Guide for Growing Onions at Home

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Story at-a-glance -

  • Whether you want to grow sweet onions, red onions, shallots, chives or scallions (aka green onions), they’re a cool-weather crop, so they can be sown indoors in early spring or planted outdoors when the weather includes light frosts
  • Onion types that come in white, yellow and red can be cultivated from transplants, sets or seeds and range from tiny pickling onions to large Spanish cultivars and may be shaped like a globe, a top or a spindle
  • There are long-day, intermediate-day and short-day onions, which should be planted in the correct geographical areas to optimize their growth, and form bulbs on varying day lengths
  • Eating allium vegetables like onions has been shown to improve various aspects of health and may even help prevent, suppress and treat several types of cancers

By Dr. Mercola

If you ever find yourself in the kitchen wondering what to dream up for dinner, you know how often onions play at least a supporting role in the final presentation. Chopped in salads, sautéed with bell peppers or emanating their savory essence in soups and casseroles, the versatility of onions makes cooking less of a chore and more of an adventure.

But that could also be said of growing onions, whether you want to grow sweet onions, red onions, leeks, shallots, chives or scallions (aka green onions). Because they're a cool-weather crop, they can be sown indoors in early spring or planted outdoors and covered with about a quarter inch of soil. It's amazing how quickly they sprout; you should see their green sprouts emerging from the ground in seven to 10 days.

Onions, says master gardener Brandon Marshall, can be grown from seed, sets or transplants. Here's an interesting tip: Round onion sets will produce a flattened onion, while a tear-shaped or elongated onion produce a round onion, Marshall says. Ironically, large sets grow green onions while the smaller ones are left in the ground to form bulbs. Rodale's Organic Life explains the differences:

Transplants are pencil-sized seedlings started in the current growing season. They're sold in bunches, usually through nurseries and mail order. They form bulbs quickly — 65 days or so — but are more susceptible to disease, and the cultivars are limited.

These should go into the ground four to six weeks before the last hard freeze of the spring (which even for veterans can be difficult to determine). Try mounding up loose soil and inserting no more than an inch of the bulb itself, which may leave several more inches above the ground.

Sets, with an even more limited list of available cultivars, are dry, immature bulbs grown the year before. They're easier to plant, earliest to harvest and least prone to disease. The downside is a tendency to bolt, i.e., flower prematurely. A pound of onion sets produces about 50 feet in a row.

Onion sets are often identified only as white or yellow rather than a specific cultivar name, which makes variety identification a bit of a mystery unless you know what they are and where they came from. Half-inch-long bulb sets are least likely to bolt. Once an onion bolts or flowers, use it first and as soon as possible, because the green flower stalk that emerges through the center of the bulb prevents storage for more than a week or two.

Seeds usually have the widest variety availability; however, the growing time, obviously, increases to about four months to maturity, so especially in the coldest growing zones, you can start plants indoors under grow lights or cold frames (in which case they can be planted even earlier) to get the most out of your season.

When the seedlings are 2 or 3 inches tall, you can begin moving them to the transplanting stage by first hardening them off — getting them used to outdoor temperatures — by exposing them on a back porch or otherwise "in between" area, gradually.

Choosing Varieties and Planting Onions

There are a number of onion types that come in white, yellow and red. Size-wise, they can range from tiny pickling onions to large Spanish cultivars. They can be shaped like a globe, a top or a spindle. It's said that the sweetest varieties are flatter, or rather like an oval with the stem on top. According to Rodale's Organic Life,1 onion varieties include a perennial bunching type of scallions, called Allium fistulosum, that are practically disease free and insect proof.

And a "multiplier" potato onion from the A. cepa Aggregatum group develops a bulb cluster, so every time you harvest them, you have bulbs to replant for a virtually limitless supply. When you get your onion transplants, try to get them into the ground as quickly as possible. If you can't plant due to rain or whatever, spread them out in a cool, dry area. If they start drying out, it's fine; as members of the lily family of plants, they'll live another three weeks. Once planted, they shoot new roots almost immediately.

