How to Grow Onions:
From Sets and Seeds to Storage
★ SaveLearning how to grow onions is one of the most practical skills a home gardener can pick up. Onions go into almost everything in the kitchen, they keep for months with simple curing, and one well-tended bed can supply a family well into winter.
Onions mostly need a good start, steady moisture, and patience. The few things that trip up beginners are day length and how onions are started, and this guide explains both.
From My Garden
Each year we look forward to the "sweet onion" season. Unfortunately, the season doesn't last very long, and sweet onions such as our favorite, Walla Walla Sweet, don't keep for very long. They tend to start to spoil quickly.
So we enjoy them while we can!
Charlotte
When should you plant onions?
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Free Onion Growing Charts
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Day Length: The Most Important Onion Choice
Onions start forming bulbs when the days reach a certain length, so you must choose a type that matches where you live. The wrong type gives small bulbs no matter how well you care for them. Matching the type to where you live is the first step in how to grow onions well.
| Type | Daylight to bulb | Best region | Example varieties |
|---|---|---|---|
| Long-day | 14 to 16 hours | North (above about 40°N), including the Pacific Northwest | Walla Walla, Yellow Sweet Spanish, Patterson |
| Intermediate-day | 12 to 14 hours | Middle of the country | Candy, Super Star, Cabernet |
| Short-day | 10 to 12 hours | South (below about 35°N) | Texas Grano 1015Y, Red Creole, Yellow Granex |
Seeds, Sets, or Transplants?
| Method | Ease | Variety choice | Lead time |
|---|---|---|---|
| Sets (small dormant bulbs) | Easiest | Limited | None; plant in spring |
| Transplants (young plants) | Easy | Good | None if bought; grow your own 8-12 weeks indoors |
| Seed | More work | Widest | Start indoors 8 to 12 weeks before planting out |
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. Sets are a good choice if you are just learning how to grow onions.
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 planting is a key part of how to grow onions, because the plants need time to grow leaves before the bulbs start to form.
Early leaf growth makes big bulbs. 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.
Soil Preparation
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. Loose, rich soil is the foundation of how to grow onions with large bulbs.
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.
How to Plant Onions
- 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.
| Method | Depth | Spacing for big bulbs | Spacing for green onions |
|---|---|---|---|
| Sets | 1 inch, point up | 4 to 6 inches | 1 to 2 inches |
| Transplants | 1 to 2 inches | 4 to 6 inches | 2 to 3 inches |
| Seed | 1/4 to 1/2 inch | 3 to 4 inches after thinning | 1 to 2 inches |
Watering and Feeding Onions
Onions need about an inch of water a week. 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. Steady water is a simple part of how to grow onions that pays off at harvest.
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.
How to Grow Onions Big and Sweet
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.
Common Onion Problems
| Problem | Likely cause | What to do |
|---|---|---|
| Bolting (flower stalk) | Cold spell after plants are growing; stress | Choose bolt-resistant varieties; use small sets; use bolted onions soon |
| Small bulbs | Wrong day-length type; crowding | Choose the right type; thin to proper spacing |
| Split or double bulbs | Uneven watering | Water steadily; mulch |
| Tunnels or rot at the base | Onion maggots | Rotate crops; clean up debris |
| Silvery streaks on leaves | Thrips | Control weeds; encourage helpful insects; insecticidal soap |
| Soft bulbs after harvest | Wet soil; poor curing | Improve drainage; cure well |
Companion Planting for Onions
★ SaveCarrots 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.
How to Harvest, Cure, and Store Onions
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. Curing well is the final step in how to grow onions that keep all winter.
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 harvesting onions and how to store onions.
★ SaveOnion Relatives to Grow
★ Save- 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 planting shallots.
- 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.
- Garlic: Planted in fall, harvested next summer. See how to grow garlic.
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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