How to Plant Onions: 3 Ways To Grow Big Crops

There are a few different ways you can grow onions, all with their own benefits. Learn how to plant onions right in this simple, quick guide!

How to plant onion transplants


Knowing how to plant onions is so important for food growers. If there is one veggie you want to grow a lot of in your backyard garden, in raised beds, or in the ground, it’s your own onions! Not only are they important in food, but they’re also important in intercropping and pest control.

Fresh onion is an important ingredient in just about every meal–soup, stew, baked chicken, lasagna, and even pizza. Without onions, food tends to be fairly bland and tasteless. You need onions for cooking, so it’s definitely in your best interest to plant a lot of these aromatic veggies in your garden.

Here are three ways to plant them in your garden like a pro. Follow any of these to grow onions every year!

Preparing for a Successful Onion Harvest

How to plant onion transplants
How to plant onion transplants. Source: Distant Hill Gardens

Especially if you want to start onion seeds, you need the right conditions. Let’s do a little primer on how to prepare and maintain your planting area. We’ll discuss what your onions need to thrive and discuss base growing conditions so you can have giant bulbs from the garden.


Onions need rich, well-drained soil to produce bulbs, regardless of the method you use to plant them. Because you need to keep the soil consistently moist, if your soil has trouble retaining moisture, you can add a little bit of coco coir or peat moss to the soil. Compost and worm castings will also help with moisture retention.

If you live somewhere with compacted native soil, it may be best to cultivate your onions in raised beds or grow bags. Compacted soil is one of the key issues people can face when they endeavor to grow onions.


When it comes to soil nutrients and your onions’ ability to uptake them, soil moisture is key. Without water for certain nutrients to bond to, onions have a hard time growing and developing properly. The base rule is to keep the soil moisture consistent.

Check the planting area regularly to determine if additional irrigation is needed. If the soil is dry up to your first knuckle, water the area. If you have heat waves at any point during your onion cultivation, be more vigilant than usual.

Fertilizing Onions

While any fertile garden soil is enough to support your bulbs and provide a healthy harvest, you can add some fertilizer to the bed to give them a little boost. Add a little bit of organic 10-20-10 fertilizer about 2-3 inches below the planting area before you plant seeds, transplants, or sets.

While it’s not completely necessary, you can feed your onions throughout the growing process. Providing a high nitrogen fertilizer every 2 to 3 weeks and watering it in can help. For this, we at Epic Gardening recommend Espoma Bio-tone Starter Plus Organic Plant Food, which has an NPK of 4-3-3. Try to use a fert that sources naturally if you prefer to use another one.

Onion Spacing and Depth

Transplanted onions
Transplanted onions. Source: nikkorsnapper

Properly spacing your onions is one of the most important things to know ahead of planting time. We’ll discuss specifics in the methods described below, but the general rule for onion planting is to space your onions 2 to 4 inches apart, with at least 18 inches between onion rows. This is especially important if you plan to grow onions from seed.

Which variety you choose to grow will change your spacing slightly. For instance, if you’re growing spring onions or onions specifically for their greens – like Evergreen Bunching Onions or Tokyo Onions – you can space them 1 inch apart. If you’re working with large sweet onion varieties, space them 4 to 5 inches apart. The onion bulb that results should be your guide.

Plant your sets or transplants so the onion tops are sticking out above the ground with just a little bit of the skin poking above the top of the soil. Don’t plant them too deeply, as this can hamper their development.

Pests and Problems

Onion root maggots are some of the most destructive pests of both immature and mature onions. Start by controlling the onion fly, and you prevent the maggots. When you plant your onions, cover them with floating row covers or sprinkle diatomaceous earth around the base of your plantings. Rotate your allium crops every few seasons, so they don’t have a home base.

If you plant your onions just a few weeks before the last frost, your plants may not have time to form bulbs, and you’ll get flower stalks and onion flowers instead. Remember to time your plantings correctly at about 30 days before the last winter frost.

Finally, if you plant your onions in a sub-par potting mix, clay soil, or soggy soil, they could rot before producing big bulbs. Keep your garden moist but not overly wet. The soil can dry a bit between the times you water onions. Amend poor soils with compost or worm castings.

How to Plant Onions From Seed

Onion seeds
Onion seeds with onions. Source: Vilseskogen

Fancy yourself a gardening pro? Planting onions from seed can be difficult, but it can yield onions that mature more quickly. To plant them from seeds, you’ll want to get the seeds in the ground roughly 30 days ahead of your last frost date.

Continue to sow the seeds every few weeks. You can sow them all the way through the fall–in some parts of the south, the prime time for growing onions from seeds is fall through early spring.