When planting sets, push the flatter side down into damp soil just until the pointed part of the top is showing, and 4 or 5 inches apart. Plant as soon as the ground can be worked, and try to make sure the temperature doesn't fall below 20 degrees F. Rows can be planted as close as 12 inches apart or as far apart as 18 inches. Again, planting too deep keeps onions from forming large bulbs.

Always water immediately after planting, and keep them relatively damp as harvest time gets closer. In between, careful weeding so that foreign plants that are stronger don't choke out what you're trying to grow, especially when plants are young, is important. It may be extremely tempting to put dirt on top of forming onion bulbs, but don't! When you see onion bulbs that appear to be lying there, lifeless, remember that their roots are still underground getting the nutrients they need.

You'll know bulb onions are ready when their tops fall over. It's a natural indication that they're fully mature. Then, pull them up, allow them to dry, separately, clip the roots and cut all but about an inch off the tops. Keep them cool and dry for storage. More pungent onions store longer; sweeter onions need to be used up sooner.

The Right Onion Varieties for Your Location

At the risk of making onion-growing seem complicated (it isn't) remember that the more information you start with, the greater your chances that your first growing season will be a whopping success story rather than ho-hum. That brings us to another three categories onions fall into:

  • Long-day onions, which covers all of Oregon, Nebraska and Pennsylvania and cuts through Illinois, Indiana and Ohio. These form bulbs with 14 to 16 hours of daylight, usually in zone 6 and colder regions of the north, and are planted in late winter and early spring.
  • Intermediate-day onions hit a wide swath, taking in the top half of California and Texas, and cut through Mississippi, Alabama and Georgia. They form bulbs with 12 to 14 hours of sunlight and grow best in zones 5 and 6. They're also planted in the fall where winters are mild and early spring in northern regions.
  • Short-day onions are the bottom quarter of the states in a semi-circle, so every one of the southern states — generally zone 7 or warmer — fall into this category. They form bulbs within 10 or 12 hours of daylight, require mild winters and when planted in the fall, mature in late spring. In the north, bulbs are smaller.

According to Marshall, flavor and pungency are often what determines the type of onion to look for. Sweet, white onions are long-day varieties while strong-flavored yellow onions fall into the intermediate or short-day categories. When planting, select an area in full sun so your onions won't be shaded by other plants. As far as soil type, it should be well-drained and loose, because compacted soil restricts bulb development. Plant in well-drained, highly organic soil.

Reducing the amount of tilling you do and avoiding synthetic fertilizers and nitrogen will help increase the soil's organic matter naturally. Rotating your onions with other crops regularly also reduces the loss of soil nutrients. The minerals and micronutrients already present in your soil negate the insistence that chemicals, including herbicides, are needed to grow healthy crops, though adding compost is universally beneficial.

You Have Your Annuals and Perennials; Onions Are Biennials

Onions begin to form bulbs based on day length. If you plant the seeds close together, you can be harvesting and eating scallions for salads or even crunchy snacking within eight to 10 weeks. If your goal is to grow your onions as large as possible, they should be planted 2 to 3 inches apart. Pulling every other one works well; you get scallions sooner and larger onions later. It's also important to note that onions are biennials.

Here's a brief tutorial based on The Spruce,2 breaking down the differences in the way plants behave season to season:

  • Annuals require annual planting, as they complete their entire life cycle in a single year, going from seed to plant to flower and back to seed, then dying off.
  • Perennials go from seed to seed in one season but don't die at the end; however, sometimes colder climates make them behave like annuals and they die. Just as often, some annuals like tomatoes and snapdragons "volunteer" to pop up the following year.
  • Biennials are deemed short-lived perennials, typically taking two growing seasons to complete their life cycle. In the first growing season, plants produce only foliage. In the second, they produce flowers and set seed, often early in the season.

