extra nitrogen later in the season can cause the onion to produce more green stems instead of bulb growth. 

Bloodmeal and fish fertilizer are good options for onions. 

Onion seedlings

How to Grow Green Onions

Green onions are a perennial, and once they are started from seed, they will grow for years.

Plant green onions in bunches. As they grow, you can divide the clumps and harvest what you need.

Bunching onions don’t need a lot of nitrogen like bulb onions. They prefer phosphorus. 

Bunching onions can be direct sown outdoors as soon as the soil can be worked.

Green onions can also be grown indoors.

Tips for Growing Leeks

Leeks can be started 8 to 10 weeks before last frost. Or direct sown when soil is around 55º to 65º F.

Leeks can be started from seed the same way as bulb onions. However, unlike bulb onions, you can plant leeks deep and add mulch around the stems. The white part of leeks is the part used in cooking, so plant seedlings 6 to 8 inches deep, so they develop a longer white stem. You can also add additional mulch as they grow. 

Keep leeks well watered.

When to Harvest Onions

Bulb onions can be harvested at various stages. From the first hint of a bulb to the giant 3 pounds. When onions are smaller, they will have a milder taste, and they are often called spring onions.  

Spring Onions are onions that are harvested early in the growing season before they have formed a full bulb. They are grown from regular bulbing onions, not bunching onions. Spring onions have a milder flavor than their full-grown version. 

To have onions for long-term storage, wait until the onions are fully developed to harvest. When an onion is fully developed and done growing, its top will start turning yellow and then fall over. 

To harvest fully grown onions, choose a dry day. Loosen the soil with a garden fork and pull the onions out of the ground.  Let them sit in the sun for a few days if it is going to be dry. Then allow the onions to cure before storing.

harvested red and yellow onions

How to Store Onions

To store onions for long-term storage, wait to harvest them until the top turns yellow and brown and then flops over. Once they have reached this stage, they will be ready to harvest and cure. 
Curing allows the outer layers of the onions to dry out before storing. Place the onions in a place where they will be dry and can cure for 2 to 3 weeks until the top of the neck is completely dry and the outer skin is brittle.
Good airflow is important so that the outer layers dry to a papery skin. If they are stored when wet, the onions will start rotting.
Don’t remove any outer layers at this point. You can brush off excess dirt, but don’t wash the onions. Once they have completely cured, then you can cut the tops off 1 to 2 inches above the bulb. You can also leave the tops on if you want to braid them. 

Flowering onions will not keep as long as other onions. So use these first. Don’t plan on storing them long-term. 
Stored onions will start sprouting if the temperature gets too high. Store in a cool place, 32º to 40º F.

Pests to Onions

Onion thrips  - Don’t plant onions next to grains. Interplanting onions with carrots can be helpful since onion thrips will also attack carrots but not do as much damage. Planting from seed can keep you from infecting your soil with thrips.

Onion maggots - Rotate crops and don’t plant onions in the same spot every year. Onion maggots are host specific, which means they only attack plants in the onion family. If the hatching maggots don’t find a food source, they will die.

White Rot - White rot comes from a soil-borne fungus. Starting from seed will ensure that you aren’t planting infected starts. Plant your seeds in sterile potting soil.

For home gardens, the use of mulch and planting with a variety of other vegetables, herbs, and flowers, can have a positive effect. Don’t use toxic chemicals that kill the natural enemies of onion pests. Also, keep weeds away, so they don’t compete with the onions. 

Flowering Onions

Onion flowers are beautiful, but if you want a large onion bulb, you don’t want your onions to flower. If an onion flowers, it has stopped growing a bulb and is going to seed. An onion usually takes 2 growing seasons to go to seed. So if temperatures fluctuate enough that the onion goes dormant and then starts growing again, it will think that it has gone through a whole growing season. Once your onion starts flowering, it’s stopped growing, so go ahead and harvest it. And use it soon since it won’t store for long.

Onions are the staple ingredient that needs to be grown in every vegetable garden. Growing onions from seed will give you a reliable harvest that will become the base of many delicious recipes. 
Now that you know how easy it is to grow onions, it’s time to add some to your vegetable garden. 
Happy Growing!

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