Onions are a high yield crop. Twenty to 50 onions can grow in 1 to 1½ square feet of space. But onions are easily grown in odd spaces alongside both slower and faster growing vegetables.
Green onions can be ready in 20 to 30 days after planting. Dry bulb onions can take 100 to 175 days to reach maturity.
Here are a few tips for growing onions:
• Seeds, sets, or transplants. Onions can be grown from seeds, sets (young, small dormant bulbs grown the previous year), or transplants. Growing onions from seed can take as much as five months. You will find seed for many varieties or cultivars of onions. Sets are easer to plant than seeds or transplants. Sets mature in as little as two months and are less susceptible to disease. But, cultivar selection is limited for sets. (Avoid onion sets with bulbs larger than a dime–they are likely to bolt.) Transplants are small seedlings that look like scallions. Transplants require about two months to reach maturity.
• Bulb or bunching onions. Select bulb or bunching onions depending upon your intended use. Bulb onions can range from the small pearl onions to very large Spanish types. Bulbs are white, yellow, or red at harvest. Bunching onions–also calle scallions or green onions–are grown for their tender, green top stalks. They are harvested before bulbs fully form.
• Long or short day. Onions grow tops in cool weather and form bulbs in warm weather. Temperature and day length control the timing of bulbing. Long-day onions require long hours of daylight–14 to 16 hours per day–to reach maturity. Long-day onions grow best in northern latitudes. Short-day onions grow best in mild-winter southern latitudes. They grow through the fall and winter and form bulbs when daylight increases to 12 hours per day in early summer. (Onions will be slow to grow if temperatures linger in the 30°s and 40°sF.)
• Garden site. Onions grow best in loose, well-drained sandy loam. Turn lots of well-aged compost and manure into the onion bed in advance of planting; turn the soil to at least 8 inches deep. Onions prefer a soi pH of 6.0 to 7.5.
• Starting seeds. Sow onion seeds indoors 8 to 12 weeks before the last average frost date. Sow seeds in pots, flats, or trays. Thin seedlings to one inch apart when they are four inches tall. Sow seed outdoors two weeks before the last average frost date in spring or four weeks before the first expected frost in autumn. Avoid sowing onion seed directly in the garden until the soil temperature has reached 50°F. Outdoors sow onion ½ inch deep and 1 inch apart. Later, thin to four inches apart. Growing onions from seed will give you the widest choice of varieties.
• Setting out starts. Seedlings (starts) can be transplanted to the garden in early spring as soon as the soil can be worked–usually about 2 to 3 weeks before the last frost when the soil temperature is at least 40°F; the air temperature should be at least 45°F. Set bulb onion seedlings one to two inches deep–depending on the size of the bulb–and four to six inches apart. Set starts for scallions one inch apart. To encourage development of bulbs, soak them in compost tea for about 15 minutes before planting. About four weeks after planting, gently push back the soil atop bulbs; this will help them to grow larger. Onions grown from transplants mature more quickly than onions grown from seed.
• Planting sets. Choose sets that have bulbs about ½ inch in diameter. (Larger sets may go to seed before producing decent-size bulbs. Sets with smaller bulbs may not grow well.) Plant bulbs with the pointy end up; the rounded end is the rooting end. Set bulbs ½ to one inch deep and four to six inches apart–depending on the size of the bulb at maturity. Onions sets are often labeled “red,” “white,” or “yellow”–you may not know the exact variety you are growing.
• Food and water. Onions are heavy feeders. Feed onions with a rich fertilizer, such as fis emulsion, early in the season to develop large plants and bulbs. (Or you canuse an organic fertilizer, 5-10-10.) Give a second feeding about a month after the first feeding or side-dress rows with a band of aged compost. Keep onions evenly watered early in the season. They require constant moisture during the bulb enlargement stage; dry conditions early on will cause bulbs to split. Give each plant about 1 inch of water each week (about 1.6 gallons). Transplants require more water than sets. At midsummer–or about a month before harvest after bulbs have formed and when the necks of the onions begin to soften, cut back on food and water and allow bulbs to mature in drier, less fertile soil.