Make sure that the seeds you use are no more than one year old. Onion seeds lose their viability quickly if they are stored. Seeds that are older than 1 year are far less likely to grow, making it less likely you’ll grow onions from seed. If you really want to increase your chances of growing onions, use fresh seeds for optimum results!

How to Plant Onions as Transplants

Onions and leeks
Onions and leeks. Source: Distant Hill Gardens

If you already have onions growing in your backyard, you don’t really need to plant new seeds. You can transplant onions from existing onion plants or buy bareroot onion bundles for transplant. You can even start the seeds indoors to be transplanted later.

For those in the northern US: You will be starting onions at least 8 weeks before the last frost of winter. This will ensure that when it comes time to transplant the shoots 6 weeks later, the ground is not too cold and the risk of frost is much lower.

For those in the southern US: Set out the seedlings in either the fall or winter.

Plant the seeds in a seed-starting container or flat. Once they begin to grow, thin out the seedlings until they are roughly ½” apart and the rows are roughly 2″ apart. This makes them easier to separate come transplanting time and ensures the surviving seedlings grow healthy and strong.

A few weeks before they are going to be transplanted, it’s time to toughen the seedlings. Place the tray outdoors, in the shelter (protected from sun and wind), leaving them there for a few hours every day. Leave them there for longer and longer until they are accustomed to a full day of sunlight.

Transplant day: Lift them carefully, shake off the soil, trim back any wispy parts of the plant until the seedling is about 6 inches long, dig a trench for the seedling, and plant. Ultimately, you want the onions to be planted about 4 to 6 inches apart, but for now, plant them a bit closer together. This will allow you to harvest green onions (while they’re still young).

How to Plant Onions as Sets

Onion sets laid out
Onion sets laid out. Source: UnconventionalEmma

For beginners who want the easy route, planting onion sets is the perfect option. They are small onion bulbs or immature onions that have been forced into dormancy while they’re still immature. They will continue to grow as soon as you plant them in your garden, yielding a crop of onions in no time. Most of the time, when people grow sweet onions, this is how they plant them.

Your plant date should be between 14 and 30 days before the last frost date. If you live in a part of the country (or world) where winter is mild, you can plant onion sets throughout both the fall and winter.

The sets need to be planted in shallow furrows and covered with only a small amount of soil–just enough so you can still see the pointed tips. Plant them about 4 to 6 inches apart or closer if you want to harvest onions as young green onions.

The only downside to using onion sets is that your onion varieties are limited. Many of them are sweet varieties, though.

When you’re shopping for onion sets, DO NOT buy the largest ones in the bin. These may end up going to seed instead of producing large onion bulbs. Instead, look for sets that are roughly ½” in diameter. These are the ones that will produce the best quality onions!

Important: Day Length and Onions

When the days start to grow long, the onions start to grow! That’s a rule of thumb you can use to know when to plant and grow onions. This will also help you determine when to start harvesting onions. The onions that grow in the south are different than those grown in the north. This is because there are more daylight hours in the north.

When you’re shopping for onion seeds, find the right seeds according to your region. There are only a few varieties of onions that can be grown anywhere; the rest need to be planted in the right region in order to grow properly. For instance, short-day onions are suited to days with 10 hours of daylight.

Frequently Asked Questions

Sprouted onion sets
Sprouted onion sets. Source: 2_Sheds

Q: What is the proper way to plant an onion?

A: This piece contains three ways you can plant them. All methods are viable ways to grow onions wherever you live.

Q: What is the best month to plant onions?

A: The month you plant depends on where you live. Use your last frost date to determine when to plant. The method you use will also change when you plant. Look at the piece above to determine which method and timing is best suited to your situation.

Q: Can you grow onions from an onion?

A: You can! In fact, two of the methods above touch on this topic. Alternatively, you can chop the base of an onion off, prepare it, and plant it, or simply plant a sprouted onion.

Q: How long does it take for a onions to grow?

A: While it depends on the method of planting you choose, most onions grow within 90 to 100 days.

Q: How many onions will one plant produce?

A: It’s a 1:1 ratio. One onion seed produces one onion.

Q: How often should onions be watered?

A: You’ll want to give your onions a medium amount of water at about 1 inch per week, and keep the soil moist. However, if conditions are already pretty moist, you may not need to water that much. Check the soil to your first knuckle. If it’s dry, water.

Q: How do I get my onions to grow bigger?

A: Choose onions that are right for your region, and plant them at the right time. Then ensure they aren’t overcrowded by other onions or weeds. Water them adequately, and you’re on the path to delicious, juicy onions.

Q: Do you cover onions when planting?

A: Yes and no. You want to plant them deeply enough so most of the bulb is underground, but you want to ensure the onion stalks point out just above the soil surface.

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