If you’ve never grown onions, you’ll be pleasantly surprised because they’re super easy to grow, hardy through winter, and ready to harvest anytime you are. Young plants are harvested as green onions or scallions. But the longer you wait, the bigger the bulb — at least until they reach full size at maturity.
Here, I’ll tell you everything you need to know to grow onions successfully. You can also download my How Do I Grow Onions? one-sheet and keep the free resource handy for your reference.
Seeds, Transplants or Sets
To grow onions, gardeners can begin with seeds, transplants or sets.
Onion seeds can be direct-sown, or you can raise your own transplants by starting the seeds indoors under grow lights. Transplants are also available for purchase, but the most popular way to grow onions in the home garden is from onion sets, which are bunches of immature bulbs that were started in the previous year.
The reason why planting sets is such a popular planting option is that sets are the easiest to plant, earliest to harvest, and least prone to disease problems. The downside is, this option offers the most limited varietal choices. Still, you almost always find choices for red, yellow or white onions.
Choose the Right Onions for Your Region
Where you live will determine which type of onions to buy. In the South, short-day onions are what you need, and in the North, long-day varieties are best suited for your region. Then there are day-neutral onions, also known as intermediate-day onions, which have a wide range.
Short-day and long-day onion plants form bulbs in response to the number of daylight hours, so it’s important to pick onions that are suited to where you live.
Short-day onions require 10 hours of daylight to bulb. A short-day onion plant will form a bulb in a region with long days, but the bulb will be small because the plant did not have adequate time for top growth before bulbing began. Among the many varieties of short-day onions are Vidalia, Texas Sweet, Red Grano, Red Rock and White Castle.
Long-day onions need 14 to 15 hours of daylight before they begin to form a bulb. Such long days begin in the North around the summer solstice, which falls on June 20, 21 or 22 in any given year. In the South, the days simply never get that long, and long-day onion plants will never bulb. Walla Walla, Rossa di Milano, Sweet Spanish, Blush and New York Early are a few examples.
Day-neutral onions will bulb no matter the day length and are exceptionally sweet. They are a great choice for regions that fall near the middle, in zone 5 and 6. Day-neutral onion varieties include Sierra Blanca, Cabernet, Expression, Scout and Candy
If you’re picking up sets at your local garden center, you don’t have to think about it because they’ve already taken that into consideration. But if you’re ordering seeds or sets online, it’s worth it to double check.
To remember which type of onion to buy, I use a mnemonic device: “S” is for short-day and the South. For long-day onions, I think of Long Island. That is definitely not in the South.
When and Where to Plant Onions
Onions develop mostly top growth during the cooler months and then shift their energy to bulb formation once the weather turns warmer. In cooler climates, plant onions in spring. In warmer climates, plant in fall or winter.
Raised or mounded beds work best for onions, but either way, make sure your soil is loose and well-drained with plenty of organic matter. The term “sandy loam” best describes this ideal soil for growing happy onions.
Location & Spacing
If you’re planting sets, simply poke a hole in the soil and place the small bulb into the hole so the tiny roots are well covered. An inch deep is about right, but not so deeply that the green leafy part is buried.
You can plant sets close together with the idea of coming back in subsequent weeks to harvest some at the green onion or scallion stage. Thin the green onions so that the spacing between the remaining plants will be about 4-6 inches. Envision fully mature onions growing nearly shoulder-to-shoulder in your rows.
If sowing seeds or planting transplants, follow the same recommendations for thinning and final spacing.
Once they’re planted, onions aren’t fussy. In fact, just keep their beds weeded and they thrive on neglect. Just make sure the soil stays moist. Soil that’s too dry will cause your bulbs to split. An inch of water each week, from rainfall, supplemental watering or a combination of the two, will be adequate.
Onions don’t require much when it comes to supplemental nutrients. In fact, any supplemental fertilizer high in nitrogen will result in more leaf growth at the cost of smaller bulbs. But due to their short, compact roots, it’s even more important that the soil is fertile. Compost-rich soil will serve your onions well through their growth.
Onion Pests & Diseases
Onions are fairly pest and disease free. However, onion maggots sometimes find their way into the soil where they bore up through the bulb and feed on the stems. Thrips are another pest that attacks the foliage in summer. Silvery blotches on the foliage are a good sign they’re around, but thrips are so small, you likely won’t see them. A good defense is to keep your garden well maintained since thrips overwinter in weeds and plant debris.
As bulbs grow larger, the tops pop out above the soil — and that’s OK. The tops should stay exposed; don’t bury them in soil or cover them in mulch.
In summer, once the foliage falls over, bend back the foliage to ground level to divert the plant’s energy to the bulb. Once the top turns brown, pull the bulbs up — but leave the tops attached.
Keep bulbs dry and allow them to bake in the sun for about a week. Once the skin is dry or cured, remove the tops and store the bulbs in a cool dry place where they’ll last for months.
Do you grow onions? Let us know in the comments below.
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Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
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