how to grow onion sets

How to Grow Onion Sets + Simple Tips for Larger Bulb Onions in 2024

Learn how to grow onion sets with this onion growing guide! Follow along, grow your own sets and enjoy a bountiful onion bulb harvest this growing season!

Onions are one of the easiest and first plants to grow in the early spring season.  They’re one of the first seeds I start, and in my family of big eaters, we grow a lot of onions! They can literally go on everything! So how do we grow them?

Well, you can start onion seeds, or buy onion sets! Both options have advantages and disadvantages, but for this post, we’re going to focus on growing sets! 

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What is an onion set?

Onion sets are essentially dry, immature bulbs that were cultivated in the prior season, providing a convenient way to kickstart the growth of bulb onions for the upcoming spring season. By utilizing onion sets in your gardening, you can expedite the process of producing healthy and flavorful onions, ensuring a bountiful harvest when spring arrives.

How to grow onion sets

While onion transplants are available in garden centers during late winter and late summer, growing your own onion sets is a simple process that allows you to utilize onion seeds that tend to spoil quickly. Here is a brief guide on how to grow onion sets:

Step 1: Choose a sunny spot with well-drained soil rich in organic matter.

Step 2: Sow your seeds densely to promote smaller bulb growth, lightly covering them with 1/4-1/2 inch of soil.

Step 3: Let the seeds germinate without thinning the onion seedlings.

Step 4: Refrain from fertilizing and water moderately to maintain moisture without excessive leafy growth.

Step 5: Harvest when most bulbs reach a minimum size of 1/2 inch. Bulbs between 1/2 inch and 1 inch are ideal for growing large onions the following year. Smaller bulbs under 1/2 inch may lack sufficient energy for bulb development, while larger bulbs over 1 inch are more likely to go to seed.

Step 6: Trim the tops and store in a breathable mesh bag in a cool, dark location until you are prepared to plant in late summer or the next year.

Growing onions from seeds or sets?

Growing onions from seeds offers a wider range of onion varieties to cultivate, unlike commercial farmers who typically limit their selection. Notably, onions grown from seeds tend to yield larger bulbs, but onion seeds often have poor germination rates.

Conversely, starting with onion sets can expedite the harvest process, especially when time is limited or seed-related mishaps occur. While sets can rescue a harvest, they come with a drawback: the risk of triggering flower stalk growth, which hinders bulb development but can provide seeds for the next planting season.

Knowing how to grow onion sets can help you overcome the disadvantages of onion seeds and sets, and ultimately give you the best results this growing season. 

Growing onions from sets

So how do we grow onions from sets?  Here are some steps to ensure a successful harvest.

Preparing your soil

Onions are considered heavy feeders in the garden, meaning they require a significant amount of nutrients to produce large, healthy bulbs. To ensure successful growth, it is recommended to enrich the soil with a generous amount of fertilizer. Start by preparing the soil thoroughly, incorporating several inches of nutrient-rich compost or well-rotted manure into the top 6-8 inches of the planting area. This will provide the onions with the necessary nutrients for robust growth and development.

Also Read: Ep 10 – Soil vs Dirt: Know the Difference to Prepare your Garden

Planting Onion Sets

Onion sets can be planted in late winter/early spring planting for summer harvest or late summer/fall planting to harvest onions the following spring. (If you are planting in fall, make sure to cover with mulch or grass clipping to protect your onions from hard freeze). 

To plant onion sets, make small holes in soil, about 1 inch deep and 6 inches apart in your prepared garden bed.  Place your onion sets in the hole with the pointy end facing upwards. Because these are transplants, be careful not to bury them too deep. Just cover the sets with soil and press gently to secure them in place. 

Caring for your onion sets

Onions need about an inch of water per week, whether that comes from rainfall or irrigation. Mulch around the bulbs with grass clippings or straw to help keep weeds down and moisture in. Fertilize every 3 to 4 weeks with a balanced fertilizer until the bulbs begin to form. Once the bulbs start to form, switch to a high-phosphorus fertilizer (like bone meal) to help them finish growing.

Also Read: 10 Best Companion Plants for Onions + 2 to Avoid

Harvesting Onions

Onion sets are ready for harvest when most of their tops have fallen over and turned brown. Carefully pull them out of the ground and lay them in a dry, well-ventilated area to cure for a week or two. Once the tops are completely dry, you can cut them off about an inch above the bulb and store your onions in a cool, dark place.

Additional tips for growing onions

If you are looking to cultivate large onions, consider the following additional tips to optimize the growth of the finest bulbs! Implementing these strategies can contribute to the successful development of robust and sizable onions in your garden.

Tip #1: Understand Onion Varieties

To cultivate larger onions successfully, understanding the key role of daylight hours in triggering onion bulb formation is crucial. Onions are classified into short-day and long-day varieties based on day length requirements. Short-day varieties thrive in regions with shorter days like southern states, while long day onions excel in areas with longer summer days like northern states. Planting a long-day type of onion in the south will hinder the maturity of your onions. As a note, there are also intermediate day varieties that have a day length requirement between long day and short day. 

Options for Onion Varieties

If you are looking for a good option for long-day and short-day onion varieties here is a quick list

Long-day onion varieties

Intermediate-day Onion Varieties

  • Hybrid Candy
  • Red Candy Apple
  • Superstar

Short-day Onion Varieties

  • Tx 1015 Supersweet
  • Yellow Granex
  • Red Burgandy
  • Crystal Wax White Bermuda

Tip #2: Planting Depth

As a general rule, onions prefer to be planted shallowly and become stunted when buried deeply. Planting depth also affects the size of your onions. For larger bulbs, plant onion sets with their tips barely below soil level. When the onion bulb starts forming, you can gently remove soil from around the bulb. 

Tip#3: Fertilizer

Onions are considered heavy feeders due to their nutrient requirements. To ensure optimal growth, it is beneficial to provide a generous amount of balanced fertilizer before planting. Once the bulbs begin to form, a consistent supply of phosphorus is essential to support bulb development. 

When I plant my onion sets, I prefer to create a trench that is approximately 3 inches deeper than the set’s placement. This trench is then filled with a mixture of balanced fertilizer and blood meal. By doing so, the roots have access to a nutrient-rich environment as they extend downward, aiding in robust leafy green growth. After filling the trench with the fertilizer mix, I proceed to cover it up and plant my onion sets, setting the stage for healthy onion growth.

And that is it! With a little bit of planning and care, you can easily grow your own onions from sets. Whether you choose to start with seeds or sets, growing onions is a fun and satisfying experience that will provide delicious additions to all your meals. Happy planting! 

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