How to Grow Big Onions from Seed: These Tips Make a Huge Difference

There are many reasons to start growing onions from seed. One compelling reason is to ensure big bulbs you can use in your delicious recipes. We'll give you all the crucial tips you need to grow big onions from seed to harvest.

• 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.

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.

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.

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. 

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.

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.

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. 

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. Knowing how to grow onions from seed will give you a reliable harvest of big onion bulbs 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!