By: Liz Baessler
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. Keep reading to learn more about how to grow sweet onions.
The key to successful sweet onion growth 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.
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Sweet onions (Allium cepa) aren't as pungent as other onions due to their low sulfur content. They are thin skinned and juicy due to their high water content, and this makes them fragile and more perishable than other onions. All onions are either short day, intermediate day or long day. Short-day onions start forming bulbs when the day length reaches 10 to 12 hours and are best when planted in the south for the largest bulbs. Long-day onions start forming bulbs when the day length reaches 14 to 16 hours and are best planted in the north. Intermediate-day onions form bulbs at day lengths of 12 to 14 hours and grow best in the central U.S. Onions are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10.
Growing sweet onions is growing in popularity for home gardeners across the U.S. Sweet onions have a mild flavor, rather than a sweet flavor and grow large bulbs. In order to grow the sweetest onions, your soil must have low sulfur levels. Too much sulfur in the soil increases the pungency of onions.
Most sweet onions are short-day onions. The bulbs of these onions grow in cool conditions with days that are 10 to 12 hours long. The larger the leaves on short-day onions grow, the sweeter and larger the bulbs will be. Most sweet onions grow in the southern states near 32° north latitude. The onions are planted in the fall and grow through the winter to reach their largest size for a springtime harvest.
If you live in a northern climate, you can plant intermediate-day onions for fall planting. As long as the temperature doesn’t reach 0°F (-17 C), you can plant these in USDA zone 7 for sweet onions.
Certain onion varieties are sweeter than others, but they still need low levels of sulfur in the soil for a mild, smooth flavor. The following varieties are the most popular sweet onion varieties:
Whether you’re growing short-day onions in the south or intermediate-day onions in the north, plant them in late summer or early fall. Transplants are the easiest for planting, but you can also use seeds, as long as you sow them eight weeks before planting time. No matter if you use seeds or transplants, preparing the soil is essential before planting. Use the following tips for preparing you soil and planting your seeds or transplants:
If you planted short-day onions, they should be ready for harvesting from April to June. The intermediate-day onions will mature from May to July.
The sweet onions are low in sulfur and high in water, which gives them such a mild flavor they can be eaten raw. They also have thin skins and so don't store very well.
Onions are very hardy and can be frost tolerant.
Correct timing is important with bulb onions, if you don't plant them at the right time they won't do very well. The best onions are grown from transplants set out in October or November and wintered over to mature in June and July. Plant transplants right away, if possible. Transplants are happy if you apply a light mulch to help conserve moisture for uniform growth.
Onion seed takes a month to germinate at 40 degrees, but only 2 weeks at 50 degrees, so you don't necessarily lose much time by waiting until the soil has warmed up a little.
Onions are quite drought tolerant, but a lack of water makes the bulb smaller and more pungent. For best flavor and largest bulbs you must keep the soil moist at all times, so there is no interruption in growth.
Low nitrogen. Moderate potassium. Moderate phosphorous.
Onions don't need a lot of nitrogen, but they do like potassium and phosphorus.
Onions can be grown easily in containers. Make sure they have good drainage and the pot is at least 12" deep. Fill with a mixture of potting soil and compost, and water thoroughly. Make sure that your onions have access to plenty of sunshine. Keep the soil moist.
Crisp and juicy sweet onion.
Learn how to plant and grow onions, leeks, and shallots.
Onions (Allium cepa) have relatives that include garlic, chives, leeks and shallots.
Storage onions grown in Minnesota generally are “long-day” types that require 14 or more hours of daylight to form bulbs. All onions require full sun for best growth. Overcast skies and cool temperatures during the growing season will delay bulb formation.
Sweet or mild onions are "short-day" onions. Although you can grow them in Minnesota, they will generally develop small bulbs. Bunching onions, including scallions and Egyptian walking onions, have green stalks.
Soil testing and fertilizer
Direct seed as soon as the soil is workable in the spring.
After seedlings emerge, thin to 3 to 4 inches apart.
Some seed companies sell onion transplants. They can tolerate light frosts, and you can plant them when temperatures reach 50° F.
You can also raise your own transplants by starting seed indoors 10 to 12 weeks before planting outside.