• Weeding. Keep onion beds well-weeded. Onions are shallow rooted. Cultivate often and shallowly. Pull weeds by hand close to bulbs to avoid up-turning plants. Use a sharp hoe only to cut off weeds at soil level. Because onions leaves are thin and strappy they do not block the sun from the soil which, in turn, allows weed germination. Onion beds require more weeding than other vegetable beds.
• Mulch. After the soil has warmed, place a 1- to 2-inch layer of mulch around onions to discourage weeds and conserve soil moisture. Use aged compost or chopped leaves around onions. Keep the mulch back from bulb tops once they start to develop. (To grow large onions, keep both mulch and soil pulled back from the top two-thirds of developing bulbs.)
• Harvest. New growth from the center will stop when bulbs start forming. When bulbs are ripe, leaves will begin to yellow and fall over. After about three quarters of tops have fallen over, use the back of a rake to horizontally bend over the remaining tops. The bent leaves will cause the plant to divert the rest of its energy to the bulbs and away from leafy growth. After the tops turn brown in a day or two, lift the bulbs with a garden fork on a sunny day, and leave them to dry in the sun. When bulb outer skins are dry and the tops withered in about a week, wipe off any soil, and cut away the tops. If the weather is damp, allow the onions to dry in an airy place. You can loop the leaves through the mesh of a fence or framed chicken wire or braid then into a garland to dry.
• Storing. Keep onions in a cool, dry place to prevent rotting. Hang them in mesh bags or braids. Cured onion bulbs will store from one month to a year depending on the variety.
- Tips for Growing Large Onions in the Garden
- Best Varieties for Growing Large Onions
- Yellow Granex
- Sweet Spanish
- Giant of Zittau
- Walla Walla Onions
- Planting Onions from Sets, Seedlings, or Seeds
- Seeds to Seedlings
- Caring for Growing Onions
- Types of Onions
- Planting Onions
- Growing Onions
- Watering Onions
- Harvesting Onions
- What Are Sweet Onions – Learn About Sweet Onion Growing
- How to Grow Sweet Onions
- How to Plant Onions
Tips for Growing Large Onions in the Garden
Freshly Harvested Purple Onions
Here are a few great tips for growing large onions when backyard vegetable gardening!
Learn which onion varieties are best when you want to plant and grow large onions in your garden.
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Provided with good soil preparation, little seasonal care is needed for onions.
To grow a highly satisfactory crop, just give the plants what they need, and they will handle the rest.
Come harvest time, you won’t be shedding tears over the loss of your crops!
Large onions are finicky about the soil they grow in.
- The plants prefer fertile, loose, friable soil that is well-drained and has lots of organic matter.
- Sandy loams are ideal garden beds for growing large onions.
- Before planting, turn in good amounts of compost or well aged manure into the garden bed.
Best Varieties for Growing Large Onions
If you have ever been anywhere in the South, you have probably heard of Sweet Vidalia onions.
By federal law, to be considered a Vidalia onion, the crops must be grown within 75 miles of Vidalia, Georgia.
However, sweet onions can be grown in other states.
White Onions Growing in the Garden
The key to growing sweet onions is soils, variety, and stress.
The pungency of onions is controlled by sulfur containing compounds in the soil.
To produce sweet tasting onions, soils must be low in sulfur, a type of onion should be chosen which does not gather sulfur, and throughout its growth period, the crops must be allowed to be stressed for nutrients or water.
Another sweet, mild onion from Vidalia, Georgia is the Yellow Granex.
This is an excellent onion for all warm regions, although your crop may be slightly stronger tasting than those grown in the peach state.
Sweet Spanish keeps well, lasting into midwinter.
This red onion, which also has a white counterpart, is globe shaped and grows very large.
They are mostly mild and often weigh two pounds.
The Italian bottle onion, which is sometimes called Torpedo has large bulbs and is mildly flavored.
Braided Onions for Storage
Giant of Zittau
The Dutch have a large onion called Giant of Zittau.
Besides being large sized, it is the longest keeper of all.
If you live in a climate where onion crops have to be planted in late spring and are not harvested until fall, it might be of advantage to grow a long keeper.
Stuttgarter Giant is also imported from the Netherlands.
It is simply delicious when used fresh for cooking or salads and is ideal for storage.
