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Want to plant and grow onions in your own garden? Here’s everything you need to know.

The common onion is an essential ingredient in foods all over the world. It adds sweet, pungent flavor and packs a nutritional punch. Various compounds in onions are linked to good heart health, cancer fighting properties, improved bone density, and more.

Onions are also easy to grow in your vegetable garden. They only require a small amount of space, and only moderately fertile soil. There are so many different cultivars available, you are sure to find one you love.

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About Onions

Onions (Allium cepa) have been cultivated so long that they have no wild ancestors remaining. Scientists estimate that at the very beginning of agriculture, onions were first collected from the wild and planted in central Asia. 

Onions are biennials or perennials. They grow from seed in the first growing season and do not flower before the plant goes dormant. In the second season they produce flowers and seeds.

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Gardeners mostly treat onion plants as annuals. Onions grow from seeds, but many gardeners buy them as either young seedlings or as “sets.” Sets are small onion bulbs that were forced into dormancy before maturity.

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Choosing Onions

It is important to plant a type of onion that will grow in your area.

Onions are categorized “long day,” “short day,” or “day length neutral.” These categories refer to the length of sunlight they need each day to form bulbs.

Long day onions need around 14 hours of sunlight each day. They are suited for northern latitudes, above the 35th parallel. Long day onions will not form bulbs in the south. 

Short day onions need 10 to 12 hours of sunlight, and are best suited for the deep south. They mature too soon in the north, and cannot attain full size. Day neutral onions can be grown anywhere.

Color, flavor, and storage ability are other factors to consider. Each growing zone includes numerous choices for each category, so always start with day length and work from that point.

Red onions have red skin and white flesh, possibly with a red tint. They have a fairly sweet flavor and are often used raw (or quickly pickled).

White onions have white skin and flesh. These onions are typically pungent and are mostly used in cooking, though a raw dice often tops tacos.

Yellow onions, also called brown onions, have brown skin and yellow tinted flesh. There are yellow onions that are pungent and others that are quite sweet. Yellow onions are often used as a general purpose ingredient, either raw or cooked.

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Sweet onions are large, yellow, mostly short day onions. These are the onions that made Vidalia (Georgia) a household name. They are great used in raw applications like salsa and salad, or sliced for a sandwich.

For a fresh flavor, you could even plant scallions. Also known as bunching onions or Welsh onions, these do not form bulbs, but produce clusters of perfect pencil thin, spring onions. You can also harvest regular onions at the pre-bulb stage for green onions. These are great for a raw garnish on all kinds of dishes, but can also be cooked (like in stir fries).

If you want to go with something unusual, consider an heirloom onion. Potato onions grow in bunches of small to medium bulbs. Tree onions, or Egyptian walking onions, are perennial bunching onions that do not form bulbs. They form clusters of tiny onion plants on stalks that eventually fall over and self-plant.

How to Plant Onions

Ten weeks before your last spring frost date, sow onion seeds indoors ¼ inch deep. Use seed starting mix in shallow trays, about 2 inches deep with good drainage. Moisten the soil well. Cover the seed tray to retain moisture.

Maintain a daytime temperature of 68 to 77 degrees with a slight cool down overnight. Onions start to germinate in about a week. When about 80 percent of the seeds have sprouted, remove the humidity cover.

Keep the seedlings under bright light for 12 hours a day. Transition to natural light and air conditions by hardening them off gradually for several days before planting into the garden.

Prepare the onion bed by adding a generous amount of compost and raking it smooth. Create furrows 1 inch deep, spaced 12 inches apart. In early spring, about three weeks before the last frost date, plant onion sets or seedlings root side down. Space them 5 inches apart within the rows.

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How to Grow Onions

Onions are heavy feeders that do not compete well with other plants. It is important to maintain a weed free onion bed throughout the growing season. They also grow best with consistent moisture. Apply a 2-inch layer of mulch after planting to help with both weed control and soil moisture retention.

Onions require nitrogen to produce big, healthy leaves early in their growth. Three weeks after planting, apply 1 cup of ammonium sulfate or ammonium nitrate fertilizer per 20 feet of row. Repeat every 2-3 weeks. Water right away after each fertilizer application.

If you opt for organic fertilizer instead, the application differs slightly. Half of the organic fertilizer should be tilled into the soil prior to planting. The other half applied to the soil surface 4 weeks after planting.

Stop fertilizing when the neck (the area of the stem near the soil) begins to soften. In a 12 week growing season you will make two or three fertilizer applications.

Onions need 1 inch of water per week. Water once or twice a week with soaker hose or drip irrigation.

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If possible, avoid overhead sprinklers that can cause the spread of disease. Discontinue irrigation when the onion tops begin to lay down at the end of the season.

How to Harvest and Store Onions

Onion tops fall over when they are fully mature. Gently pull the plants and bring them into a warm, dry location to cure.

Lay the onions in a single layer, not touching one another. It takes between two days and a week to fully dry onions. When the necks have shrunken and skins have tightened, clip the onion roots close to the base of the bulb. Then cut the tops an inch above the bulb.

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Pull out any onions that have not fully dried, or are damaged. All the others may be stored in a cool, dry location with good air circulation. Lay them in single layers in shallow baskets, mesh bags, or shallow cardboard boxes with holes punched in the bottom.

See our onion recipes for delicious ways to use them up.

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