If you’re interested in how to plant onion bulbs you’re not alone. As a component of countless dishes, onions are a staple in the kitchen. They caramelize into delectable sweet and savory bites, provide zing to salads, and add depth and flavor to soups and stews.
When you decide to grow onions, you’ll be met with the question every gardener faces: should I grow onions from a bulb, seed, or transplant?
While all three methods are viable, bulbs are the easiest way to plant onions. Bulbs are readily available, and they require less time and space than seeds do. If you’ve never grown onions before, don’t worry. Learning how to plant onion bulbs is an easy process.
What is an Onion Bulb?
Before you learn how to plant onion bulbs, you need to know that they are. Onion bulbs, also called onion sets, are dormant onions grown the previous year. Rather than being grown to maturity, they are pulled up at an immature size and put into conditions that induce dormancy. Basically, they’re hibernating.
While they make look lifeless, they are alive. They’re simply waiting for the right conditions to break their dormancy and start growing. When you plant them in the proper environment, they’ll resume their growth, rewarding you with large, fresh onions.
Most garden stores carry onion sets in the spring, and online retailers offer a large variety of onion sets via mail order. Once you receive your onion sets, it’s best to plant them right away. If you need to wait, store them in a cool, dark location.
Why Daylength Matters
If you think the only difference between onions is their color and shape, think again. Bulb formation in onions is notoriously sensitive to daylength. Therefore, onions are divided into three main categories based on how much daylight they require for proper bulb formation.
- Short-day: daylength of 10-12 hour
- Intermediate-day: daylength of 12-14 hours
- Long-day: daylength of 14-16 hours
If you plant a long day variety and your days are only 12 hours long in the summer, your onions won’t form bulbs. So, pick a variety that matches your latitude. Short-day onions are well suited for locations closer to the equator, and long-day onions are better suited for those closer to the poles.
When to Plant Onion Bulbs
Depending on your climate, you can plant onion bulbs in the fall or spring. For fall plantings, aim to plant a few weeks before freezing temperatures arrive. To protect the plants from the winter cold, drape row cover over them.
If you live in an area where winter temperatures regularly fall below 20ºF, it’s best to wait until spring arrives to plant. Once temperatures are regularly above freezing, you can plant your onion bulbs outside. If your soil is soggy from spring thaws or rain, wait until it dries out.
Where to Plant Onions
Onions need full sun, plenty of water, and good drainage. If you plant them in a soggy location, the bulbs will likely rot. If you’re dealing with poorly draining or clay soils, consider planting onions in raised beds, since these provide a well-drained environment.
How Deep to Plant Onion Sets
Before planting your onion sets, make sure you give your bulbs a good look. Figuring out which way to place the bulbs in the ground is a key part of earning how to plant onion bulbs. You want to place the sets in the ground with the root side down and the shoot side up! The root side is flat and rough, while the shoot side is pointy. You might even see a small green tip beginning to emerge from the bulb.
After ensuring you’re planting the onions in the right orientation, place them in the soil with the top of the plant peeking out of the ground. Following these directions, the bottom side of the onion sets will be about 1 inch under the ground. Make sure the sides of the bulb are covered before moving onto the next plant.
Plant the onions 4-6 inches apart in rows 12 inches apart. You can plant the bulbs closer together if you are planning to thin out plants to use as green onions.
Other Ways to Plant Onions
As mentioned above, you can plant onions from seed. This process involves more time, space, and work than planting onion bulbs. However, seeds cost less than bulbs and it’s easier to find seeds of unique varieties.
If you want to give your onions a jump start on the season, you can grow transplants. While it’s still cold outside, plant seeds in potting soil in trays or containers. Aim to plant the seeds 4-6 weeks before you want to transplant them outdoors.
When it’s time to transplant, gently pull out an onion seedling from the potting soil. Plant the seedling’s roots 1/2 inch deep. Continue planting the seedlings 4-6 inches apart.
You can also plant onion seeds directly in the ground. To do so, wait until the ground has warmed up and the temperatures are above freezing. Plant seeds 1 inch apart and 1/4 inch deep, with 12 inches between the rows. Once seeds germinate, thin the plants to 2-4 inches apart.
One more way to plant onions is to buy onion seedlings and transplant them. These seedlings look like small green onions. Don’t be fooled – they will turn into big, bulbous onions under the right conditions.
To plant onion seedlings, place the root end of the plants 1/2 inch under the soil. Space plants 3-4 inches apart, with 12 inches between rows.
How Deep to Plant Onion
Since some of the most common questions about onions involve planting depth, let’s review how deep to plant onion.
- Onion bulbs/sets: rooted end 1 inch underground, with shoot tip just barely visible
- Onion seeds: 1/4 inch deep
- Onion seedlings: rooted end 1/2 inch underground
Get Out and Plant
Now that you know how to plant onion bulbs and how deep to plant onion, you’re ready to try your hand at growing these sweet and pungent vegetables.
If you plant onions sets in the spring, the bulbs will reach maturity within a few months. When the onions are the size you’d like, just them pull from the ground and enjoy!