How to Grow Onions from Seed

comments (12) March 10th, 2009

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Click To Enlarge Photo: Lynn Karlin

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Starting onions seeds in flats
Lingering cold temperatures and soggy ground keep me out of the garden most years until mid-May, so I must start my seeds inside in January or February (at least two months before the last frost), or there won’t be quality bulbs at harvest, especially since I grow the large, sweet Spanish types.


More expert advice on sowing and transplanting...

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I sow seeds 1⁄4 inch deep in flats filled with soilless potting mix. I cover the flats with a plastic top to keep in moisture, and place them on the heated floor of the sun­room. Onions germinate in just a week at around 70°F. Once their stringlike tops poke through the soil, I change the growing conditions. I remove the plastic top and move the flats to my cool attic where they’re placed under fluorescent lights, one warm white and one cool white bulb per fixture. I keep the lights just above the leaves, adjusting the lights as the plants grow. I feed the seedlings with a water-soluble fertilizer at half strength every other time I water, being careful not to keep them too wet. Finally, I thin them to one every 1⁄4 inch or so.

Transplanting into six-packs
I transplant the onions into six-packs when the leaves are 5 or 6 inches tall, usually by mid-March. Of the hundreds of seedlings I transplant each spring, I dread doing those onions. I grow at least 36 plants of a half-dozen varieties, and since onion seedlings are fragile, this job is somewhat tedious. Some people skip this step by going from flats directly to the garden, but by allowing each one its own growing space, I give my onions the best possible chance for root development.

I carefully overturn the flat to expose the seedlings’ roots. Handling the plants by the leaves, I gently tug to separate the roots of individual plants. Then I tuck each plant into its new cell without damaging the long, threadlike roots.

With scissors, I snip the leaves back to about 4 inches to keep the plant from being too top-heavy and to give more nutrients to the roots instead of the leaves.

I continue watering with half-strength soluble fertilizer and keep the seedlings under the grow lights. If your weather allows, you could move your transplants to a cold frame.

Onions are cold-hardy plants and can stand chilly temperatures. Here in Maine, I usually wait until mid-April to put them in my cold frames. These are located on the south side of the garage where they receive the full sun essential for growing robust onions.

Onion seedlings ready for transplanting Separate the onion seedlings
When onion greens are 5 to 6 inches tall, it's time to transplant them into individual cells. Begin by carefully overturning the seedlings onto your hand.   Gently separate the roots of the seedlings from each other, holding each seedling by the green tops.

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posted in: onions

Comments (12)

VaneelaCharon writes: One of the great info for gardeners!!
Posted: 1:43 am on May 25th
Reechoclara writes: I love this info, it's truly valuable. thanks for sharing this!!!
Posted: 5:45 am on March 31st
APZ writes: Am delighted to have found this site. Am new into farming infact havent lasted even 3 mnths but have got the resiliance to give it a shot. Have started with onons and seams tricky,My onion starts shot up quickly and then flopped over and the foliage disintegrated. What did I do wrong or what should I do. Thenks for the article
Posted: 12:17 pm on October 10th
chuck50 writes: I have successfully grown onions from sets, with this drought not many producers in our area are planting for sets. I am planting seeds for the first time, both in the garden and in a flat to use as sets. I will see which works best here in central Texas! Thanks for the article

Posted: 4:16 pm on November 5th
AmandaMomRoss writes: My onion starts shot up quickly and then flopped over and the foliage disintegrated away. I read that the planting depth could have been to shallow or its possible I overwatered. Any thoughts? BTW: thanks for the article! This is my 4th year gardening and I still feel like I know next to nothing! Glad to have found a website with decent articles.
Posted: 10:25 am on April 2nd
Ruth writes: Good one, myseasons. I haven't exactly flunked Onions 101, but I've never gotten higher than a C-. Onions are one of those tricky crops for me (along with eggplant and melons). But I keep trying, and I hope you will, too. Even undersized onions have a lot of flavor.
Posted: 9:11 am on January 20th
myseasons writes: I have failed Onions 101 three times now. With this instruction sheet I will try one more time. Wish me luck.
Posted: 11:03 am on January 19th
gardengrower writes: Thanks for the great information. I'm going to link to your article on my gardening blog. Can't wait to try my onions from seed this year, last year even my sets didn't do well.
Posted: 10:56 am on January 19th
HunkieDorie23 writes: I am starting my onions from seed this year and appreciate the pic. I already started with individual seed trays but will try the trays next year. I planted my onions in rows of two last year and it worked great. Planning to do it again this year.
Posted: 9:52 am on March 30th
maybee writes: We live in Mississippi and last early summer planted onion seeds. Went out today to start cleaning the old beds and we have about 20 green onions growing. What a surprise!
Posted: 4:55 pm on March 16th
lettucegrow writes: The best article on growing onions yet. I have done ok. A little less than ok actually. Small bulbs but still tasty. I want big bulbs! I'll be using your tips this year. Thank you.
Posted: 2:47 am on October 18th
Lorithyme writes: This is a great article - so complete. Thanks
Posted: 12:47 pm on March 14th
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