Ultimate Guide to Growing Awesome Onions

A few years ago my sister, a budding gardener, called me from Philadelphia, where she was thinking about growing onions, and asked me, “When you plant one onion, how many onions do you get?”

Wow! This one question completely re-framed things for me. I had never quite thought about each vegetable in this way.

And when I told her she would get one onion from planting one onion, she said, “Oh, then I’m not going to plant them. It won’t be worth it.”

Of course I had to laugh when she said this, because one of my favorite yearly rituals is growing onions!

But, our exchange was a wonderful reminder that what’s “worth it” to grow our gardens is a purely personal decision.

Since you’re here reading this post, I’m going to assume you feel enthusiastic about growing onions this season.

I don’t know about you, but most of the recipes we cook at home start with onions and garlic in the pan with a little olive oil. We cook most nights of the week, so that means we use a lot of onions throughout the year.

That’s why they’re one of my favorite vegetables to grow in my garden.

I usually plant between 300-500 (mostly storage) onions so we can eat them all winter long. A fun challenge I set for myself is to never have to buy onions at the store.

Some years are more successful than others!

One thing I can say is that over the years, I’ve mastered the art of growing amazing onions. Every July, my onion harvest is pretty epic for a home garden, as you can see from the photos in this post.

Unfortunately, a lot of gardeners confide in me that they struggle to grow onions successfully. I understand, because onions there are some very specific techniques you need to emply to grow onions to brag about.

The goals of this post are to share the best practices for growing onions and get you excited about including them in your garden plan this spring. And you don’t have to grow 500 like me to join the onion enthusiast club!

This post contains affiliate links.

8 Tips for Growing Great Onions

#1: Plant Early

If you’re having trouble with your onions producing nice big bulbs, it could be because you’re planting them too late. The amount of daylight your garden gets during onion growing season will dictate when they start forming a bulb.

If you garden in northern latitudes, your summer days are much longer than gardeners to the south of you. That means you should plant long day onions.

These onions start forming a bulb when the days are 14-16 hours long. This type is best for latitudes between 37-47 degrees in the US.

I live in Wisconsin in zone 5, so I usually notice my onions starting to really produce a bulb around the summer solstice.

By then, my garden is receiving over 15 hours of sunlight every day. See, there are some advantages to living in the north!

Short day onions are best suited for southern gardens located between 25-35 degrees. They’ll start producing a bulb when the day length is 10-12 hours.

There are also intermediate day onions, which you should plant if you live between 32-42 degrees latitude. They’ll start the process of bulbing up when the day length is between 12-14 hours.

Johnny’s Selected Seeds has an informative map of the US highlighting which areas should grow each type of onion here.

Why does this matter? As I wrote above, in zone 5 here in Wisconsin, my onions really start growing bulbs around the summer solstice. The amount and size of the green leaves on the plant at that time will help determine the size of the bulb I harvest in July.

Let me repeat that – if your onion leaves are puny, you’ll get puny onion bulbs.

That’s why it’s critical to plant your onions early – four to six weeks before your average last frost. If you plant them too late in spring, the onions won’t have time to grow enough green leaves to encourage large bulbs.

This is one of the most common reasons gardeners aren’t happy with their onion size. That’s why I put it first on the list. If you don’t know your average last frost, enter your zip code into this site to find out.

Depending on the weather, I usually plant my onions the third week in April.

#2: Evaluate Your Options for Planting Onions

When you’re making your onion growing plans it’s important to understand the three options for selecting plants.

Option #1: Start Your Own Seeds

I’ve gotten the best results by far when I’ve started my own onions seeds in the winter. Starting your own seeds gives you the ability to pick exactly the right kinds of onions for your situation – long day, short day, storage, sweet, red, yellow and more.

And you’ll have access to a plethora of interesting varieties – way more than you would likely find at your local nursery. Onions are one of the first seeds to get started in the winter, about 10-12 weeks before your last frost.

In my house onions kick of the seed starting season around the last week in February.

If you’ve never started seeds you’re really missing out on one of the most fun parts of gardening. You can find lots of seed starting inspiration here.

