Onions: Answers to Frequently Asked Questions About Growing, Storing, and Cooking

Onions
Curing Onions

Onions can be grown from seeds, transplants, or onion sets (small onion bulbs approximately 3/8-3/4 inch in diameter). Each strategy has advantages and disadvantages. Growing from seed provides access to more varieties, but the onions take longer to grow. Transplants often produce the largest bulbs, but they are more expensive (unless you start them yourself). Sets are quicker and easier to grow, but they are more likely to bolt (produce flowers early).

With regard to flavour, onions can be divided into two types: sweet and pungent. Sweet onions include varieties such as Walla Walla, Vidalia, and Sweet Spanish. These are the varieties that are often sliced raw for use in salads and hamburgers, though they are also good cooked. More pungent varieties, which are usually cooked and can be stored for much longer, include Western Yellow, New York Bold, and Northern Red.

What are short day, intermediate day (day neutral), and long day onions?

Onions are very sensitive to the number of hours of daylight they receive, so it’s important to select varieties that will thrive in your region. Otherwise, your onions will produce only small bulbs or no bulbs at all.

Short day onions grow best in the South, at latitudes of up to 36 degrees. Short day pungent varieties include Red Creole, White Creole, California Early Red, and Ebenezer, and short day sweet types include Texas Super Sweet, Yellow Granex, White Bermuda, and Texas Early Grano.

Intermediate day (more commonly known as day neutral) onions grow best at latitudes of 34-38 degrees north, but they can cope with a broader range of latitudes overall. Pungent intermediate varieties include Super Star, Southport Yellow Globe, White Portugal, Italian Red, Stockton Red, and Australian Brown. Sweet varieties include Red Candy Apple, Candy, and Walla Walla.

Long day onions grow best at latitudes above 36 north. Pungent long day varieties include Early Yellow Globe, New York Early,  Ailsa Craig, Red Zeppelin, Copra, and Yellow Globe Danvers. Sweet long day-friendly varieties include White Sweet Spanish and Yellow Sweet Spanish.

What are the health benefits of onions?

Onions are low in calories and rich in fiber, and they contain vitamin C, calcium, potassium, and other health-protecting substances. The sulfur compounds that give onions their strong odour are particularly beneficial for maintaining health.

Research suggests that health benefits associated with regular onion consumption may include cardiovascular protection, increased bone density, anti-inflammatory action, and reduced risk of several cancers. However, you should eat at least half a medium onion per day to achieve these benefits (sporadic consumption, while better than nothing, is not as effective for reducing the risk of various health problems). Pungent onion varieties such as Western Yellow, New York Bold, and Northern Red contain greater concentrations of the chemicals that reduce cancer risk.

When can I plant onions?

If you’re growing onions from seed, plant them early in the spring, as soon as the soil can be worked (approximately 4-6 weeks before the last frost). Plant as early as possible to allow plenty of growing time for large bulbs. You can start onion seeds even earlier (10-12 weeks before the last frost) if you provide some cold protection, such as a polytunnel, cloche, or cold frame. Plant the seeds 1/4 to 1/2 inch deep.

Onion seeds don’t keep for as long as seeds of many vegetable types, so use fresh seed. The ideal germination temperature range for onions is 60-65 Fahrenheit (15-18 Celsius), though they will germinate at temperatures ranging from 50-80 Fahrenheit (10-27 Celsius).

If your climate is mild, you can also plant onions in the fall. Fall crops should be planted 6-12 weeks before the first anticipated frost. Fall plantings will usually produce bigger bulbs, because the growing time is extended. You may need to provide some protection, such as a cold frame.

You can also extend the growing season by growing your own transplants indoors starting 8-12 weeks before conditions are suitable for planting outdoors. Don’t put the transplants out too early; exposing them to cold temperatures followed by warm temperatures can trick them into thinking they’re experiencing their second spring, so they bolt (produce flowers and seeds). Wait until temperatures warm up before transplanting to prevent this problem.

If planting sets,  check to make sure they’re firm and not afflicted with disease or rot. Don’t use any bulbs that are brittle, soft, or moldy. Plant onion sets 2/3 into the ground, pointy side up, in March or April, depending on your local climate.

Water thoroughly after planting.

Where should I plant onions?

Plant onions in a sunny spot, ideally with some wind protection.

