Last Updated on August 25, 2023

How to Grow Large Onions – Complete Beginners Guide For Growing Big Onions At Home In Your Garden

Onions are a popular and easy crop to grow, but achieving large onions can be a bit trickier. In this guide, we’ll walk you through everything you need to know about how to grow large onions, from choosing the right variety for your area to key growing tips.

Step 1: Selecting the Right Variety

One of the most important factors in growing large onions is choosing the right variety for your location. Onions are sensitive to day length and will form a bulb when the temperature and number of daylight hours reach certain levels. Varieties are generally classified into three categories: long day, short day, and intermediate day.

• Long-day onions start bulbing between 14 to 15 hours of daylight and are best for northern gardeners. These onions will have time to produce lots of top growth before the daylight triggers bulbing.
• Short-day onions, on the other hand, start bulbing at 10 hours of daylight and are best for southern gardeners. If short-day onions were grown in the north, they would bulb too early and be generally smaller.
• Intermediate-day onions (sometimes called day-neutral onions) start bulbing at 12 to 13 hours of daylight and can be grown in either the north or south.

Step 2: Choosing Onion Sets, Seeds or Starts

There are three options for starting your onions: purchasing onion plants, using onion sets, or starting from seed. Onion plants, also known as onion starts, are started by a nursery or onion company and mailed out to you in bunches of 50. They are typically ready to be transplanted into your garden when they have a diameter of about a pencil.

Onion sets are dry, immature onion bulbs grown the season before. These are generally used for growing green onions, not large onions. They are often available at big box stores, but it’s important to note that the variety may not be labeled. Typically onions grown from sets won’t grow very large.

Starting from seed is a more involved option, but it allows you to choose the variety you want to grow and gives you a larger quantity of plants.

Steps 3. When to Start Onions

Onion seeds should be started indoors about 10 to 12 weeks before your last expected frost. Onion seeds germinate at an optimal range of 65 – 85 F. This should germinate in about 4 days at a steady 77F. See our seed germination reference chart here.

You can transplant your onion plants into the garden 2 – 4 weeks before your last expected frost. Make sure you harden them off a week before transplanting. For your specific planting dates enter your last expected spring frost date below. (You can get find your last frost date by entering your zipcode in PlanMyGarden)

Onion Planting Calculator

Step 3: How to Grow Large Onions – Key Growing Tips

There are a few key tips to follow to ensure they grow to be large and healthy. First, make sure to choose a sunny location with well-draining soil. Onions prefer a soil pH between 6.0 and 6.8 and will do best in a soil that is rich in organic matter.

Onions can be planted at 9 per square foot. See our square-foot planting chart here.

It’s important to keep your onions well-watered, but be sure not to over-water them. Onions have shallow roots and can be easily damaged by excess water. Water your onions with about 1 inch of water a week, or slightly more if very hot and dry weather.

Fertilizing your onions is also key to their growth. To promote leaf growth and encourage larger bulbs, use a fertilizer that is high in nitrogen when growing onions. The more leaves an onion plant has, the larger the bulbs will be. You can also add compost or well-rotted manure to your soil to provide additional nutrients.

Step 4: Harvesting and Storing Your Onions

Onions are ready to be harvested when the tops of the plants start to yellow and fall over. Carefully dig up the onions, being careful not to damage the bulbs. Brush off any excess soil and let the onions cure in a dry, shady spot for a week or two. Once they are cured, cut off the tops, leaving about an inch of stem, and store them in a cool, dry place.

Where to Buy Onion Seeds?

Our favorite seed company is High Mowing Organic Seeds. They have a fantastic selection of onion seeds, including varieties suitable for a range of growing conditions and climates. Whether you’re looking for sweet, mild onions or strong-flavored ones, you’ll find what you need at High Mowing. In addition to their great selection of onion seeds, they also offer a wide range of other organic seeds for vegetables, herbs, and flowers.

Grow Large Onions Is That Easy!

With these tips, you should be well on your way to growing large, healthy onions. Remember to choose the right variety for your area, start your onions from seed or purchase plants, and follow key growing tips like watering, fertilizing, and harvesting at the right time. With a little bit of care, you'll be able to enjoy the delicious taste of homegrown onions all season long.

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