Option #2: Buy Plants

In this option we’re talking about onion plants, not sets. If you’re not going to start your own plants, this is a good second option.

When you buy onion plants from a farmer or nursery, they’re grown from seeds that were started a few months before, most likely in a greenhouse.

This makes them superior to onion sets, which we’ll talk more about below. The benefit of planting a good-sized onion plant is that it can compete with the weeds better (onion hate weeds!) and will usually produce a better sized bulb than an onion set.

You might be able to find onion plants at a local nursery or farmers market. Buying from a local farmer is always the best option because you can check to make sure the varieties being sold do well in your area.

If you’re buying from a store, use your phone to look up the varieties and confirm they grow well in your region.

You can also order plants online. I’ve purchased onion plants from Dixondale Farms in the past and was very happy with the results. They even carry some of my favorite varieties.

Never buy onions that don’t have any variety listed, or just say red, white, or yellow on the tag. Who knows what you’re getting!

Option #3: Buy Onion Sets

Onion sets look like tiny little onion bulbs. They are generally started the previous year. This means when you plant them it’s technically their second season growing outside.

This is problematic because onions are biennial, which means they flower in the second year. 

A flowering onion is very pretty, but it’s not going to produce a very big bulb because it’ll be focusing all of its energy on that flower. If you’ve ever planted onion sets and been surprised when they flower, now you know why!

At one of my local nurseries the onion sets are never labeled with a variety. But instead are named Red, White, and Yellow. Variety is important, so no-name onions are another reason to stay away from onion sets.

If you’re committed to trying to grow quality onions I highly recommend sticking to the first two options in this list.

And a final note, you cannot plant onion seeds directly into the ground in spring. They won’t have enough time to turn into bulbs. You need to use a plant or a set.

Step 1: Choose the Right Type for Your Latitude

As we discussed in tip #1, you need to choose a variety that’s appropriate for your latitude. Your choices are long, intermediate, or short day. You can use this map of the US here assist you in figuring out the best type for your garden.

Step 2: Choose Fresh or Storage Onions

The second step is to think about when you want to use your onions. Do you want to eat them in the weeks following harvest? Or do you want to store them long term so you can use them in favorite recipes all winter long?

If you’re only growing a handful of onions in your garden you probably want to choose a fresh eating variety. These tend to be sweeter and have more water content, but they don’t store well over the long term.

I usually grow one variety of sweet, non-storage onion in my garden. I keep them separate and try to use them up before my other onions, ideally during the month or two following their harvest.

If you want to keep your onions for many months you should choose a storage variety. This is the majority of the onions I grow in my garden. Depending on how you prepare and store them you can expect them to last from 3-6 months or more.

Finding storage varieties can sometimes be a challenge when you don’t start your own seeds, but you may have luck finding storage variety plants at your local farmers market or nursery. I’ll share some options for ordering online below.

Step 3: Choose Your Color

There are three colors of onions – red, white, and yellow. The differences between the colors are more connected to the varieties than the actual color.

Some store better than others, some are mild, and some more potent. Read the variety descriptions to aid in your decision-making.

I encourage you to plant a mixture of different colors and varieties so you can find your favorites. In my garden I tend to plant about 2/3 yellow onions and 1/3 red onions.

Other ways to find varieties that perform well in your region are to talk to fellow gardening friends, chat up local farmers at the Saturday market, and visit a reliable nursery in your area.

Favorite onion varieties:

Fresh Red: Long Red Florence

Fresh Yellow: Ailsa Craig

Storage Red: Cabernet, Redwing, Red Baron, Red Carpet

Storage Yellow: New York Early, Patterson, Cortland

I order most of my garden seeds from Johnny’s Selected Seeds and High Mowing Seeds.

For ease of browsing I’ve gathered up my favorite onion varieties in several different places so you can shop in the way you prefer.

Find my favorite onion varieties, as well as other tools and supplies, on Amazon here.

Check out my onion variety list on Etsy, as well as my other favorite lists here.

My husband prepping the onion bed in our community garden plot.