Can I grow onions in the shade?

Most sources assert that onions require full sun (at least 6 hours of direct sunlight per day), but a few say that they will do alright in partial shade (several hours of sun per day). You’re likely to have better results in partial shade if you plan to harvest your onions early as green onions rather than waiting for mature bulb formation.

What type of soil do onions need?

Onions grow best in loose, moisture-retentive soil that has been enriched with organic matter (compost). They do poorly in heavy, dry, compacted, acidic, or waterlogged soils.

Before planting onions, add 2 inches of compost or aged manure to the soil, and dig it into the top 6-8 inches.

How far apart should I plant onion seeds or sets?

Plant onion seeds an inch apart and then thin to anywhere from 3-6 inch spacing between plants, depending on how large you want the bulbs to get (you can eat the larger thinnings as green onions).  Wider spacing produces larger bulbs, but soil fertility is also a factor, as you can grow large bulbs with closer spacing in more fertile soils.

If planting sets, space them 3-4 inches apart.

How long will onions seeds take to sprout?

Onion seeds typically sprout in 1-4 weeks, depending on the temperature. They’ll sprout more quickly in warmer temperatures and take longer to germinate in cooler temperatures.

Can I grow onions in a container?

Yes. Onions don’t have very long roots, so they do well in containers, provided they have appropriate soil, care, and growing temperatures. I’ve had success growing medium-sized onions in containers just 6 inches deep.

What ongoing care do onions need?

Put row covers over seedlings or provide some other form of cold protection if you anticipate a hard frost.

Thin seedlings out to the appropriate spacing a few weeks after they’ve sprouted (3 inches apart for small bulbs, 6 inches for large bulbs, in between for medium bulbs). You can eat the thinnings as green onions.

Water regularly, particularly during hot, dry weather, but not to the point where the soil becomes waterlogged. Reduce watering several weeks before harvesting, because allowing onions to dry out before harvesting will improve their storage quality. Stop watering completely when the tops of the onions turn yellow and die back.

Weed regularly. Onions don’t grow extensive foliage, so they can’t shade out and compete effectively with weeds. Be careful when weeding – onions have shallow roots that can be easily damaged.

Applying a mulch once temperatures have warmed up can be beneficial for preventing weed growth and retaining moisture.

Remove bulbs that show signs of disease or bolting (flowering).

Onions grow best when they experience cool temperatures (55-75 Fahrenheit) during their early growth stages and warmer temperatures (70-80 Fahrenheit) when they’re producing larger bulbs.

Should I fertilize onions?

Recommendations are mixed regarding onion fertilization, with some sources recommending it and others saying that onions don’t need much in the way of added nutrients, provided you start with fertile soil that has been enriched with organic matter (compost).

If your soil is nutrient poor, your onions will benefit from the addition of potassium (which can be added in the form of greensand), phosphorus (which can be added in the form of bonemeal or rock phosphate), and nitrogen (a liquid seaweed fertilizer will provide all three nutrients). Or ask for onion fertilizer recommendations at your local garden center.

There are two schools of thought with regard to nitrogen-rich fertilizers for onions. Some recommend regular applications throughout the growing process, while others say that using them once onions have begun forming larger bulbs can actually be detrimental, as they encourage the plants to put more energy into leaf production than bulb growth. Given this lack of consensus, you may need to experiment to see what works best for your garden. I grow onions in containers, starting them in a rich potting mix that has some added fertilizer combined with a couple of inches of worm compost. After that, I don’t fertilize at all, and I’ve been happy with the results.

How long do onions take to grow?

When grown from seed, onions mature in 5-8 months. Growing them from sets takes 3-4 months. Onions tend to grow a bit faster in warmer temperatures and slower in cooler temperatures once they’ve reached the bulbing stage.

When do I harvest onions?

You can harvest onions at any stage if you plan to consume them quickly, but if you want to store them for longer, you’ll need to let them mature fully and cure them (dry them completely indoors). For mature onions, harvest when three quarters of the leaves have turned yellow and fallen over.

When the tops fall over, stop watering the onions to let them dry out before harvesting. Then dig them up carefully to avoid damaging the outer skin, as damaged onions will rot more quickly.  Harvest your onions on a dry day to reduce the risk of rotting.

