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How to Grow Onions: Complete Guide to Plant and Harvest

Tips and techniques to grow this kitchen garden favorite at home.

Onions

Learning how to grow onions isn’t difficult. Easy to cultivate with a long storage life, backyard gardeners are growing onions (Allium cepa) successfully in most areas of the United States.

We rely heavily on these glorious globes for their sweet to pungent flavor and enjoy them fresh, dried, or cooked in a number of dishes including salads, soups, and souffle.

Onions are so popular, it’s easy to forget that the plump bulbs serve up a super-sized portion of vitamin C, they’re low in calories, and are high in minerals and dietary fiber. Other wellness benefits include cardiovascular health and cold and allergy prevention.

Fun Fact: Records indicate that onions were grown in Ancient Egypt, and eventually arrived in Rome and became known as the word onion (from the Latin word UNIO, which means large pearl).

Starting from seed is often the only way to get classic, gourmet heirloom onions. Planting instructions are included with each ​seed ​packet and shipping is FREE!

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Onion

Onion Seeds

Sow onions indoors up to twelve weeks ahead of outdoor transplanting.

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Common Name: Onion

Botanical Name: Allium cepa

Family: Amaryllidaceae

Plant Type: Biennial, vegetable

Hardiness Zones: 5-10 (USDA)

Sun Exposure: Full sun

Soil Type: Loamy, well-drained

Soil pH: Neutral

Height: 12-18 inches tall

Plant Size: 6-12 inches wide

Bloom Time: Summer

Native Area: Asia

 

Quick Guide: Planting, Growing & Harvesting Onions

  1. Choose a variety suited to day length in your areas (see above)
  2. Direct seed or plant sets
  3. Plant into well-prepared soil in full sun
  4. Keep well watered, fertilize regularly
  5. Harvest by pulling bulbs and drying them for 2 weeks
  6. Common pests and diseases are thrips, wireworms, botrytis rot and downy mildew

Onion Plant Care

Light

For proper growth, onions require full sun for at least six hours each day. The more sunlight, the better when growing onions.

Soil

Proper soil is essential for growing onions successfully. It is essential that the soil be extremely well-drained, perhaps even sandy, and that it contains a significant amount of organic matter.

A loose loam will work just fine. And a soil pH that is neutral to slightly acidic is ideal.

Water

Onions require regular watering to keep their bulbs from swelling. Allow them one inch of water per week.

Overwatering or allowing the bulbs to sit in wet soil, on the other hand, can cause bulb rot. A light layer of mulch can help to keep soil moisture in place.

Temperature and Humidity

To germinate, onion seeds require temperatures of at least 40 degrees Fahrenheit. Onions grow best in temperatures ranging from 68 to 77 degrees Fahrenheit.

Humidity is usually not an issue as long as soil moisture requirements are met.

Fertilizer

Onions are known to be rather heavy feeders. Applying a fertilizer high in nitrogen to them every few weeks will encourage leaf growth, which in turn will result in the production of large bulbs.

Follow the directions on the label to find out how much to use. The foliage growing cycle is finished once the soil around the bulb starts to crack, at which point fertilizer is no longer needed.

Overwintering

It won’t be necessary to overwinter the onion plants because they are typically grown as annuals. Consider planting a fall crop in raised garden beds if you live in a warm climate.

Through the winter, they will aid in maintaining a more constant temperature for the dormant onions.

What is the Difference Between Bulbing onions, Green Onions, and Spring Onions?

Onions come in a lot of varieties, with different colors such as red, yellow, white, and sweet onions all having subtle flavor differences.

Depending on the age at which they are harvested, traditional bulbing onions can be enjoyed in a variety of ways.

Immature onions, also known as scallions, can be harvested before they form large bulbs and are used as green onions.

Allowing them to mature a little longer and form a modest but still underdeveloped bulb qualifies them as spring onions.

You can make bunching onions by gathering a group of young green onions, spring onions, or scallions.

There are also varieties of slender, non-bulbing onions that are true ‘green onions’.

Types of Onions

Aside from size and color, different onion varieties are classified as short-day, long-day, or day-neutral onions. They are put into groups based on how many hours of daylight they need to make good bulbs.

It is essential to select and cultivate onion varieties that thrive in your region. Otherwise, the onion bulbs may not develop properly.