Walla Walla Onions
The Walla, Walla onions are big as softballs and have been popular for more than a century.
This is a giant sized onion that everyone can grow, even in northern gardens.
It is so sweet, juicy, and crunchy; it could be eaten like an apple.
Tastes great served fresh in salads or on hamburgers.
Not all onions store equally well.
Sweet onions such as these should be used within a few weeks of harvesting.
In many areas, late summer and fall seeding or planting is a general rule for onions.
Another full-sized variety is the English import, Ailsa Craig.
This huge white onion variety is pear shaped.
The bulbs can be over four inches in diameter.
Best if used when freshly picked but can be stored for several months.
Planting Onions from Sets, Seedlings, or Seeds
Young Onion Plants Growing in the Garden
Onions can be planted from seeds, seedlings, or small onions called “sets”.
- If you want to grow large onions in one season, it is best to start your onion crop from sets or seedlings.
- Plant the onion sets in the garden in late spring, in well-prepared compost-enriched garden soil.
- Onion sets should be planted about 1 inch deep.
- The rows should be spaced about 16″ apart.
- You can plant the sets fairly close together in the rows, as you can thin some of the green onions out of the rows as they grow, and let the remaining onions develop into large onion bulbs.
Seeds to Seedlings
- Growing large onions from transplants is also works well.
- You can grow the onion seedlings yourself from seeds planted indoors about 2 months before it is time to plant outdoors in your area, or you can purchase the seedlings from a local garden center or online.
- Onion transplants or seedlings work well for growing large onions in your garden.
- Your local nursery can tell you which varieties of onions will grow best in your area.
Caring for Growing Onions
Their shallow root systems make onions sensitive to fluctuations in soil moisture.
Though the onion plants do not need a whole lot of water, regular watering every week is required. Mulching controls weeds and maintains soil moisture.
Onions do not require a lot of nutrients, but their root systems are so small, they need to grow in highly fertile soil to absorb what they do need.
In late spring, side dress the growing large onions with compost.
If needed, apply an organic fertilizer such as fish emulsion once a month.
Do not overdose on nitrogen because too much will produce lots of leaves but small bulbs.
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Dried or fresh, raw or cooked, onions are a foundational part of a variety of soups, salads, breads, and casseroles. Onions are easier to grow than you might think, and they’re a great plant for tucking into spare corners and along the edges of garden beds. Here’s how to grow ’em:
Types of Onions
Onions come in a wide variety of shapes, sizes, and colors. The white, yellow, or red bulbs range in size from small pickling onions to large Spanish cultivars; they can be globe-, top-, or spindle-shaped.
Most types can be pulled young as green onions, but there’s also a perennial bunching type called Allium fistulosum that’s practically disease- and insect-proof and produces superior scallions.
Each bulb of the multiplier or potato onion (A. cepa Aggregatum group) multiplies into a bulb cluster. So with every harvest, you’ll have bulbs to replant for a continual supply.
The Egyptian or top onion (A. cepa Proliferum group) produces a bulb cluster at the end of a long stem with a second cluster frequently forming on top of the first. It also has an underground bulb, which is often too pungent to eat.
You can grow onions from transplants, sets, or seeds. You can buy transplants, which are seedlings started in the current growing season and sold in bunches, from nurseries or through the mail. They usually form good bulbs over a short period of time (65 days or less), but they’re subject to diseases. The choice of cultivars is somewhat limited.
Sets are immature bulbs grown the previous year and offer the most limited cultivar choices. They’re the easiest to plant, the earliest to harvest, and the least susceptible to diseases. However, sets are also more prone to bolting (sending up a flower stalk prematurely) than are seedlings or transplants.
If you plant onion sets, the sets may be identified only as white, red, or yellow rather than by variety name. Most growers prefer white sets for green onions. When buying sets, look for 1/2-inch-diameter bulbs because they’re the least likely to bolt.
Growing onions from seed offers the great advantage of a wide choice in cultivars. The challenge with starting from seeds is that your crop will take up to 4 months to mature. Gardeners in cold-winter areas will need to start their onion seedlings indoors.
