Skip to Content

How to Grow Onions Indoors (Detailed Step-by-Step Instructions)

Onions are a staple in every family kitchen, and lots of people would love to grow them on their own. Learning how to grow onions indoors is the solution for those interested in vegetable gardening but don’t have the land to do so. Here are some tips for figuring out how to grow onions at home.

Ways of g rowing onion indoor.

When you want to start growing vegetables indoors, onions are a natural and fun first choice. You may have even already grown onions in your home without even realizing it.

Onions are easy to grow and an exciting addition to any meal when you know you grew them with your own hands. Onions are larger vegetables, so this is something you need to keep in mind when you are growing them indoors.

Container is likely your main issue, particularly if you have experience gardening because growing onions is easy. Not every gardener has the luxury of a backyard or deck and patio, so indoor onions will be the way to go. And you can do it. Use this step-by-step guide to growing onions indoors, and you’ll feel like a pro in just a few weeks.

1. Decide What Type of Onion You Wish to Grow

Variety of onion in plate on table.

There are many different ways to grow onions indoors. Additionally, there are different kinds of onions. You can get onion sets, grow onions from seeds, or use leftover onions from your own produce bin. You also want to consider whether or not you want long-day or short-day onions in your indoor garden, and this decision will be determined by your sun exposure.

The long-day onion needs a lot of sun, typically between 14 and 16 hours daily. You’ll want to and need to grow these during the longer days of the year, in the spring and summer.  

Northern climates are ideal for long-day onions on the longer days of the year. Southern climates may have warmer days but less sunlight and fare better with short-day onions. If you have that kind of sun exposure, long-day onions are delicious and worth the work. Short-day onions are just as delicious and only need 6 to 8 hours of sunlight a day.

As with any other produce, you can also purchase day-neutral onions and worry less about your climate when it comes to sun exposure.

2. Know What Your Onion Needs

Woman watering onion in pot.

Knowing what environment to put your onions in means everything to the success of your onions. Onions need the right container size, the right kind of soil, and the right kind of soil. As already mentioned, they also need the right light source as well.

For lights, you can grow onions with grow lights. This is helpful if you want to grow onions in the winter or long-day onions when you don’t have the climate for them. You can also find a windowsill or light space in your home that attracts the most sun for your plants to help them get their six to eight hours of sun.

You may also want to keep lights underneath your onion plants. Every plant likes to be warm. That is the impact of greenhouses. Putting lights underneath your onions will help them to germinate the way that you want them to. They don’t need this warmth all day. You can put your onions on the top of the refrigerator overnight when they are not in the sun to keep them warm while they are in their germination stage.

Additionally, you need a container and soil that will provide the right kind of drainage for your onion plants. Your container should have holes in the bottom for water and soil drainage. You want to see water in the dish that holds your container after you water your onions. You don’t want to keep that water sitting around, though.

You also need to have fertilizer on hand. You can fertilize your onions a few weeks after you seed them. If you are using onion bulbs or onions to grow them, you can start fertilizing sooner. That is a step later in the game of growing onions indoors. Start your onion project with the right tools ahead of time.

Containers are the last thing you need to consider, and you want to have the right ones before you start planting. Onions can grow very big, so you want to have containers that are approximately six or ten inches in depth. You also want to have enough space for three inches between onions as well.

Don’t be afraid to use your larger containers here. Every gardener has a large container they don’t know what to do with, and onions can be your answer for that one. Onions grow well in almost anything, plastic, glass, fiberglass, or ceramic. Whatever you want to grow them in will work. Even an old plastic water jug has worked.

3. Prepare Your Containers

Sprouted onion in pot near windowsill.

It’s time to start putting soil in stuff! Start filling those containers with good potting soil. It doesn’t have to be expensive, and it will be almost a waste if it is. Onions can grow in almost anything they really want to grow.

You do want to be sure that the soil has a pH of at least 6.5. You want to leave approximately one inch of space between the top of the soil and the top of the container. Get a dish to put your container in so that excess water and drainage have a place to go when it needs to.

4. Plant Your Onions, Seeds, Starters, and Sets

Gardener hands planting onion in pot.

There are three ways to plant onions. You can start with onion seeds and start from scratch to have your own onion. You can use a starter onion, such as onion from dinner scraps, or you can purchase onion sets to begin your own onion garden.

For seeds, scatter the seeds on the top of the soil, and cover them with one-quarter to one-half inch of soil. Make sure that you plant them a few inches apart. Three inches will do. You can plant them further apart if you want. The larger the container, the better if you have a lot of seeds to work with. Onions are very large when they grow, and they will want the room to grow.

For containers with partitions, the kind you would typically use for seedlings if you grow seeds often, you want to put two seeds in each partition. You can expect at least one seed to germinate, but both might surprise and delight you. Your seedlings will take approximately four weeks to grow to a few inches high, but could take as long as six weeks. You can move them to containers when they get this big.