Green onions that don’t bulb are less particular about the precise time of day, but all onions prefer lots of sunlight.

Short Day Varieties

Short-day onion varieties are the ones that form bulbs once daylight hours increase to 10 to 12 hours per day.

They are perfect for southern gardeners because their growing seasons are characterized by consistently shorter daylight hours than their northern counterparts.

If short-day onions are grown in the north, they may go to flower too early in the season as the days get longer. This can cause the bulbs to grow too slowly and be too small.

Short-day onions are said to be sweeter than long-day onions. Short-day onions come in a number of varieties, including Red Burgundy, White Bermuda, Southern Belle, and Yellow Granex (Vidalia).

Long Day Varieties

When given 14 hours of daylight or more, long-day onions form bulbs. If the days aren’t long enough, they won’t grow bulbs.

Long-day onions do best in the north of the United States and Canada.

Some excellent long-day onion varieties include Yellow Sweet Spanish, Walla Walla, Southport White Globe, Rossa di Milano, Red Carpet, Cortland, and Sweet Spanish White.

Neutral Day Varieties

For those gardening in the middle of the United States, day-neutral or ‘intermediate day’ onion varieties are ideal because they produce bulbs in 12 to 14 hours of daylight.

Consider Cabernet, Gladstone, Sierra Blanca, Talon, and Red Amposta for intermediate-day onions.

 

How to Plant and Grow Onions

Site Preparation

Onions grow best in full sun and deep, fertile, well-drained soil with plenty of organic matter.

Work in a generous amount of organic compost or well-aged animal manure prior to planting. All varieties are shallow rooted and need constant moisture near the surface.

Bulbs form in response to day length. If you choose a type inappropriate for your area, it may make small, premature bulbs or no bulb at all.

Long-day onions, Sweet Spanish included, need 14-16 hours of daylight and are grown in northern climates.

Short-day onions, Bermuda included, need 10-12 hours of daylight. These start making bulbs early in the year. Select an onion type that is suited to your zone for best results.

How to Plant Onion Seeds

Starting from seed is often the only way to get classic, gourmet varieties. Sow indoors up to twelve weeks ahead of outdoor transplanting.

Set out two weeks before the last frost date. You can also purchase onion sets from nurseries and local garden centers. Sets are easy to start and mature quickly, but varieties are often limited.

When planting onions allow 2-4 inches between seedlings in traditional rows spaced 1-2 feet apart. Add a layer of mulch and weed thoroughly.

Feed young plants with a high-quality organic vegetable fertilizer and apply Liquid Fish & Seaweed later in the season to improve crop yields, taste, and appearance.

Seed Saving Instructions

Biennial. Plants cross-pollinate and should be isolated by 1 mile from other onions going to seed. Select only the best bulbs and store 3-6 months at 32-45˚F. Plant out bulbs in early spring and allow them to form seed heads. When the heads start to dry, cut off the tops, dry further, and thresh.

How to Harvest and Store Onions

Onions are ready to harvest when the tops fall over naturally, usually 80-100 days after direct seeding; 30-40 days from onion sets.

Spring-planted onions are usually ready to harvest by mid-summer.

When onions reach maturity, their tops turn yellow and begin to fall over. Bend the tops down or even stomp on the foliage at this point to hasten the final ripening process.

Loosen the soil around the bulbs to promote drying. In dry weather, harvest onions by late summer. Wet onions do not cure well and may rot in storage.

Pull bulbs from the ground and let them air-cure for two weeks. Watering prior to harvest makes pulling easier.

After curing, store the bulbs in a cool, dry place such as the cellar. Do NOT keep them in a plastic bag.

Common Insect & Disease Problems for Onion Plant

To prevent insect and disease problems rotate crops and avoid planting onions in the same area for three years. Thrips are suspected if the bluish-green leaves become bleached and wither.

Damage to the lower portion of the stem is often caused by onion maggots. Wireworms (the larvae of click beetles) can also cause serious damage as they feed on all underground parts.

Note: Monterey® Take Down Garden Spray combines the fast knockdown of pyrethrin and the residual activity of canola oil to kill ALL stages of insects, including eggs. Best of all, it breaks down quickly in the environment and may be used on edible crops up to and including the day of harvest.

Common diseases include botrytis rot and downy mildew, both of which are characterized by soft tissue and mold.

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