How to Plant Onions: 3 Ways To Grow Big Crops

straw basket of homegrown yellow and red onions on a marble kitchen countertop
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Last Updated on 5 months by Michelle

Growing onions is a rewarding gardening experience that yields a versatile culinary ingredient right from your own backyard. An onion botanical known as Allium cepa, the common onion thrives when you set the stage right: think full sun, well-drained soil, and a little TLC.

As you gear up to plant onions, you have a variety of methods at your disposal, whether starting with onion sets, onion seeds, or even the base of an onion bulb you might otherwise toss after cooking.

As you prepare your garden, choosing the right type of onion is key for success. Short day onions and long day onions are named for the amount of daylight they need to form bulbs. Meanwhile, green onions, also known as scallions, can be harvested earlier as they do not form large bulbs. Planting onions like the yellow sweet Spanish variety, known for their sweet flavor profile, can lead to delicious outcomes. Fertilizer onions correctly, maintain the soil moist, and practice crop rotation to prevent plant diseases and pests such as onion root maggots.

Your adventure in onion production involves several stages from sowing to harvest. Plant onion sets or onion seeds in early spring, typically four to six inches apart in rows to facilitate good air circulation and bulb development. Throughout the growing season, ensure the soil remains rich in organic matter and keep the soil moist without overwatering. Come late summer, when the tops turn yellow and fall over, it’s your signal that the onions are ready. Harvest onions with care, cure them to extend storage life, and you’ll have home-grown onions to enjoy.

straw basket of homegrown yellow and red onions on a marble kitchen countertop
Cured Yellow and Red Candy onions from my garden

Understanding the Basics of Onion Growth

Growing onions can be a satisfying experience in your garden. To start, choosing the right onion variety is crucial. You have short day onions which are ideal for southern regions, and long day onions suited for the north, with day-neutral varieties thriving in most areas. Your local seed companies can help you select the best type of onion for your USDA hardiness zone.

When you’re ready to plant onions, the timing is critical. Early spring is ideal, as onions need cooler temperatures at the start. Prepare a sunny spot in your garden because onions require full sun for proper growth. Well-drained, loamy soil is best to allow for good air circulation and easy bulb development, and incorporating organic matter can enhance soil health.

Onion seeds can be sown directly into the soil, but many gardeners prefer using onion sets or onion transplants. When planting onion sets, space them about four inches apart in rows that are 12-18 inches apart to ensure enough room for bulb growth. A fertile soil helps, and you might consider a light fertilizer designed for onions.

Watering onions is also key – aim to keep the soil moist but not waterlogged. Herbicide-free grass clippings or a light layer of mulch help retain moisture and control weeds. As the season progresses, you’ll notice the green tops grow tall and strong, signaling healthy bulb development.

Lastly, when the tops of your onions turn yellow and fall over, it’s time to harvest onions. Use a spading fork to gently lift them from the ground, and allow them to dry in a mesh bag with plenty of airflow before storage. With these basics in mind, you’ll find that onions can be a must-grow vegetable for gardeners of any level.

A close up picture of a store display of onion seeds.
Onion Seeds at my Local Walmart

Choosing the Right Onion Varieties

Selecting the appropriate onion variety is crucial for a successful harvest. Knowing your region’s climate and daylight hours will help you determine which type of onion will thrive in your garden.

Short-Day Onions

Short-day onions are ideal if you’re in the southern part of the country, where daylight hours are shorter. They initiate bulb formation with 10-12 hours of daylight, making them suitable to plant in early spring. Varieties like the ‘Yellow Granex’, often sweet in flavor, are perfect if you’re aiming to grow onions that are milder and good for salads. Plant onion sets approximately one inch deep and four inches apart in well-drained soil to encourage healthy growth.

 

Long-Day Onions

Long-day onions, on the other hand, require about 14-16 hours of daylight and are your go-to if you live in the northern regions. They tend to form larger bulbs and are typically more pungent, making the ‘Yellow Sweet Spanish’ onion a popular choice. For optimum bulb development, make sure to plant these onions in full sun and keep the soil moist. Sow seeds in a sunny spot in fertile, well-drained loamy soil.

