How to Grow Onions:
From Sets and Seeds to Storage

Free Onion Growing Charts

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Onion companions

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Onion relatives

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Day Length: The Most Important Onion Choice

Onions start forming bulbs when the days reach a certain length, so the first step in how to grow onions is to choose a type that matches where you live. The wrong type gives small bulbs no matter how well you care for them.

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Seeds, Sets, or Transplants?

Choosing how to start is an early step in how to grow onions. Sets are the easiest way to start, and sets or transplants are the best way to grow large onions in one season. Seed gives you the widest choice of varieties and costs the least.

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When and Where to Plant Onions

Plant sets and transplants as soon as the soil can be worked in spring, about two to three weeks before your last frost. Both handle light frost.

Early leaf growth makes big bulbs. This is the heart of how to grow onions well. Each leaf an onion grows becomes one ring in the bulb, so a long, cool early season pays off.

Give onions full sun, at least six hours a day, where taller crops won't shade them later.

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Soil Preparation

Good soil is half of how to grow onions. Onions have shallow roots and need loose, rich, well-drained soil, ideally a sandy or silty loam with a pH of 6.0 to 6.8. Work in 2 to 3 inches of compost or well-aged manure and a balanced fertilizer before planting.

Heavy clay stays wet and causes rot; grow onions in a raised bed if that's what you have. Loosen the top 6 to 8 inches and remove clods and stones so bulbs can swell freely.

For sweet onions, avoid sulfur-rich amendments; sulfur makes onions more pungent. Sweet onions also stay milder with steady water and feeding.

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How to Grow Onions: Planting Step by Step

• Sets: Plant 1 inch deep, pointed end up, 3 to 4 inches apart, in rows 12 to 18 inches apart.
• Transplants: Trim roots to about half an inch and tops to about 4 inches, then plant 1 to 2 inches deep.
• For big bulbs: Space plants 4 to 6 inches apart.
• For green onions too: Plant sets close together, then pull every other one as a green onion and let the rest grow into full-size bulbs.

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Watering and Feeding Onions

Onions need about an inch of water a week. Steady watering is a simple part of how to grow onions that many people miss. Their shallow roots dry out fast, and dry spells while bulbs are swelling mean smaller onions. Mulch 2 to 3 inches deep to hold moisture and keep weeds down.

Feed early, then stop. Side-dress with compost or a nitrogen fertilizer (like fish emulsion) in late spring to grow strong leaves. Stop feeding once bulbs start to swell, since late nitrogen gives leaves instead of bulbs and onions that don't store well.

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Growing Big, Sweet Onions

The keys to big, sweet onions are the right variety, good soil, and no stress. Choose a variety that matches your day length, give it rich, loose soil low in sulfur, and keep water and feeding steady.

• Walla Walla Sweet: Softball-size, sweet, and juicy enough to eat like an apple. A long-day onion that grows well in the North.
• Sweet Spanish: Big red or white globes, often up to two pounds; mild, and keeps into midwinter.
• Ailsa Craig: A huge, pear-shaped onion over four inches across; best fresh but stores a few months.
• Yellow Granex: The sweet onion grown as Vidalia in Georgia. Best in warm, short-day regions.
• Giant of Zittau and Stuttgarter Giant: Large onions that store very well.
• Torpedo: An Italian bottle-shaped onion, large and mild.

Sweet onions don't keep long. Use them within a few weeks, and grow a good keeper too for winter.

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Common Onion Problems

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Companion Planting for Onions

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Carrots are the classic onion companion: onion scent helps keep carrot fly away. Cabbage, broccoli, tomatoes, lettuce, and strawberries are also good neighbors.

Keep onions away from beans, peas, and asparagus. See our companion planting chart.

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How to Harvest, Cure, and Store Onions

The last step in how to grow onions is harvest. Harvest when the tops turn yellow and fall over, usually when about half the bed has flopped. On a dry day, loosen the bulbs with a garden fork and lift them gently; don't bruise them.

