Have you ever tried to grow onions from seed? It’s the best way to get huge, crisp bulbs of the most delicious varieties of onions.
Onions are a great storage crop and will easily store for months in a dark pantry or basement.
Planting onions from seed will give you the biggest onions and offers you the most variety to choose from. Plus, it’s just fun!
In this post, I’ll share why growing onions from seed is the best option, what varieties you can grow, and how to start the seeds and plant seedlings outdoors. You’ll also get all my hard-won tips and tricks to growing, harvesting, and curing onions.
There’s a lot to talk about, so let’s get started!
Why You Should Grow Onions from Seed
Growing onions from seed isn’t the only option. Garden centers sell onion sets and sometimes bundles of onion plants (they look like a bunch of scallions from the grocery store).
However, I prefer growing my onions from seed over sets or dried-out bundles of plants.
Growing onions at home from seed is great for several reasons.
- more variety to choose from
- easy to grow
- economical
- control over the full process
- larger bulbs
Onions are biennial, meaning they grow one year and put their energy into growing a flower that will go to seed the following year.
Onion sets have spent their first year growing a small bulb, which is what you will plant. That means the year they grow out in your garden is their second year, or their flowering/seed-setting year.
Instead of putting their energy into growing a large edible bulb, they will put their energy into going to seed.
Additionally onion sets aren’t usually labeled, so you don’t even know what variety of onion you’ll get.
Bundles of onion plants are not a bad option; however, be wary of buying these at big box stores, as I’ve found they don’t care for the bundled onion plants very well and allow them to dry out too much.
For myself, I enjoy growing onions from seed because I think it’s fun and I like trying different kinds of onions each year!
Not sure where to plant your onion seedlings? Read How to Choose the Best Location for Your Garden.
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What Variety of Onion Should You Plant?
When onions grow from seed in our gardens, they will first grow bushy, healthy green onion tops before forming their bulbs at the base of the plant. The bulb-forming process is triggered by the amount of daylight plants are exposed to.
Depending on where you live, you can choose from long day, short day, or intermediate day onions. A fourth category of perennial onions is also available for home gardeners, although you plant small, immature bulbs instead of from seed.
- Long Day Onions: bulb formation begins when day length reaches 14-16 hours; best for northern locations (37° latitude and above) with long daylight hours in the summer (think summer solstice)
- Short Day Onions: bulb formation begins when day length is 10-12 hours; best for locations below 35° latitude with short daylight hours and mild temperatures in the winter
- Intermediate (Day Neutral) Onions: bulbs form at 12-14 daylight hours; best for locations between 32° and 40° latitude and can be grown almost anywhere
- Multiplier onions: onions that grow a cluster of small bulbs that can be harvested and replanted over and over
If you’re in the US, find your state on the map to figure out which onion you should grow.
Long day and intermediate day onions are usually planted in the spring, while short day onions are planted in the fall to grow during winter.
If you’re on the border between intermediate day and long or short day, intermediate day would be the safer bet.
Onion Varieties I’ve Grown
I grow long day onions even though I could also grow intermediate day onions.
Walla Walla’s are my go-to’s, but I’ve also tried Red Long of Tropea/Florence, Globo, Ailsa Craig, and Red Amposta.
Red onions are my favorite to make quick pickled onions with. The color is so pretty!
I also grow Egyptian Walking Onions. They are so fun to grow in the garden!
Here’s a list of seed companies I recommend if you want to purchase seeds online.
- Botanical Interests
- High Mowing Organic Seeds
- Pinetree Garden Seeds
- Baker Creek Heirloom Seeds
- Johnny’s Selected Seeds
- MIgardener
- Quail Seeds
When to Start Onion Seeds
Onions need to be started about 10-12 weeks before last frost.
I actually like to plant my onions a month ahead of last frost, so I typically start 10-11 weeks before the end of March, which ends up being around the middle of January.
I know it seems so early, but you want your onion plants to be as thick and well-developed at planting time as they can be in order to get the largest bulbs possible.
See this post for more on finding your last frost date and when its safe to plant.
Starting Onion Seeds Indoors
Onions are easy to start indoors, although they can take a little while to germinate (7-14 days).
Make sure to use fresh seed as germination rates for alliums decline rapidly after the first year. You don’t have to use a heat mat unless your seed starting room is cooler than 60°F. My seeds germinate fine in my home without a heat mat.
Because onions are not fussy about being transplanted and have strong, thick roots that are easy to separate, you can either sow all your seeds in a flat like a berry clamshell container or in cell trays.
I’ve done both and they have different advantages.
If I’m planting in a flat, I’ll sow all my onion seeds, and then separate them later when it’s time to transplant.
Planting in a flat helps keep the soil evenly moist, which is great for germination and young seedlings.
Also, for old seed, planting in a flat ensures you have some seeds come up without wasting individual cells on seeds that didn’t germinate.
However, planting in individual cells makes it much easier to plant onions out. You won’t have to spend so much time separating the plants, but can just pop the whole cell in the ground.
Find everything you’ll need for seed starting in this post on essential seed starting supplies.
How to Sow Onion Seeds
- Sow 2-3 onion seeds per cell in a seed-starting tray.
- Plant them about 1/4″ (6 mm) deep in the soil.
- Mist with water and cover with a humidity dome.
- Keep the soil moist until the seeds germinate.
It’s exciting once they first come up. You’ll see that onion seedlings look like grass!
To keep onion seedlings happy, give them 14-16 hours of light per day indoors and start feeding the seedlings with a diluted liquid fertilizer once a week after they get their second set of leaves.
Read more on planting from seed and caring for seedlings in my complete seed starting guide.
To Clip or Not to Clip?
Many onion growing guides mention trimming your onion tops as they grow. Supposedly, this helps onions grow better.