Intermediate-Day Onions

Intermediate-day onions are versatile, developing bulbs properly with 12-14 hours of daylight, which makes them suitable for a wide range of climates, from USDA hardiness zones 5 to 6. Varieties like ‘Candy’ can offer you a balance between size and flavor. These onions can be planted closer together if you wish to harvest green onions, or give them more space to produce big bulbs suitable for storing. Use a light layer of organic matter to keep moisture consistent and remember to practice crop rotation to prevent plant diseases.

By choosing the right onion variety for your area and managing soil and light conditions effectively, you can enjoy a bountiful and flavorful onion harvest. Whether you decide on red, white, or yellow onions, providing enough space, sun, and proper soil will help your onion plants thrive throughout the growing season.

Bed of Fall Planted Short Day Onions
Fall Planted Short Day Onions in Early Spring in my Market Garden

Preparing the Soil for Planting

Before you begin the rewarding journey to grow onions, it’s crucial to prepare the soil properly. This ensures your onion to plant will have the best environment for bulb development, fostering healthy growth from the start.

 

Soil Testing

Firstly, test your soil to establish its current nutrient levels and pH balance. You can purchase a soil test kit or seek a professional soil analysis. For optimal onion production, your soil should have a pH between 6.0 and 6.8; this range is key for onion plants to access the nutrients they need.

Adjusting Soil pH

If your soil is outside the ideal pH range, it needs to be adjusted for successful growing onions. Add lime to raise the pH or sulfur to lower it. Mix these amendments into the soil several inches deep where onions will form bulbs. This will help you to prevent common problems such as onion root maggots which thrive in very acidic soil.

 

A front end loader scooping up a large load of cured leaf mold. A large pile of lead mulch is in the background.
The Black Gold at Keep Gastonia Beautiful. I Use This to Add Organic Matter to my Soil.

Enriching the Soil

Incorporating organic matter like compost or well-drained soil amendments is vital. It improves soil structure, ensuring good air circulation vital for onions, and provides essential nutrients. A couple of inches of compost worked into the top layer of soil can significantly enhance your onion growing experience. Additionally, apply a balanced fertilizer as recommended by the soil test results to provide necessary nutrients for bulb growth.

A garden bed with organic fertilizer sprinkled on the surface is in the foreground, while a backhoe with a bucket of composted leaf mulch is in the background.
Amending Soil With Fertilizer and Organic Matter Before Planting.

Planting Onions

When you decide to plant onions, choosing the right method and time are crucial for a successful harvest. Whether you start with seeds, sets, or transplants, ensuring proper soil conditions and spacing will lead to a bountiful crop of onions.

 

Germinating Seeds Indoors

For those of you eager to grow onions from seed, start by germinating onion seeds indoors. Fill trays with fertile soil, sprinkle the seeds across the surface, and lightly cover them with about a quarter-inch of soil. Keep the soil moist and maintain a temperature above 50°F for the best germination rates. Starting onion seeds inside allows you a head start and leads to stronger onion plants ready to transplant in early spring.

 

Using Onion Sets

Many gardeners find planting onion sets the easiest method to grow onions. These small bulbs are planted outside in the garden at a depth of about an inch deep, with spacing around 4 inches apart in rows. Position them in a sunny spot with well-drained soil to encourage bulb growth. Onion sets can mature faster than onions from seed, making them a popular choice for gardening enthusiasts.

Caring for Onion Plants

Once you’ve planted your onions, proper care is crucial to ensure healthy growth and a bountiful harvest. Paying attention to watering, fertilizing, and managing weeds will support your onions through their growing season.

Watering Regularly

Onions have shallow roots which make consistent watering essential, especially during dry spells. Aim to keep the soil moist to help the onion bulbs form properly. Provide about an inch of water per week, either through rainfall or supplemental watering. Watering onions is particularly important during bulb formation.

