Have you ever tried to grow onions from seed? It’s not the most popular option in traditional home gardening, but I have seen the method gain popularity as more and more homesteaders try it for the very first time.
Onions are a great storage crop and will easily store for months in a cool, dry spot. Anyone interested in buying less food from the grocery store can and should grow onions.
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’m going to cover why growing onions from seed is the best, what varieties you can grow, and how to start the seeds and plant them outdoors. I’ll also share plenty of tips and tricks that I’ve learned about growing onions, as well as briefly cover harvesting and curing onions.
There’s a lot to talk about, so let’s get started!
Why You Should Grow Onions from Seed
You technically don’t have to start onions from seed. Nurseries and some online garden companies offer home gardeners onion sets and also bundles of onion plants (much like you would buy a bundle of green onions from the grocery store). Sometimes you’ll also find actual onion plants for sale.
However, I prefer growing my onions from seed over sets or bundles.
Growing onions at home from seed has many advantages.
- more variety to choose from
- easy to grow
- economical
- control of the process, especially if growing organically, peat-free, or plastic-free, etc.
- 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 only come in “red,” “yellow,” or “white,” so you don’t even know what variety of onion you’ll get.
Bundles and started plants are not bad options, 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.
However, I would encourage you to try growing your own onions even if you have to use sets or pre-started plants because it is just a rewarding and delicious crop to grow.
For myself, I enjoy growing onions from seed in late winter because I think it’s fun and I like trying different kinds of onions each year!
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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: bulb formation begins when day length reaches 14-16 hours; best for northern locations with long daylight hours in the summer
- Short Day: bulb formation begins when day length is 10-12 hours; best for southern locations with short daylight hours and mild temperatures in the winter
- Intermediate (Day Neutral): bulbs form at 12-14 daylight hours; best for locations in between northern and southern states in the U.S. and can be grown almost anywhere
- Multiplier onions: similar to shallots, these onions grow a cluster of small bulbs that can be harvested and replanted over and over; some plants (like Egyptian Walking Onions) grow tiny onions at the top of the plant that can be replanted and grown into small to medium size onions
I personally grow long day onions, but I could also grow intermediate day onions if I wanted to. Walla Walla’s are my go-to’s, but I’ve also tried Red Long of Tropea/Florence, Globo, Ailsa Craig, and Red Amposta.
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
- Hoss Tools
- Park Seed
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.
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.
I do recommend using a humidity dome if you have one.
Onions also are not fussy about being transplanted, and additionally have strong, thick roots that are very easy to separate.
For this reason, you can opt to over-sow onion seeds in individual cells or plant them all in a flat like a berry clamshell container. I’ve done both and they both have different advantages.
If I’m planting in a flat, I’ll sow all my onion seeds, and then separate them into individual cells or grow them together until outdoor transplanting time. Planting in a flat helps keep the soil evenly moist, which is great for germination and young seedlings. Also, if you are using seed that is over a year old, planting in a flat ensures you have some seeds come up without wasting individual cells on seeds that didn’t germinate.
Planting in individual cells makes it 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.
There’s no single right way to start onion seeds. As long as you keep the soil moist until the seeds germinate, and then wait for the top of the soil to dry out between waterings once the seeds are up and growing, your seedlings will be fine.
It’s exciting once they first come up. You’ll see that onion seedlings look a bit like grass!
Make sure your onion seedlings are getting 14-16 hours of light per day indoors and start feeding onion seedlings with a diluted liquid fertilizer once a week after they get their second set of leaves.
For more on seed starting, check out my 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 really 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. I haven’t had an issue with this, but I’m only planting out a hundred onions or so and I have the patience to slowly tease the onions apart when needed.
I just wanted to mention this common “tip” and give you my thoughts on it. Clip if you want to, or not! You have the freedom to garden however you want!
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. However, if they are a bit smaller than that, you can still 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 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.
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. Add a thin 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 pretty sad once they’re transplanted, but don’t worry, they will soon grow into large, healthy onions!
Onions can handle a little bit of competition when they are young, however, as they grow and mature, they will require more space and nutrition and will not enjoy being interplanted or being surrounded by weeds.
For the largest onions, make sure your onion beds are well-weeded. I also recommend using a thick mulch to suppress weeds and keep the soil cool and moist.
Caring for Onion Seedlings
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.
Watch for pests and protect your onions with insect netting if needed.
Harvesting Tips
- Clip a few onion leaves throughout the growing season and use the leaves like green onions in your dishes.
- Onions are ready to harvest once the tops fall over and bend at the neck.
- 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.
Grow Onions and Grow Your Green Thumb
Onions are not the fastest growing crop out there, but they are relatively easy to grow once you know how. I hope this guide has given you the knowledge and confidence you need to grow onions from seed to harvest.
I’m on my fifth year of growing onions, 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.
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!