Rodale's Organic Life3 notes other interesting tidbits:

  • You can cut the tops of chives in season to encourage production, and dig up portion of the roots to keep indoors for winter harvest. You can also chop and freeze them, and they're almost as good as fresh.
  • Keep your garden cleared of weeds and mulch (or even lengths of folded aluminum foil) around plants to discourage pests like thrips, aphids, carrot flies and Japanese beetles.
  • When purchasing onion varieties, whether they're sets or seeds, read the label to determine whether you're buying sweet or hot, as well as the daylight requirements.

Onion Compounds Lower Your Cancer and Other Disease Risks

If you ever wondered if onions offer health benefits, you'll be glad to know they certainly do. As an allium vegetable, one of the most important aspects of the phytonutrients in onions involves the affect they have on cancer. One of the most interesting aspects of the pungency of onions is that the more flavor they have, the more effectively they combat cancer.4 In fact, to lower your cancer risk, eat more onions.

Several antioxidant, cancer-fighting compounds in onions include the flavonoid quercetin, anthocyanins and sulfur compounds like onionin A (ONA), S-allylcysteine (SAC) and S-methylcysteine (SMC), and diallyl disulfide (DDS), as well as potent phenols and flavonoids. Other flavonoids, glutathione, selenium compounds and vitamins E and C are also compounds that adversely affect cancer cells.5

Quercetin, studies indicate, decreases cancerous tumor initiation and inhibits the proliferation of cultured ovarian, breast and colon cancer cells. It's also associated with a decreased risk for brain cancer6 and a lower risk of lung cancer, even if you smoke.7 Onionin A, as another example, has been found in studies to be protective against epithelial ovarian cancer, the most common type of ovarian cancer, because it slows its growth, and was also found to slow the activity of other cancer types.8

In fact, onions deliver more flavonoids than many other foods, which is amazing because these powerful phytonutrients have been linked to a decreased risk of diabetes and heart disease as well as cancer. They also have the distinction of being antiallergenic, antibacterial, antiviral and anti-inflammatory.

More Studies on the Cancer Prevention Properties of Onions

Multiple studies report that several different types of cancer cells are diminished when scientists place the cells with onions extracts. One study9 at Cornell University noted that color and variety have a lot to do with the types of cancer they're most effective against. Shallots, Western Yellow, pungent yellow and Northern Red onions have more anticancer chemicals than other varieties tested. The study explained:

"Phenolics and flavonoids are types of phytochemicals — antioxidant chemicals that protect plants against bacteria, viruses and fungi. Phenolics and flavonoids, like other antioxidants, help prevent cancer by mopping up cell-damaging free radicals and inhibiting the production of reactive substances that could damage normal cells."10

Lead study author Dr. Rui Hai Liu noted that shallots and the yellow onion varieties are effective against liver cancer cells, and the two yellows are best against colon cancer cells. In fact, Western Yellow onions had 11 times the number of flavonoids compared with the onion with the least, the Western White variety.

A 2016 study also associated allium vegetable consumption with decreased risk of cancer, particularly cancers of the gastrointestinal tract, as well as "decreased bioactivation of carcinogens, antimicrobial activities, and redox modification,"11 meaning a reduced oxidation rate. Onions and other allium vegetables are observed to prevent or suppress several types of cancer, including:

Kidney

Oral cavity

Breast

Endometrial

Ovarian

Lung

Gastric

Colorectal

Esophageal

Liver

Stomach

Prostate

If you love onions, it's great to know you need go no further than your own backyard garden to get some of the most powerful, cancer-killing foods on Earth. It should be noted from the same study that:

"Altering dietary habits may be a practical and cost-effective means of reducing cancer risk and modifying tumor behavior. Approximately 30 to 40 percent of cancers are preventable by appropriate food and nutrition, physical activity, and maintenance of healthy body weight. 

This means choosing foods that help to maintain a healthy body weight, reducing consumption of foods such as … processed meats that may increase cancer risk, and increasing consumption of foods that may decrease cancer risk."12

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