Fertilizing

Start with a fertile soil and enrich it with organic matter before planting. Once the onion plants are established, you can fertilize them regularly. Use a nitrogen-rich fertilizer early in the growing season to promote good leaf growth, which is necessary for bulb development. As the onions start to form bulbs, you can ease off on fertilizing.

 

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Weeding and Mulching

Keep the area around your onions free from weeds, which compete for nutrients and space. A light layer of mulch, such as herbicide-free grass clippings, can help suppress weeds and maintain soil moisture. It also provides the bonus of adding organic matter to the soil as it breaks down.

 

Thinning Onion Seedlings

If you’ve sown onion seeds too densely, you’ll need to thin the seedlings. This means pulling out the extra plants to allow about 4-5 inches apart between mature onion plants. Thinning helps to prevent overcrowding, ensuring each onion bulb has enough space to form bulbs properly.

Battling Pests and Diseases

When growing onions, it’s essential to stay vigilant against various pests and diseases that can affect your crop. Effective management involves a combination of proactive prevention and organic treatment methods tailored to the common afflictions that target onion plants.

Common Onion Afflictions

Pests:

  • Onion Maggots: These are a primary threat, especially when onion bulbs are young.

  • Thrips: They cause onion leaves to silver and desiccate.

  • Bulb Mites: Invisible to the naked eye but can wreak havoc on your onion bulb.

Diseases:

  • Botrytis Leaf Blight: Indicated by small white spots that can envelope the entire leaf.

  • Downy Mildew: Shows as pale patches which could lead to distorted or stunted growth.

  • Purple Blotch: This disease is marked by small, water-soaked lesions that turn purplish.

Preventive Measures

Crop Rotation:

  • Avoid planting onions in the same location year after year to thwart onion maggots.

  • Practice crop rotation for at least three to four years to disrupt pest life cycles.

Cultural Practices:

  • Maintain well-drained soil and good air circulation among your onion plants to decrease disease risk.

  • Sow seeds at the appropriate time (i.e., early spring for onion sets) and space them a few inches apart to ensure they’re not overcrowded.

Companion Planting:

  • Use companions like marigolds or carrots to deter onion flies.

  • Healthy diversity in your garden can naturally reduce pest populations.

Organic Treatment Options

Biological Controls:

  • Beneficial nematodes can be introduced to the soil to target onion root maggots.

  • Soft-bodied pests like thrips can be controlled with insecticidal soaps.

Organic Sprays:

  • Baking soda mixed with water can serve as a foliage spray to curb fungal diseases.

  • Neem oil is an all-purpose organic treatment effective against various pests and diseases.

Remember to remain persistent with your observation and treatment actions to keep your onion harvest healthy and plentiful.

Harvesting and Storing Onions

When your onion plants’ tops begin to fall over, it’s time to think about harvesting and storing the precious bulbs you’ve nurtured through the growing season.

Recognizing Harvest Time

Keep an eye on your onion plants as the growing season progresses. It’s time to harvest your onions when the tops start to yellow and fall over, usually in late summer. This is when the bulbs have finished growing and are entering their dormant phase. Wait for the right moment, as premature harvesting can lead to less flavorful or poorly storing onions.

Harvesting Techniques

Gently loosen the soil around your onions with a spading fork, being careful not to damage the bulbs. You want to get under the bulbs and lift them out of the ground to minimize damage and reduce the chances of onion root maggots. Brush off the excess dirt, but don’t wash the bulbs to avoid moisture getting trapped, which can lead to rot during storage.

Close up of a Freshly Harvested Onion on a plastic table outside.
Freshly Harvested Candy Onion from My Garden

Curing and Storage

Once harvested, cure onions for two to four weeks in a dry, well-ventilated area away from direct sunlight. A good air circulation is crucial, as it allows the outer layers to dry out and the green tops to wither, creating a protective layer for the bulb. After curing, store onions in a mesh bag in a cool, dry place; a light layer of loose soil can help keep them dry. They’re best kept at a temperature range of 32 to 40°F (0-4°C) to prolong shelf life.

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