I haven’t found this to be true.
Onions are plants. Plants get their energy by photosynthesizing through their leaves. If they have fewer leaves, then they won’t be getting the most energy they possibly can.
Less energy = less growth.
It can be helpful to clip your onion leaves at planting time if they are tangled and just hard to work with, but in general, I recommend leaving them alone.
Even though I plant out a hundred onions or so every year, I don’t mind teasing the onions apart when needed.
I just wanted to mention this common “tip” and give you my thoughts on it.
Clip if it makes sense for you to do so. Your garden, your rules.
Hardening Off Onion Seedlings Outdoors
After several weeks of growing indoors, your seedlings will be ready to plant outside.
Ideally, you’ll want your onion seedlings to be about as thick around as a pencil.
But don’t stress if they are a bit smaller than that. Onions bounce back pretty quickly. Plant them out and they will be fine.
Just make sure the ground is thawed, drained, and not too cold. You’ll also need to keep an eye on the weather if you are planting before your last frost date and cover the plants with frost fabric like this kind if temps get below freezing.
But before you can plant the onion seedlings out, you will need to harden them off first.
Hardening off is the process of gradually acclimating plants to more and more sun and wind exposure.
Outdoor growing conditions are much more harsh than indoor growing conditions, and you don’t want to shock your baby plants and kill them before they’ve had a chance to acclimate.
To do this, place your seedlings outside in a semi-protected location like a porch or close to the wall of the house where they can get sun for at least one hour.
Bring them back indoors after an hour of direct sunlight.
Increase the amount of sun exposure the seedlings get every day by one to two hours until the seventh day. Then leave plants in direct sun all day.
I have also hardened seedlings off the “lazy way” by putting them in a shaded location (my back porch) all day for the first day, then in a dappled shade location (the corner of the porch) for the second day, and so on until they are able to be in full sun all day long.
I do bring them inside every night, though. Can’t let those babies get too cold!
Prepping the Soil and Planting Onions
When onion seedlings are fully hardened off, choose a spot in the garden that gets 8 hours of sunlight a day or more.
Then prepare the soil where they will be planted by loosening with a broadfork if needed.
If your “garden” is still technically a lawn, try sheet mulching. Sheet mulching is a quick and easy way to start new gardens without digging everything up.
See how to sheet mulch in this post.
Once your soil is ready, add a 1 inch (or a couple centimeters) layer of compost and mix in a balanced, slow-release organic fertilizer like Plant-Tone or Garden-Tone.
Although you wouldn’t think it, onions do benefit from nitrogen to grow large.
The larger and bushier their onion tops grow, the bigger the bulb will be once bulb formation is triggered.
Onions are heavy feeders, so adding both compost and slow-release fertilizer will make your plants very happy.
Finally, plant the onions 4-6″ apart in rows. You can space onions as far as 9″ apart if you want the largest onions. I usually shoot for medium to large, so the 6″ spacing works best for me.
They may look floppy and sad after transplanting, but don’t worry, they will soon recover and grow into large, healthy onions!
Onions do not enjoy being planted too close to other plants or being surrounded by weeds.
For the largest onions, make sure your onion beds are well-weeded.
I also use a thick mulch to suppress weeds and keep the soil cool and moist. Crucial for those hot summer months when the onions are nearly ready.
And if you’re looking for more veggies to plant in early spring, read this post on the best spring crops to plant with your onions.
Caring for Onion Plants
Once onion seedlings are in the ground, care for them like you would your other vegetable plants. They will need regular water, good airflow, supplemental feeding (if you’re wanting mega onions!), and protection from pests.
Water onion seedlings by drip, if possible, to avoid wetting the leaves too frequently.
Feed with a balanced organic liquid fertilizer like Neptune’s Harvest every other week after planting for the largest onions.
Watch for pest damage and protect your onions with insect netting if needed.
Harvesting and Curing Seed-Grown Onions
You don’t have to wait for the onions to be completely finished growing before you can start harvesting.
Onions can be enjoyed at any time during their growing process.
Clip a few onion leaves throughout the growing season and use the leaves like green onions in your dishes.
Pick a fresh bulb and cook as you would with any storage onion.
However, if you do plan on harvesting them for storage, it’s best to wait until they’ve finished growing.
Onions will be ready to harvest for storage once the tops fall over and the neck is bent like in the photo below.
They should have sizable bulbs and their outermost skins will have started to dry and may be tan in color.
To get the longest shelf life out of your storage onions:
- Stop watering onions a week before you plant to harvest.
- Harvest onions on a dry day.
- Cure onions in a warm, dry location out of direct sunlight for a couple weeks. Necks should be completely dry and not squishy at the end of the curing process.
- Braid onion tops together if desired, or clip off the tops for storage.
- Keep as much of the onion “paper” intact to protect them in storage.
I love braiding the onion tops together and hanging them in my basement pantry to save shelf space.
But I do also clip some tops right after harvest to use for seasonings and soups. See how I keep onion tops from going to waste in this post.
Once onions are done growing, keep gardening by succession planting. Find out what you can succession plant after your spring crops are done in this post.
Grow Onions and Grow Your Green Thumb
Onions are not the fastest growing crop out there, but they are so rewarding to grow, even if you’re not a homesteader!
I hope this guide has given you the knowledge and confidence you need to grow onions from seed to harvest.
I’ve been growing onions since 2021, and I learn more about growing these amazing vegetables every year.
Every year I feel like my onions grow a little better and I grow as a gardener, too.
Gardening, like growing onions, is a slow, but rewarding process. Wherever you are in your gardening journey, enjoy every last bit of it!
Are you growing onions this year? Do you have any tips that you want to share? Answer these questions and leave any other comments in the comments section below!