Learning how to grow onions from seed is surprisingly simple! The first time I tried this method, I was shocked at how incredibly easy it was. Not only was it easy, but it barely took up any space in my seed starting area and it produced so well!
If you need a quick and easy method to start your onions from seed, you have come to the right place.
Growing onions from seed can feel intimidating, especially when you can buy those cute little bulbs and plant those but I promise you, this is the easiest (and most economic!) way to grow onions.
Onions are quite slow to grow (90-100 days) so you need to start them well ahead of when you plan to plant them. This means that you need to know which zone you are in so you can plan accordingly.
Here is a link to figure out your growing zone but I will tell you when to start your onions based on each zone down below.
This post will teach you how to grow onions from seed.
The first step: Knowing when to start your onion seeds
Depending on where you are in the world, you will have different seed starting times. Let’s go over the zones and when you would need to start your onion seeds based on your zone.
If you are in zone 2, 3 or 4, you will need to start your onion seeds indoors sometime in January to get a good harvest. Due to the short growing season, 16 weeks before the last frost is a good number to aim for so you get those big bulbs!
If you are in zone 5, you can also start your seeds in January but you can transplant yours into the garden way earlier than us up in the north. For zone 5, your typical last frost would be mid April so you could transplant your onions outdoors beginning of April.
If you are zone 6 and up, your growing season is long enough that you can direct sow your seeds a few weeks before your last frost and they should have enough time to get big and juicy!
Regardless of which zone you are in, you need to harden your plants off prior to planting them outside (another post to follow!) You will also want to transplant them out into your garden 2-3 weeks before the last frost (when your soil is workable.)
The second step: Preparing your soil
I am a firm believer that keeping things simple is the best way to accomplish your goals. This is especially true if you are new to gardening. If this is your first year starting your own onions from seed, we are not going to mess around with making our own soil.
For starting your onions, we are going to pick up a bag of seed starting soil and work with that. This one is the highest rated on amazon so that is what we are going with.
Next, it is important that we kill any of the bug eggs that may be hiding in there. We can do this by pouring boiling hot water over our soil and then letting it dry. I like to do this the night before I plan to start my seeds so it is ready to go in the morning.
This may seem like an unnecessary step but trust me, you do not want a gnat infestation in your house!
After you let your soil dry, you can run it through a strainer to collect any of the big pieces. This step is only needed if you notice a lot of big clumps or pieces of wood. If you are using a seed starting mix, it shouldn’t be that chunky.
Next, it is time to moisten it again. Find a container (I just use my seed starting tray) and add water. You want it to be damp but not dripping wet.
These are the 3.5 x 5.5 containers I use most years.
Container time!
It is time to choose your container! I have not noticed that onions need a dome covering them to germinate and if you start them early, you shouldn’t be in a rush for the to germinate. This means you get to choose whatever container you want! There are a few things to consider.
- You want a large surface area since we will be sowing a lot of seeds in one container.
- It needs to be deep enough so the roots can establish really well. An egg carton would not be an effective seed starting vessel as it is quite shallow.
For onion starts, I always use a large plastic container. The containers strawberries come in are a great option if you are on a budget, otherwise I use the trays you can get from a garden centre that are 3.5 x 5.5 inches. Unfortunately they do not have any on amazon to link to.
Fill your container with the soil and lightly press down. You do not want it to be compacted.
Now it is time to add the seeds!
You may have heard of people soaking their seeds in water prior to planting. I have never done this with onion seeds and I have always had great germination rates. I want to reiterate that I like to keep things simple and if a step isn’t needed, I choose not to do it lol
Sprinkle the seeds as evenly as you can over the top of the soil and then cover with 1/4 inch of damp soil. Lightly water it using a mister or a gentle watering can as it is easy to flood the seeds (squishing them all together or into the nooks on the sides.)
Onion seeds do not need light to germinate so you can leave this in a warm spot in your house (on top of the fridge) to sprout, moving it to your seed starting set up as soon as they poke through the dirt.
Make sure you are watering them regularly using the bottom water method (pouring water into the bottom of the tray and allowing the roots to suck up the water- versus top watering) to make sure the roots establish themselves well.
Finally, add a fan to your seed starts to mimic the wind blowing them. This will ensure they are strong enough to survive outdoors. A few hours a day is more than enough in the beginning, working your way up as time allows.
When onions go wild!
You’ve put in the work and your onions have sprouted! Now they are going wild! You will need to start giving your onions regular haircuts. Once they hit 4-6 inches tall, you are going to use a sharp pair of scissors to trim them down to approximately 3 inches tall.
This will allow the onions to put more emphasis on growing a strong root system and becoming very hearty! You can use the tops in different recipes, just like you would a chive. How exciting is it to have your own fresh produce in the dead of winter!
Harden those onion babies off
When you are 2 weeks from your transplanting date, (check out my garden planning template printable above to keep yourself organized!) it is time to harden your onions off. This means that you slowly get the onions used to being outside so when they do finally go outside, they don’t get stunted or die.
The method is similar with all plants. The first day, you place your onion starts outside close to where they will be planted and you leave them out there for 1 hour.
The next day you leave them out for 2 hours. The following day you leave them out for 3 hours and so on until they are able to stay outside all day.
It's transplanting day!!
You’ve cared for your onions for months and they are strong and healthy. It is now time to plant them! For every square foot of garden space, you can plant 9 onions. For my 4 x 8 foot garden plot, I can plant 288 onions! That is more than enough for storage, salsa, hot sauce, pasta sauce, and processing.
Onions can also be planted next to beets, brassicas (cabbage, cauliflower, brussel sprouts and broccoli,) carrots, dill, kohlrabi, leeks, lettuce, strawberries and tomatoes.
They do not do well next to asparagus or peas. (More info on companion planting to come!)
Finally, I have had a bad experience with magpies eating my onions. It was so incredibly frustrating to lose all my baby starts to birds so I would highly recommend covering your garden beds with onions in them with a cloth row cover.
This row cover will also protect your tender seedlings from frost when you plant them out prior to your zones last frost day.
That's it! This post was all about how to grow onions from seed.
Thank you so much for reading! Please let me know if you have any questions! I love to hear from you over email and in the comments 🙂
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