Some sources recommend leaving onions outdoors for hours or even days to begin the drying process if harvesting on a dry day with no risk of rain. However, if there is any likelihood of moisture, bring your onions indoors immediately.

Mature onions are usually ready to harvest in the summer or early fall.

Why are my onions bolting (producing flowers) early?

This often happens when onions are exposed to temperature variations that trick them into acting as though winter has passed (for example, if onions experience a cold snap in the spring and then a return to warmth, or transplants are put outdoors early so that they experience cold after warm indoor temperatures and then warmth again). Bolting for this reason is more common when growing onions from sets or transplants than from seed.

Onions may also bolt because they are the wrong type for a particular area.

To reduce the risk of early bolting, provide protection during cold snaps, don’t transplant onions outdoors until temperatures have warmed up, and grow varieties suitable to your latitude (some onions prefer shorter days; others longer days) or day-neutral varieties.

If your onions bolt, harvest them immediately. You can still eat them – just remove the woody stem core when preparing the bulb.

Although onions that have bolted can be eaten, they won’t store well, so you should use them soon after harvesting.

How do I cure and store onions?

Cut the top greens off, leaving 2 inches of greens above the bulb, and remove all but 1 inch of the roots (don’t remove the top greens if you want to braid your onions for storage). Gently brush soil from the bulb, but be careful to leave the outer skin intact, as it protects the onion and increases its storage life. Handle onions gently, because bruised or otherwise damaged onions will rot faster.

If it’s warm and dry when you harvest your onions and there is no risk of rain, you can leave them outdoors for a few hours or days. If there is even a slight chance of rain, bring them into a sheltered area immediately.

Cure onions in a warm, dry area on screens, slatted shelves, or sheets of newspaper  for 2-3 weeks, and then put them in a dark, dry, cool place in boxes, mesh bags, or wicker baskets for long-term storage. Don’t store onions in the fridge, as the moisture will encourage them to sprout.

You can freeze onions for longer storage as well. Just wash, peel, and chop them and then place them in freezer bags (you don’t need to blanch them). Squeeze out any air, seal the bags, and spread them out to freeze quickly. Once they’re frozen, you can stack the packages on top of one another for greater space efficiency. Frozen onions can be used in cooked meals such as soups and stews, casseroles, and ground meat mixtures.

How long will onions keep?

Sweet onion varieties should be used up relatively quickly (within 1-2 months), whereas more pungent onions will keep for many months in the right conditions. You can expect them to last for 3-4 months in the pantry, and even longer in a cooler space (some sources say up to 12 months in ideal conditions), but don’t store them somewhere moist like the fridge. Onions will maintain their quality for the longest time in a cool, dry space.

Chopped, raw onions will keep for at least 3-6 months in the freezer.

Chopped, raw onions stored in a sealed container in the fridge will keep for around 7-10 days.

How do I save seeds for onions?

Onions are biennials, which means they produce flowers and seeds during their second year if you leave them in the ground. When the seed heads have matured, cut the stalks and dry them completely before removing the seeds. You can dry them in paper bags and then shake the bags to release the seeds when they’ve dried.

Select seeds from onion plants that have produced the largest bulbs.

Onion seed only keeps for a few years at most, and justonly a year or so. Using onion seed within 12 months is recommended.

How do I prepare onions?

Onions can enhance almost any dish. You can add raw sliced onions to sandwiches, salads, and burgers, or bake, roast, or fry them to increase their sweetness.

Always sauté onions on low or medium heat, as high heat can cause them to develop a bitter taste.

For onion recipes, see the National Onion Association’s Recipes page.

How many whole onions are in one cup of chopped onions?

One medium onion yields approximately one cup of chopped onion.

For more vegetable and herb gardening articles, see the main Gardening page.

References:

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    • Herriot, C. (2010). The Zero-Mile Diet: A year-Round Guide to Growing Organic Food. Madeira Park, BC: Harbour Publishing.
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    • Lang, S. (2004). Onion a day keeps doctor away? Cornell researchers find some onions do indeed have excellent anti-cancer benefits. Cornell University.
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    • The George Mateljan Foundation. (2016). The World’s Healthiest Foods: Onions. WHFoods.com.
    • Tozer, F. (2013). The New Vegetable Growers Handbook. Santa Cruz, CA: Green Man Publishing.
    • University of Nebraska-Lincoln. (2016). Freezing Onions. Food.unl.edu.