Growing Onions

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In my area I like to have my onion seeds and leeks started before the end of February. Don’t worry if you don’t get the seeds started in time, simply purchase onion plant starts (plants) or sets (bulbs). Sometimes you can find leeks as starts too.

Two onions from last years garden.

Grow The Right Type Of Onion For Your Area

Make sure you plant the correct type of onions if you want to get full sized bulbs. If you are going for scallions or green onions it does not matter. In our area we need to plant long day varieties. Long day varieties are planted above the 36-degree latitude and require over 14.5 hours of daylight to bulb. Short day varieties are planted south of 36 degrees and require 12 to 13 hours of light to initiate bulbing.   Day neutral varieties can be planted anywhere in the United States. They need to be started in a warm environment. The optimum temperature is about 75 degrees. I start them in a greenhouse but you can start them in your house. If starting them in your house, once they are sprouted and growing they should be transported to a cold frame outside where they will get plenty of sunlight but still be protected from the cold. If you have a sun porch where they are protected from frost that would work also. Any area where they get plenty of sunlight and are protected from frost will be fine.

Using Pots To Start Onion Seed Early

I start my seeds in a good-sized, dark colored pot and I fill it with healthy soil halfway up the side. The dark color helps provide heat from the sun and leaving space at the top will protect the plants later when I take them out of the greenhouse to let it toughen up before planting. We get some cold winds here sometimes in the spring and it is helpful to have that protective windbreak from the top of the pot. I have done it with and without the protection of the pot and those with the protection do better.

Onion seed has been planted in a pot of dirt

Germinating Onion Seed

Here is the process. I sow the seed in the greenhouse. I spread all seed thickly in my pot half filled with soil. I plant the seeds ¼-1/2 inch in the soil. I do this by spreading them thickly on top of the soil and then I scatter ¼-1/2 of soil on top of them.  I then tamp it down gently and water it. They should germinate in 3-14 days depending on the temperature. They germinate best at temperatures of 70-80 degrees. However, they will germinate if it is at least above 50 degrees. It will just be slower.

They germinate and grow in the greenhouse until it is warm enough to plant them out. (As an alternative, I have also started them in my house in front of a South facing window. Use a window with old glass that allows the suns rays through.) They are moved outside where they can be hardened in a cloche or if the weather is not bad, I set them in a sunny, wind protected area for a minimum of a few days. Then I transplant them in the garden. If you are growing your new onions in a cold frame or unheated sun porch or other cool, protected area you will not need to harden them off. Don’t cut your tops off like you might buy them from a retailer. This is not necessary and will slow them down. The retailer does this to help compensate for the root damage that has happened to the ones you purchase. This way, they assume the green tops will not be pulling up as much energy from the damaged roots. Yours grown from seed will not have root damage when carefully transplanted from their pot to the garden bed. So, you won't have to be concerned about this.

Issues With Growing Them Indoors

Do not grow your starts in your house if possible. They will not get adequate sunlight and will have too much heat. This causes them to grow spindly and be weak. The new windows being used in houses now are built to decrease the ultra violet light necessary for plants so they may get so little light that they are very sickly. Even the old windows that let in UV light are usually not enough to grow a healthy plant. Additionally, the artificial light in your house at night confuses the plant, as it is not a normal light cycle for them. Turning on lights in your house when it is dark will send them a message that it is a different time of year and confuse them. If you have to grow plants inside, do it in front of a south facing window that does not have the low UV glass. Also you can add ultraviolet lights above your plants but make sure to turn the light off when the sun goes down and do not have any other light on in that room. If you give them extra light at night they will think it is summer and time to make bulbs.  You will get tiny little bulbs.

Purchasing Onion Sets

If you are purchasing onion sets (little bulbs) here is what you need to pay attention to. Make sure you get the correct variety (long day, short day, day neutral) for your area. Be aware that if the bulbs are larger than ½ inch in diameter and have been stored at temperatures of 40 degrees to 50 degrees most of them will bolt on you and you will not get nice bulbs. Onion sets need to be stored above freezing (33 degrees) or above 65 degrees. If grown between these two temperatures they will bolt. It may be hard to know if the onion set you are purchasing from a retailer has stored them appropriately. If you don’t know, make sure you purchase bulbs that are less than ½ inch in diameter as the temperature will start affecting them when they are larger than ½ inch. You may be tempted to buy the biggest bulbs but that may lead to them bolting if they were stored incorrectly.

Planting Outside

When planting onion starts or bulbs outside, make sure they have fertile, well drained soil. The best ph is about  6 – 6.8. If your soil tends to hold water and puddle, they will not be happy, but they also need adequate water. Adding compost will help in that case. Plant them about 4-6 inches apart.  Basically, give them the room they need to grow.

Onions need little besides compost and weed control. They don't compete well with weeds.  They grow best at 55-75 degreses and bulb up better when hotter.

Onion Leaf Rust

The only disease issue I have had with my onions has been a rust that I get in all my Allium plants. I found the best way to deal with it was to make sure I rotated the plants with no Alliums in that area for three years and using a well oxygenated compost tea on the leaves to compete with the fungus. I added horsetail for silica to the compost tea I made and also sprayed them seperately wth 501 biodynamic spray.


The tops of onion plants fall over when the bulbs are ready for harvesting. Sometimes I help them over if they are taking too long. I make sure many of the leaves have fallen over before I start pushing the others over. As soon as they start to fall over or sometimes before, I wills top watering them. This allows them to begin to cure in the ground. If the leaves are starting to die back and partially cured with rain on the way, I will dig them out and finish drying them in an outdoor covered area with little sunlight. To cure them well, I let the greens die back most all the way before removing them.

Curing And Storing

The drying or curing process allows the onion to dry well and be protected against disease organisms that may be present. An onion that has been cured correctly will have a dry, shrunken neck and dry outer scales. High temps and low humidity are best for curing onions.  Air movement with a fan can help if there is humidity. Once dry, the tops are removed and they are stored in a dark, cool, low humidity area.

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