Cure them for two to three weeks in a single layer in a warm, dry, airy spot out of direct sun, like a covered porch, garage, or shed. Turn them now and then until the necks shrivel and the skins are dry and papery.

Store in a cool, dry, dark place in mesh bags, baskets, or braids. Trim tops to an inch, or leave them long for braiding. Pungent storage onions keep for months; sweet onions only a few weeks. See how to store onions for more.

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Onion Relatives to Grow

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• Bunching onions: Grown for green tops, not bulbs; many are ready in 50 to 75 days.
• Shallots: Mild, sweet bulbs that grow in clusters. See growing shallots below.
• Leeks: Mild stalks for soups; they stand well in cold weather. See how to grow leeks.
• Egyptian walking onions: A hardy perennial with little bulbs on top that "walk" across the bed. See Egyptian onions below.
• Garlic: Planted in fall, harvested next summer. See how to grow garlic.

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Growing Shallots

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Shallots are easy to grow from a single bulb. Everything about how to grow onions applies, with a few differences. Each bulb you plant splits into a cluster of 4 to 12 new bulbs with a mild, sweet onion flavor. They are not the same as scallions, which are young green onions.

• When to plant: in fall where winters are mild, or in early spring in cold climates.
• Planting: separate the clusters into single bulbs. Plant each one root end down, with the tip level with the soil, 6 to 8 inches apart in rows about a foot apart.
• Soil: loose, fertile, well-drained soil with plenty of compost, pH 6.0 to 6.8. The looser the soil, the bigger the shallots.
• Water: about 1 inch a week while they grow. Soggy soil makes the bulbs rot. Keep heavy mulch pulled back from the bulbs.
• Feeding and weeding: feed with compost in spring before the bulbs swell, and keep them weeded.
• Flower stalks: snap off any that appear so the plant puts its energy into the bulbs.
• Harvest: when the tops yellow and fall over, lift the clusters and dry them in a warm, shady, airy spot. Don't cure them in hot sun.
• Storing: in mesh bags in a cool, dry place, ideally 35 to 45°F, shallots keep for 6 to 8 months. Save a few of the best bulbs to plant again.

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Growing Egyptian Walking Onions

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Egyptian onions plant themselves. Also called walking onions, tree onions, top onions, or winter onions, they grow clusters of little bulbs on top of the stalks instead of flowers. When the clusters get heavy, the stalk bends to the ground, and the bulbs root and grow a new plant. Year by year, they "walk" across the bed.

• Very hardy: a perennial that survives cold winters and is one of the first plants up in spring. Plant once, and you have onions for years.
• Planting: set the little top bulbs or the underground bulbs 1 inch deep and about a foot apart, in fall or early spring, in full sun and well-drained soil.
• Give them room to spread, or pull up the extras to keep them in place.
• Greens: cut them anytime and use them like green onions or chives. They are mildest in early spring.
• Top sets: harvest in late summer or early fall. They usually don't form the first year.
• Bulbs: dig some of the underground bulbs in late summer or fall, and leave the rest to grow next year's plants.
• In the kitchen: the whole plant is edible, with a strong onion flavor. Use the little bulbs in soups, stews, and pickles.
• Where to buy: some garden centers carry sets, and specialty seed companies sell them in fall. Or ask a gardening neighbor for a handful.

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Kids love them. Show children how the onions "walk," and they will want to tell all their friends.

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Onion Questions

How long do onions take to grow?

Most take 100 to 120 days from planting sets or transplants to full bulbs. Green onions can be ready in about 60 days.

Can I grow onions in containers?

Yes, especially green onions and smaller bulb types. Use a pot at least 8 to 10 inches deep with good drainage.

Why are my onion bulbs small?

Usually the wrong day-length type for your area, or crowding. Choose the right type and give plants room.

Should I start with seeds, sets, or transplants?

Sets are the easiest. Transplants give more variety choices, and seed gives the most.

What can I do with onions sprouting in storage?

Use them right away; they're fine to eat. Or plant them, and they'll grow green onions.

Source: University of Minnesota Extension.

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More Onion and Vegetable Guides

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