You don’t have to use all of the seeds that you have at once. Onion seeds can last for up to two years. If you want to save some of the seeds until the next season, or even the one after that,  you can keep the seeds in the freezer or a cold cellar, and they will keep just fine.

For onion bulbs as your starters, you want to put them in just a little bit deeper. Onion bulbs or the scraps from your dinner onions will work.  The section of the onion that you cut off and do not cook at dinner is all you need to start growing your own onions.  You can do this with dinner onions or green onions for the same result in gardening.

Planting the bulbs approximately two inches deep will work, but you can afford to even put them in a little deeper than that.  Cover the bulb with soil and let nature do the rest.

You can also put bulbs in water to grow them, and this is a very common way to grow onions. It is as simple as taking the bottom of the onion that you cut off at dinner and putting it in a glass of water. Onions grow on their own, you don’t even need to put them in anything if you don’t want to, but it helps. Onions will really like growing in water, and they can even thrive here until you either use them or transplant them.

Put the onions in water that is approximately three inches deep. You can use anything, but glass is best, as you will be able to see its progress and watch its roots develop. If you have children, this makes for a fun experiment, too, as watching them grow could be fascinating. You’ll see some unique gardening magic here.

Soil offers nutrients, but the onions don’t need them to survive, it just helps, and water provides its own nutrients that support growth. You want to change the water every few days to prevent rot and decay. Sunlight also helps, but onions without soil don’t need that either.

5. Fertilizing the Onions

Onion in pot sprayed by fertilizer.

You don’t have to fertilize onions to make them grow, but it helps. You’ll want to do it if you are looking for bigger onions. Fertilizing also creates a nicer onion. When you are fertilizing onions, keep in mind that they get most of their nutrients from the top of the soil. These are big vegetables and plants, but they do not go deep.

So you want to have fertilizer as close to the top of the soil as you can. Feed the soil at the top of the soil nutrients so that the onions can feed off of that when they want to. Nitrogen-based fertilizer is the best kind of fertilizer to use. However, a fertilizer that uses other nutrients such as zinc, copper, and magnesium helps as well.

Chemical fertilizers are the most common, as they will have all of the nutrients that you need to add to your soil. In addition to magnesium, zinc, and copper, you want one that has potassium and phosphorous and a ratio listed as 10-20-10 for fertilizing.

Organic fertilizers such as compost are a common way to fertilize onions. It is not as easy for an indoor garden to do this as it is for an outdoor garden. But, if you have your own compost bin inside, you can use that compost to create fertilizer for your indoor garden.

Creating your own indoor compost bin is an entirely separate conversation, but it can be started with your traditional table scraps, a dead plant-based material, some water, and an airtight container. Mix that up and keep it for a few weeks, and then you can use that on the top of your onion soil as fertilizer. You can add this compost to your onions several times throughout the growth of your onion.

For onions, the roots are going to develop within three weeks.  This is when you can start fertilizing. Any fertilizer should be added up to the mid-point of the growing season. So, if you have onions that started in early spring, you want to stop fertilizing by July or early August at the latest. Fertilizing more than you need to doesn’t really harm the onion, but it could turn them soft and make them less enjoyable.

6. Harvest the Onions

Matured onion plants in pot.

It is time to harvest your onions when your onions have sprouted at least three inches tall. You may even know by instinct alone when it is time to collect the onions. You don’t have to harvest right away, but it will be too soon to harvest if they are any shorter, unless you want smaller onions. There’s nothing wrong with that either.

In general, if it has been three or four months since you planted the onions, or if they have soft spots, they should be ready to eat. The tops of the onions will also have fallen, and that is also an ideal method of determining it is time to harvest.

Once you harvest, you’ll want to dry out or cure the onions before you store them for food. Do this by laying them out on a paper towel or old bath towel, and dry them out for approximately eight to 12 hours. You want to keep them out of the direct sun now. You will know the onion is cured when the leaves of the onions are not green anymore. You can clean them of the soil before you do.

A mesh bag or an onion bin are both popular methods of storing onions for long periods of time. You want to keep an eye on them to ensure they do not rot or sprout while they are storing. If you have cured the onion properly, you will be able to store it until the next season.

7. Avoid Pests

Onion leaves attacked by pest.

Onions are a food source and also a plant source and are as susceptible to pests as anything else in the garden. The most common pests you will see with onions are onion maggots. You won’t miss them if you have them. They are small, and they are underneath the soil, but you will still see them if you have them. Transplanting your onion into fresh soil is the best way to overcome these pests.

One thing that will help here is container gardening. The smell of onions alone is also a repellant to many traditional garden pests. Container gardening makes it a little more difficult for pests to get involved with your onions, but onion maggots may still appear, particularly if the soil is not tended to regularly.