You can also plant onions from sets, which are small bulbs grown the previous year. Most of the sets available in Minnesota are of the short-day type. If using sets, plant them as soon as the soil is workable in the spring.
Onions are cool-season vegetables that grow best in moist, cool soils. While they’re forming bulbs, however, onions prefer warmer temperatures and drier conditions. Texas Supersweet onion plants produce large, globe-shaped onions with a sweet flavor. Sweet onions should be planted as early in spring as possible if you live in a colder climate. Texas Supersweet onions are typically grown from purchased transplants.
Select a planting site for your Texas Supersweet onions that’s in full to partial sunlight and has cool, fertile and moist soil. Remove all weeds and grasses from the planting site and loosen the soil using a rototiller or pitchfork.
Plant the Texas Supersweet onions in early spring, while the soil is workable but still cool. Plant the onion transplants 1 to 1 ½ inches deep and spaced 6 to 8 inches apart. If you’re planting the onions in rows, space them about 4 to 5 inches apart in rows that are 12 to 18 inches apart.
Firm down the soil gently around the onion transplants using your hands. Water them deeply and thoroughly, applying 1 cup of dissolved starter fertilizer solution per plant.
Hoe shallowly around the Texas Supersweet onion plants to keep the soil bed free of weeds. Water the onions deeply to soak the soil to a depth of at least 6 inches once each week when rainfall is less than 1 inch.
Side-dress the onion transplants about three weeks after planting them with a nitrogen-only fertilizer. Apply 1 cup of ammonium nitrate or ammonium sulfate per 20 feet of row.
Feed the onion plants every two to three weeks with the nitrogen-only fertilizer until the stem bases, or “necks,” begin to feel soft. Water the onion plants deeply and thoroughly immediately after spreading the fertilizer along the rows.
Harvest your Texas Supersweet onions by pulling them up from the soil in July or August, when the tops fall over. Pull the onions in the morning and set them out to dry in the planting bed until late afternoon.
Allow the onion bulbs to dry for two to three weeks indoors or in a protected spot outside. Cut off the tops so that they’re about 1 ½ to 2 inches long after the bulbs dry.
Avoid setting out the onion bulbs to dry in the planting bed when conditions are hot, dry and very sunny, because the bulb may become burned. In these conditions, set the Texas Supersweet onions in a slightly shaded location to dry.
The easiest and surest way to grow good onions, either green or dry, is to use sets. Sets are small onions, less than an inch in diameter, that were grown from seed the previous year. Onion plants are sometimes used, especially for growing the mild Sweet Spanish type. Texas varieties of onions, which are normally grown in the south, do not produce satisfactory yields in Illinois.
For green onions, place the sets upright about 1 inch apart in a furrow about 3 to 4 inches deep. For dry onions, place the sets upright about 3 to 4 inches apart in a furrow 1 to 2 inches deep. Cover the sets with soil. The larger sets are preferable for green onions while medium-sized sets are best for dry onions.
For good yields of mature dry onions, plant early in the spring as the soil can be prepared.
Smut, downy mildew, purple blotch, neck rot, and pink root are some of the diseases found in onions. All white onions are susceptible to smudge. Insects that cause the most trouble are thrips and cutworms.
You can harvest green bunching onions 4 to 6 weeks after planting sets. Dry onions will be ready in 3 to 4 months, about late August or early September.
Dry onions are ready to harvest when the necks are thoroughly dry or about 95 percent of the tops have bent over. Pull the onions and place them under cover to dry. Drying will take from 2 to 4 weeks. Then cut off the tops about an inch above the bulb, place the onions in a well-ventilated container, such as a slatted crate or mesh bag, and store in a cool, dry place. The best conditions for storage are a temperature of about 32° F. and a relative humidity of 70 to 75 percent.
Frost dates are figured as average numbers of days between the first and last freeze of the spring and fall seasons. This range is not a scientific declaration, per se, but allows gardeners to use specific guidelines for when to plant and harvest.
Northern Ohio is in USDA hardiness zone 5, where temperatures don’t typically fall below 15 degrees below zero (Fahrenheit). If you live in the Cleveland area or other sections of the “North Coast,” March to early April is usually a good time to plant—but not always. The best rule of (green) thumb is to plant onion seedlings into the ground after the danger of frost has passed usually by mid-April but it could be as late as mid-May.