Always check a cultivar’s day-length requirement or recommended latitudes before you buy, because day length affects how and when onions form bulbs. Short-day onions, such as ‘Red Hamburger’, will form bulbs as soon as days reach 10 to 12 hours long. They’re suitable for southern latitudes only. Long-day types, like ‘Sweet Sandwich’ and ‘Southport Red Globe’, need 13 to 16 hours of summer daylight in order to form bulbs. They’re the type to grow in more northern latitudes.
Onions like cool weather in the early part of their growth, so plant them in spring — except in mild-winter areas, where onions are grown as a fall or winter crop. Generally speaking, onions grow tops in cool weather and form bulbs when the weather warms.
Plant onion seeds four to six weeks before the last average frost — or even earlier indoors or in a cold frame. When indoor seedlings are 2 to 3 inches tall, harden them off by exposing them to above-freezing night temperatures.
Outdoors, sow seeds thickly in rows about 1/2 inch deep. You can try mixing in radish seeds both to mark the planted rows and as a trap crop to lure root maggots away from the onions. Thin seedlings to 1 inch apart, and thin again in four weeks to 6 inches apart.
For transplants or sets, use a dibble to make planting holes 2 inches deep and 4 to 6 inches apart. Use the closer spacing if you plan to harvest some young plants as green onions. For sets, open a furrow 2 inches deep and place the sets stem pointed end up 4 to 6 inches apart, and then fill in the furrow. One pound of sets will plant about a 50-foot-long row.
The practices you use will depend on the specific crop you’re growing. In general, onions grow best if you keep them well weeded. Use a sharp hoe to cut off intruders; pulling or digging weeds up can damage the onions’ shallow roots. Once the soil has warmed, put down a mulch around and between the plants to discourage weeds and to hold moisture in the soil.
Dry conditions cause bulbs to split, so water when necessary to provide at least 1 inch of water each week; keep in mind that transplants require more water than sets do. Onions can’t compete well with weeds, so it’s important to direct water right to the onion roots.
If you’ve prepared your soil well, no fertilizing should be necessary. Always go easy on nitrogen, which can produce lush tops at the expense of bulbs. New growth from the center will stop when the bulbs start forming.
Egyptian onions, chives, and shallots require slightly different cultivation from regular onions. Here are some guidelines for growing these onion relatives:
Plant Egyptian onions in fall throughout the country; harvest some in spring as green or bunching onions. In midsummer or fall, miniature bulbs will form at the stem tip, where most onions form flowers. Pick these tiny bulbs when the tops begin to wilt and dry. Use them fresh or store in the freezer.
Plant chives and garlic chives in early spring in rich soil. They will tolerate partial shade put prefer full sun. Seeds are very slow to germinate, so most growers prefer to plant clump divisions, which you can harvest after two months. Space the clumps, each of which should contain about six bulbs, 8 inches apart.
Cut the grasslike, hollow tops frequently to maintain production. The pom-pom-like lavender flowers are very attractive, but always remove the spent flowers to reduce the chance of rampant self-seeding. Dig up, divide, and replant every third year. Transplant to containers and move indoors for winter harvests. Chives are almost as good frozen as they are fresh.
Shallots, a favorite of French chefs, have a blue-green stem that’s used when young. In addition, it has a gray, angular, mild-flavored bulb that’s related to the multiplying onion and is used like a mild-flavored garlic. Shallots will tolerate all but the most acidic soils, but dig the earth deeply because the plants put down 8-inch-long feeder roots. However, they have no lateral roots, so space them just 2 to 3 inches apart.
Propagate shallots by dividing bulb clusters. Each clove, in turn, will produce four to eight new bulbs. In February or March, plant them 1 inch deep, barely covering the tip of the clove. Keep the soil weed-free and slightly moist, but don’t fertilize. In early summer, draw the soil away from the bulbs. Harvest shallots as green onions at any time. Cutting the tops off near soil level will produce new tops, and such harvesting actually increases bulb production. Bulbs mature in about five months. Pull and store like onions.
To water onions efficiently, extend soaker hoses along the row close to the plants. Or open a small trench between rows and fill it with water. This keeps the roots supplied, while leaving most of the soil surface dry, inhibiting weed seed germination.
Watch this video to learn the basic steps for planting onions.
