from the employee owners at Gardener's Supply Co.

Growing Onions from Seeds, Sets, and Transplants

Onions grown from seeds or seedlings get larger and keep longer than onions grown from sets.

Onions must be one of the most confusing vegetables to grow for new gardeners. Does it make a difference whether you grow them from seeds or sets? Yes, it does and here’s why.

Most onions are biennial, which means that they grow vegetatively the first year and bloom the second year. Onion sets are marble-sized bulbs that grew from seed the previous year. They can be planted in cool soil early in the spring, grow quickly, and naturally want to bloom and set seed in the summer. This makes onion sets ideal for scallions and fresh harvest throughout the summer. They don’t keep very long in storage, however, and the bulbs stop enlarging as the flower stalks develop. The variety selection is usually limited, too.

If you want to choose your own varieties and store big, solid onions for the winter, grow them from seeds or buy started seedlings. In the North, look for long-day varieties that begin forming bulbs when the day length is more than 14 hours. Southern gardeners grow short-day varieties that form bulbs when days are 12 hours or longer.

Sow fresh seed about ¼”- ½” deep in sterile soil in seed trays about 8 to 12 weeks before your transplant date in mid spring. To prevent them from forming bulbs too soon, give the seedlings no more than 12 hours of light a day. Keep the tops trimmed to 3-4” high and don’t allow the soil to dry out.

Transplant your own or purchased seedlings to a garden spot that gets full sun. Plant them about 4-6” apart with the roots just under the soil and the top of the bulbs exposed. You’ll find more information on growing onions and other root crops in our how-to article called Preserving the Harvest. For a fun fact sheet on cooking with onions and storing them, click here.

-Ann Whitman, Horticulturist

7 Comments

  1. Anonymous
    March 27, 2008    

    That was a great article

  2. May 4, 2008    

    I once bought a box labled “onion sets” little, not round red onion bulbs. I plnated them, and not really knowing what I was doing let them go to seed, then they wintered over. Next year there were more of those little not-round sets. I harvested them to replant, but someone threw them out. Is this typical? I cannot find anyone who knows what I grew, nor any info online.

  3. May 5, 2008    

    Well you have me a bit stumped. Is there any chance that you planted shallots rather than onions? Shallots form a small bulb (1 to 2″ in diameter) that’s typically more oblong than round. They usually have a shiny skin with a pink-golden color. Most shallot varieties form one large bulb, or two or three bulbs clustered together very tightly (a bit like garlic cloves). Depending on the variety (and growing conditions) you sometimes get little round bulbs that look like miniature onions. Usually, if you leave shallots in the ground over the winter, they’ll sprout in the spring and form another cluster of bulbs that will be ready to harvest in late summer.
    I am a huge shallot fan. They’re more pungent than onions and less pungent than garlic. I also find my shallots keep right through until the next year’s harvest. I’m still eating last year’s right now (in May).
    This is my best guess about what you grew! — Kathy LaLiberte

  4. Anonymous
    August 27, 2008    

    I planted red onion sets this past May and they have lots of green stalks, with very little bulb to them. Any idea what I can do to boost the development of a bulb?

  5. August 27, 2008    

    I find that sometimes onion sets go to seed rather than produce an onion. Did your sets produce a stiff stalk with a flower on top? If they did, that’s why you didn’t get a good onion to form. If the set went to seed there’s nothing you can do — it will never make a good size onion.

    Sets are actually tiny onions that were grown the previous growing season. If the sets were too mature when harvested, they will just go to seed once they’ve been replanted.

    To avoid this problem, I either start my onions from seed myself, or purchase plants in the spring. Sometime you can find onion plants in a well-stocked garden center, but to be sure I get the varieties I want, I purchase them by mail. (I always order mine from Johnny’s Selected Seeds in Maine: http://www.johnnyseeds.com). The little seedlings arrive bundled together, usually in groups of 50 plants and look like very, very skinny scallions. I order 3 different varieties and share them with friends.

    One good thing about gardening is you get to try again next year! Hope this information is helpful.

  6. December 31, 2010    

    A very helpful and informative article on growing onions. I agree that you are better off growing from seed or transplant sown in the year your are growing as they do store so much better, than onios grown from sets.

    I hope you do not mind but I have added this post to digg.com as i believe more peopel should see it.

  7. April 26, 2012    

    Hello, I came across your article via pinterest. I planted red onions last fall and I thought I harvested all of them but I obviously didn’t because I have a lot growing back and I can see the bulb/onion. They are squishy right now and some are splitting into what looks like 3 onions? Will these onions get bigger and should I split up the three-somes? Thx in advance!

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