How To Grow Onions From Seed – For The Biggest Bulbs!
Learn how to grow onions from seed by starting them indoors. These detailed step by step instructions and pictures will set you up to successfully grow the biggest onions you’ve ever seen!
Since I fell head over heels in love with gardening and became passionate (or crazy) about growing as much of my family’s vegetables and fruits myself, I’ve been excited to be able to completely eliminate certain things from my grocery list!
Onions are one of those things that since I’ve figured out how to grow them successfully, I haven’t had to buy any from the store in years!
Onions are easy to grow and last for up to a year in storage, so every year, I start several hundred from seed to make sure we don’t ever have to resort to store-bought onions again!
This post contains affiliate links, which means I make a small commission at no extra cost to you. Get my full disclosure here.
what you need to start onions from seed
- Onion Seeds – Purchase onion seeds from a reputable source (I recommend Johnny’s Seeds). And make sure to choose the correct type of onion for your area. More on that below!
- Seed Starting Medium – I use Promix mixed with some worm castings, but any quality seed starting mix will be fine. Or find out how you can make your own!
- Seed Starting Containers – You can use cell trays or a flat with drainage holes, or 3-4 inch pots.
- Grow Lights – Look for lights that are listed for seed starting for the best results. More on my grow light setup.
- Heat Mat – A heat mat will speed up and aid with germination. I use the countertop above my dishwasher that gets pretty warm. Works really well!
- Fertilizer – Using an organic liquid fertilizer once your seedlings get their second leaf will give your seedlings a boost of nutrients to hold them over until they are planted outside.
how to grow onions from seed
Starting Onion Seeds Indoors
Prepare your seed starting medium by adding enough water to make it moist but now drippy wet.
Sow the seeds 1/2 inch deep and cover with seed starting mix. (You can either plant a few seeds into each cell or scatter the seeds into a larger tray or pot. It will depend on what type of tray you decide to use.)
Cover with a humidity dome or plastic wrap and set the tray on a heating mat or warm surface in your home.
Onions sprout within a week. Once over half of the onion seedlings have come up, remove the humidity dome and take the tray off of the warm surface and set it under grow lights.
The seed coating will most likely get stuck on the seedlings which is usually an issue for most other types of plants, but for onions it’s as simple as taking a pair of scissors and snipping the seed coating off.
Keep the seed starting medium moist, not too wet. When the second set of leaves appear, start bottom watering with a liquid fertilizer (I use fish emulsion) once a week.
Once the onions have grown over 5 inches tall, give them regular trims to keep them around 4 inches tall until they are planted out.
transplanting onion seedlings outdoors
Begin hardening off onion seedlings about 8 weeks before you average last frost date. Watch the weather though. If you are experiencing below average temperatures, you can wait a week or two to start this process.
Once the soil is dry enough to work with plant seedlings in a prepared bed. Bulbing onions should be planted just 1/2 inch deep, and 4 inches apart with 10 inches between rows. For more intense spacing, plant in a grid with 6 inch spacing between each onion.
Onion seedlings can look pretty sad for a week or two after transplanting, before they start to put on new growth, so don’t assume you’ve done something wrong if they look stunted at first.
onion care and harvesting
While your onions are growing, water weekly, but do not let the soil stay saturated or the seedlings can rot.
Side dress onions with a well balanced fertilizer about 2 months after transplanting and again a month later.
Next check out how to harvest, cure and store your onions.
why you should start your own onion seedlings
- Onion seeds are cheaper than buying onion starts or sets
- There is a wider variety of onions available to buy as seeds vs sets
- Onions started from seed yield larger onion bulbs
- Onions grown from sets are more prone to bolting (going to seed) than those grown from seed
fAQ about starting onions from seed
what type of onions should you grow?
It’s very important to figure out what type of onions you should grow in the area you live, otherwise you might not get any onion bulbs at all!
Onions only start to bulb up when they have a certain number of daylight hours. Different varieties of onions require different different day lengths, meaning they will only perform well at a certain latitude.
To find out what latitude you live, check out this tool.
Three Types of onions:
Long Day – Require 14-16 hours of daylight to form bulbs and grow well at 37-47 degrees latitude.
Intermediate – Require 12-14 hours of daylight to form bulbs and grow well at 32-42 degrees latitude.
Also called Day Neutral Onions because they can be grown successfully at other latitudes as well.
Short Day – Require 10-12 hours of daylight to form bulbs and grow well at 25-35 degrees latitude.
where should I buy onion seeds?
Onion seeds have a very short shelf life and generally should be replaced every year to get the best germination results.
Because of this it’s best to source onion seeds from reputable sellers. My favorites are Johnny’s Selected Seeds and Baker Creek.
when should I start onion seeds?
Onions should be started from seed 16 weeks before your last frost to grow big enough seedlings to plant out 10 weeks later.
How long do onions take to grow from seed?
Onions take a long time to grow from seed so they need to be started early. From the time the seeds are started until they are ready to transplant out into the garden is around 90 days.
I start my onion seeds January 1st which gives them plenty of time to thicken up enough to be planted out on St. Patrick’s Day (or there about).
After the seedlings are planted out, they will be in the ground an additional 100 or so days before they are ready to harvest.
When should I transplant onion seedlings outdoors?
Onions are cold tolerant plants, which means they can handle some cold temperatures after they have been properly hardened off.
The optimal time to plant onion seedlings out is about 6 weeks before your estimated last frost date. And you should start the hardening off process 2 weeks before that.
How Do You Prepare The soil for onions?
Onions do best in well draining soil with a neutral to slightly acidic ph. Loosen the soil by tilling or using a broadfork and amend it with compost or well rotted manure.
How Should Onions be spaced at planting?
Onion seedlings should be spaced 4 inches apart in rows 10 inches apart. For larger onions, space them further apart at around 6 inches.
For a more intense spacing, space the seedlings 6 inches apart in a grid pattern.
more gardening posts on the blog
Pin these steps to try later. Tag @athomeontheprairie on Instagram to share what you’ve made with us!