You can generally expect a disease-and insect-free crop. One possible pest is onion maggots: 1/3-inch-long white, legless larvae that travel in line from one bulb to the next and burrow upwards to feed on the stems. To reduce the chances of extensive damage, scatter-plant onions throughout the garden. (This interplanting can also benefit other garden plants; many Allium species will ward off pests—such as aphids, Japanese beetles, and carrot flies—from roses, lettuce, carrots, beets, parsnips, and members of the cabbage family.) Placing a thin layer of sand around onion bulbs may discourage adult flies from laying their eggs at the bottoms of the plants.
Barely visible onion thrips tend to attack during hot, dry weather in July or August. They produce deformed plants with silvery blotches on the leaves. Thrips overwinter in weeds, so reduce pest populations by keeping the garden clean. Try spreading a reflective mulch, such as aluminum foil, between rows to confuse the thrips. If you catch the problem early, you can spray plants with Beauveria bassiana or spinosad to combat thrips. As a last resort apply neem to control a serious infestation.
A disease called smut causes a swelling or hardening of leaves just about the neck, which eventually bursts and spills powdery black spores over the plant. Downy mildew, a purplish mold, shows up in midsummer during warm, humid weather. Onions are also subject to pink root, which causes roots to turn various colors and then shrivel, and neck rot, which causes tissues to form a hard, black crust. All these problems are caused by fungi in the soil and can be avoided by rotating crops and by working humus into the onion bed to provide good drainage.
Once onion tops turn yellow, use the back of a rake to bend them over horizontally. This stops the sap from flowing to the stems and diverts the plant’s energy into maturing the bulb. A day or so later, when the tops turn brown, pull or dig the bulbs on a sunny day, and leave them to dry in the sun. Lay the tops of one row over the bulbs of another to help prevent sunscald.
When the outer skins are thoroughly dry, wipe off any soil and remove the tops—unless you intend to braid them. Store in a cool, dry place; hang braided onions or those kept in mesh bags in an airy spot. Such dried bulbs will keep for about 4 months to 1 year.
What Are Sweet Onions – Learn About Sweet Onion Growing
Sweet onions are starting to become wildly popular. What are sweet onions? They get their name not from their high sugar, but their low sulfur content. A lack of sulfur means that the onion bulbs have a milder, smoother taste than other onions. In fact, the best commercially grown sweet onions come from parts of the world that have naturally low levels of sulfur in the soil, like Vidalia, Georgia. Sweet onion growing can be a little tricky, however. Keep reading to learn more about how to grow sweet onions.
How to Grow Sweet Onions
The key to successful sweet onion growing is giving the plants enough time to form really large bulbs. The best way to do this is to plant them in late summer or early autumn and let them grow through the winter. This means sweet onion plants grow best in climates that have mild winters.
The most popular sweet onion plants for winter growing are called short-day onions, a variety that still grows well during the short days of winter. These onions tend to be hardy down to 20 F. (-7 C.). Other varieties called intermediate-day are hardy down to 0 F. (-18 C.) and can survive in colder climates. If your winters are very cold, it’s also possible to start sweet onions indoors and transplant them out in the spring, though the bulbs will never get as big.
Sweet onions like well drained, fertile soil. They are heavy feeders and drinkers, so caring for sweet onions involves watering them frequently and applying regular fertilizer in the spring when the bulbs are forming. Avoid fertilizers with sulfur, as this will make the onions taste less sweet.
Short-day sweet onions should be ready to harvest in early to mid-spring, while intermediate-day varieties should be ready in early to midsummer.
Planting onions is one of my favorite April vegetable garden projects. This year I am planting Walla Walla onions, our regionally famous sweet onion. In late August we will harvest the bulbs and enjoy them raw on hamburgers, sandwiches and in salads.
Onions are planted from seed, sets or small onion bulbs or – in the case of the Walla Wallas – as small plants. They are a biennial plant which means they will grow a bulb the first year and if left in the ground over winter, they will produce a seed stalk the second year. We generally harvest them at the end of the first year for the bulb. The pungent aroma that brings tears to your eyes is caused by the release of sulfur compounds when they are cut.