#4: Prep Your Garden Beds for Planting

I’ve never tilled my garden in 17 years and I don’t recommend it because it destroys the soil structure over time and also causes a lot of unnecessary work of setting up your garden every single spring. 

Instead, I recommend you establish permanent beds and paths. This style layout will decrease the long term preparation and maintenance involved in your garden every season.

And as a bonus, with already established garden beds spring garden prep is a breeze!

Onions have shallow root systems, so they grow best in loose, crumbly soil that’s free of large chunks of debris like stones and soil clods. Because their roots stay near the surface of the soil, you only really need to worry about the top 7-8 inches.

This means there’s no reason to dig deeply, flip over soil, or disturb your soil too violently.  If you have clay soil like me, you can use a hard rake to loosen and break up the top few inches of soil before planting your onions.

One of my new favorite tools (I bought it for myself for my birthday a few years back!) is the broadfork. It’s great for heavy soils like mine because it allows me to lift and aerate the soil (kind of like fluffing it) without having to use a tiller.

I use to prep all of my garden beds before planting. And this one is made in Wisconsin!

I recommend mixing a balanced organic fertilizer into your soil before planting all seeds and plants. You can sprinkle it onto the soil with your hands and then use the rake to mix it into the top few inches of soil.

#6: Give Your Onions What They Need

How well your onions will ultimately grow is the result of temperature, soil nutrients, soil moisture, and competition. Until the plant receives its critical amount of daylight to start forming a bulb it will be focusing on growing its leaves.

#1 Need – Water!

Because onions are so shallow rooted they need frequent watering. Try to keep the top 3-4 inches of soil moist. Remember that after planting new transplants you’ll need to give them some extra water while their root systems get established.

Depending on the spring rains and weather I try to water new transplants every 2-3 days for the first week or so.

In general, most garden vegetables need one inch of water per week. More on watering your vegetable garden.

I’ve made the mistake of underwatering my onions in the past and they didn’t produce as well as I had hoped. Now I make sure to give them plenty of water.

#2 Need – Weed-Free Existence

Onions don’t compete well with weeds. Try to keep your onions beds as clean as possible and you’ll be rewarded with a stellar harvest.

I hate weeding and try to avoid it at all costs. That’s why my garden is always covered in a thick layer of mulch. It’s best to get your onions mulched as soon as possible after planting.

It can be difficult to get the mulch around the small onion transplants. I find it easier to wait a couple of weeks until they get bigger and then mulch with a layer of marsh hay. You can also use straw, grass clippings, or leaves.

An added bonus of mulch is that it traps in soil moisture so you’ll need to water less and the soil around your onions won’t dry out as quickly.

#8: Prepare Them for Storage

If you’ve grown a small amount of fresh onions and plan on eating them in the next month or so, you don’t have to worry about curing them. Just bring them in the house and start using them in your favorite recipes.

Storage onions require more of a curing process to help you successfully keep them over the winter. I actually love this part of onion growing because I feel so rich in onions when I’m lining them up in my garage.

It’s fun to think about all of the dishes I’ll be using them in throughout the winter.

Ideally, onions are cured at 80-90 degrees and low humidity for several weeks. The temperatures where I live in Wisconsin are generally in the 80 degree range in July, when I’m harvesting and curing my onions.

But, the humidity can be as thick as soup. Still, I’ve had no problems with curing my storage onions. Don’t be too worried about the humidty, there’s not much you can do to control it.

Cure your onions out of direct sunlight. At my old house I spread them out on my east-facing covered porch. Now I have a garage, so all of the onions go straight in there.

It’s best to provide them with some air circulation, so spreading them out on old screens set atop sawhorses is one option. Another option is using your metal seed starting rack. After seed starting season is over I move my metal rack into the garage and use it to cure my onions.

Don’t cut the leaves off the onions until after they’re cured. I like to bend mine at the neck when placing them on the curing rack to seal off the bulb from moisture.

A properly cured onion has a dry outer skin and a dry neck (the part of the plant where the bulb meets the leaves.)

You can check out my article on how to store garden onions or watch the video below to see exactly how I harvest and cure my onions for storage.

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