How to Grow Really Big Onions in Your Garden

Inside: How to grow big onions at home. Pick a big variety and add lots of water and lots of nitrogen.

Even if you don’t have a farm, you can still grow huge onions in your regular backyard garden. It isn’t complicated, but it does take a bit more care than just planting and forgetting. When I started vegetable gardening, I made lots of mistakes before I figured out exactly what works. I’ll show you what works so you can avoid those mistakes.

Choosing the Right Onion Variety

Not all onions can grow huge. Some types are just built to get bigger, so picking the right variety is your first step to success. For more on basic onion growing, check out my complete guide to growing onions.

Here are the best options for growing REALLY big onions:

Ailsa Craig – Long day onion, grows up to 5″ across with a mild flavor. Stores pretty well.

Walla Walla Sweet – Long day onion that can reach 6″ across! Super sweet but doesn’t store long.

Yellow Sweet Spanish – Long day variety reaching about 3″ across. Mild flavor with decent storage life.

Candy – Works in more places since it’s an intermediate day onion. Can grow up to 6″ across with a sweet flavor and stores for a moderate time.

The “day length” part matters because it tells you if that variety will work where you live. Long day onions need more daylight hours, so they grow best in northern areas. If you’re in the south, look for short day varieties instead.

Understanding Onion Types and When to Plant Them

Picking the right type of onion for where you live is SUPER important if you want those jumbo-sized bulbs. It all comes down to daylight hours.

The 3 Main Types of Onions:

If you plant the wrong type for your area, you might end up with tiny onions or just lots of green tops with no bulb at all. Trust me, I learned this the hard way!

When to Plant

You can plant onions outdoors in late April in most areas since they can handle light frosts. Just make sure to adjust your timing based on your local weather conditions. The right planting time is key to getting those impressively large bulbs.

Soil Preparation

Onions need loose, well-drained soil with a pH between 6.0 and 6.8 to grow really big. Adding compost or aged manure to your soil is super important for improving drainage, air flow, and nutrients. My organic garden soil prep step-by-step guide can help you even more.

• Know your soil type: Figure out if your soil is sandy, loamy, or clay-based. This affects how it drains and holds nutrients.
• Fix your soil accordingly:
  • Clay soil: Add lots of compost or peat moss to help water drain better.
  • Sandy soil: Mix in organic matter to help it hold moisture and nutrients.
• Try raised beds: Raised beds give excellent drainage and are easy to fill with the perfect soil mix for onions.
• Choose small sets: When planting onion sets, pick ones smaller than 0.5 inches across. Bigger sets often flower in summer, which takes energy away from the bulb and gives you smaller onions.
• Keep weeds away: Onions can’t compete well with weeds. Weeds steal water, nutrients, and sunlight that your onions need to grow big. Mulch around your plants to stop weeds and keep moisture in the soil.

Fertilizing for Optimal Growth

Onions are hungry plants and need plenty of nutrients, especially nitrogen, to grow those impressive bulbs.

• Start with a balanced fertilizer: Use a 10-20-10 fertilizer when planting to give your onions the basic nutrients they need for good roots.
• Switch to nitrogen: After a few weeks, change to a nitrogen-rich fertilizer like 21-0-0 (ammonium sulfate) or calcium nitrate. This helps grow strong, healthy tops – and bigger tops mean bigger bulbs!
• Don’t forget calcium: Calcium is SUPER important for growing dense, healthy bulbs that store well. It helps prevent rot when you’re storing them.
• Time it right: Feed your onions every 2-3 weeks until you see the soil cracking around the bulbs (this means they’re starting to form).
• Try organic options: Fish emulsion works great as a natural nitrogen source if you prefer organic gardening.

Watering: Consistent Moisture is Key

Onions have shallow roots and need steady water all season long, especially when bulbs are forming.

• Water deeply and regularly: Give your onions about 1 inch of water every week from rain or watering.
• More water during bulbing: When bulbs start forming, bump up watering to 1.5 inches per week to help them swell up nice and big.
• Don’t overdo it: Too much water can rot your onions. Let the soil dry out a bit between waterings.
• Try drip irrigation: This gets water right to the roots where it’s needed and helps prevent leaf diseases that come from wet leaves.

Sunlight: The Energy Source for Growth

Onions LOVE sun and need plenty of it to grow those impressive bulbs. For the biggest onions, make sure they get 8-10 hours of direct sunlight every day.

• Plant in a sunny spot: Choose the sunniest place in your garden for your onion bed. If you’re not sure how much space you need, calculate how big of a vegetable garden you need or my plan 2 in my 4×4 garden plans.
• Keep them unshaded: Don’t plant onions near taller vegetables or structures that might block their light.
• More sun = bigger bulbs: Here’s a simple fact – each leaf on your onion plant equals a ring in the final bulb. More sun means more leaves, which means bigger onions!

Preventing Pests and Diseases

Onions are pretty tough, but a few pests and diseases can still mess with your dream of growing those massive bulbs.

• Watch out for these pests:
  • Onion maggots: These little white larvae eat your onion roots and bulbs. They’ll stunt growth and cause rot.
  • Thrips: Tiny bugs that suck sap from leaves, making them look silvery and distorted. Bad infestations mean smaller onions.
  • Cutworms: These caterpillars come out at night and can chop down young onion plants right at the soil line.
• Common diseases to look for:
  • Purple blotch: Causes purple spots on leaves that eventually kill the leaf and reduce bulb size.
  • Downy mildew: Shows up as pale green or yellow patches on leaves.
  • White rot: Attacks roots and bulbs, making them rot – this one’s BAD news.
• How to prevent problems:
  • Rotate crops: Don’t plant onions in the same spot every year. This breaks pest cycles.
  • Clean up your garden: Remove and throw away (don’t compost) any sick-looking plants.
  • Start with healthy plants: Use good quality seeds, sets, or transplants.
  • Water the right way: Drip irrigation is best – wet leaves invite disease.
  • Welcome the good bugs: Ladybugs and lacewings eat the bad bugs, so make them feel at home.

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Harvesting and Curing Onions

When you see your onion tops turning yellow and flopping over, they’re ready to harvest. This is usually a pretty clear signal – about 75-80% of the tops will be lying down.

• Harvest on a dry day: Pull or dig your onions carefully when the soil is dry. Don’t yank them – gently loosen the soil around them first.
• Curing is SUPER important: Lay your onions out in a single layer somewhere with good air flow. A garage, shed, or covered porch works great.
• Give them time: Let them cure for 2-3 weeks until the necks are completely dry and the outer skins are papery.
• Prep for storage: Once cured, trim the roots and cut the tops to about an inch above the bulb.
• Store properly: Keep your onions in a cool, dry place with good air circulation. Ideal temperature is 32-40°F with low humidity. Mesh bags, baskets, or even old pantyhose work great for storage!

Properly cured onions can last for months, so you can enjoy your homegrown giants well into winter. Enjoy them in a cucumber and onion salad or as deep-fried onion rings. I won’t judge which one you pick 😉.

Growing REALLY big onions isn’t just for professional gardeners. You can absolutely do this in your own backyard. With these simple steps and a little patience, you’ll be growing onions so big you’ll want to take pictures. And trust me: you will!