Onions form their bulbs in response to the length of daylight. Here in the northern part of the country, long-day onions begin setting bulbs after we start getting more than 14 hours of daylight. This is why it is critical to get onions planted as early as possible in the spring so the plants have a chance to grow before they begin setting bulbs.
Not all onions will keep through the winter. Summer onions like the Walla Walla sweet onion and other mild, Spanish-types will keep for a couple of months while the winter or storage onions will keep well into the winter when stored in a cool, dry place. Use up the short keepers first and then enjoy the long storage ones through the winter.
Onion sets are much easier to grow than seed. Onion seed is very fine and must be started indoors early in the winter or in the garden the previous fall. Onion sets can be purchased in the spring and should have a base no larger than a dime. Sets larger than this will bolt or flower instead of setting a bulb. Walla Walla sweet onions are usually purchased in the spring as bareroot bunch of 25 to 50 plants.
Onions grow best in full sun in an evenly moist, sandy loam soil rich in organic matter. Onion sets and Walla Walla starts should be planted an inch deep about four inches apart in rows 12 to 15 inches apart. Onions from seed started indoors in late February can be planted in April when they are about the size of a pencil lead. It will take about two weeks for the green shoots to appear and start growing.
Once the plants are about six inches tall, apply a 10-10-10 fertilizer every three weeks until the tops start falling over. Keep the soil evenly moist until the tops begin to yellow and die then discontinue watering so the bulbs can harden off before harvest.
When most of the tops have fallen over, gently dig them and let them dry in the sun for a couple of days to harden the skins before storing them in cardboard boxes or mesh bags in a cool, dry place for the winter.
Pat Munts has gardened in the Spokane Valley for more than 35 years. She can be reached at [email protected] gardening.com.
How to Plant Onions
Welcome to Homegrown/Homemade, a video series from FineGardening.com and our sister site FineCooking.com. We’ll be following a gardener (Danielle Sherry) and a cook (Sarah Breckenridge) as they plant, maintain, harvest, store, and prepare food crops.
Episode 1: How to Plant Onions
Onions have a reputation for being hard to grow, so here are some tips to help. Onions can be started from seed or from sets (tiny bulbets). With seeds you have more choice of varieties, and you can choose varieties that are suited to your region. (In the North, plant long-season onions in the spring; in the South, short season onions, planted in the fall, are best.)
Start your seeds, transplant into larger cells, and before planting out, harden them off by putting the pots outside for several days. In the garden, onion plants should be spaced 4 to 6 inches apart in the row, with rows 1 to 2 feet apart. Plant each one about 1/2 inch deep in a small hole. After planting, trim the tops to about 4 inches.
Onions need about 1 inch of water per week, so if the weather is dry, you’ll need to water.
Episode 2: How to Care for Onion Plants
As the onions grow, the bulbs enlarge, and when that happens, it’s time to “unearth”. You simply move the soil away from the tops of the bulbs. Exposure to the air helps the papery skin firm up, preventing rot. Leave only the lowest part of the bulb in contact with the earth.
Episode 3: How to Harvest and Cure Onions
Onions need a long growing season. Plan on harvesting toward the end of summer. It’s easy enough to harvest them by pulling them out of the ground. The most important thing is to cure the bulbs, which means allowing them to air-dry for about a month with their tops on. Spread the bulbs evenly on trays or screens and allow them to air-dry in a cool, dry spot. Then cut off the tops (leaving an inch or so behind), brush off any dirt, and store until you’re ready to use them.
Episode 4: How to Preserve Onions: Caramelized Onions
Sometimes onions don’t cure properly and begin to spoil. If that happens, don’t despair; the good parts can be salvaged and turned into caramelized onions, which can be preserved in the fridge or freezer for use later. Sarah shows Danielle how to cut onions into uniform slices (and also the right way to dice onions), then cook them slowly in oil until they turn golden brown. One way to use caramelized onions is to make a quick and delicious French onion soup.
Recipe: Roasted Onions Stuffed with Prosciutto and Parmesan
Onions star in this satisfying dish. Slice off the top and bottom of the onions, then peel. Saute in oil until soft, then remove the core, and stuff with a mix of prosciutto, parmesan, cream, and herbs. Bake.
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