Growing onions from seed is easy and guarantees you get the variety of onion plants that you want. For northern gardeners, starting onion from seed is a necessity since the growing season is so short.
To know when to start onion seeds, knowing your last frost date is important. Start growing onions (allium cepa) from seed at 10-12 weeks before your last frost date. So, make sure you order your seed packets in December or January!
Types Of Onions
Which type of onions should you grow? Well, if you are looking to learn how to start onion seeds, I’m guessing that you have a shorter growing season and will need long day onions. But take a look to make sure. This map from Johnny Seed’s shows you exactly which type of onion to plant based on your location so that you order the right onion type for your zone.
- Long Day – ex. walla walla, yellow/spanish
- Short Day – ex. red creole, texas super sweet
- Intermediate Day – ex. cipollini yellow
Growing Onions From Seed
Onion seeds are a bit more temperamental than other seeds you might plant. Not only do onions take a long time to grow, but any seeds older than the past year and the germination rate will be quite low. Make sure to over seed when you sow seeds to guarantee some onions. You can thin them out later if needed.
Also, make sure you plant more than enough onions, as they are typically good keepers and the more you plant, the less you need to go to the grocery store!
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Get a Container to Plant In
Onion seedlings like a bit deeper of a container. Clamshell containers are great for growing onions from seed. This will provide enough root growth room later that you won’t have to repot before bringing outside to transplant. Just be sure to drill some good drainage holes.
How to Sow Onion Seeds
Put moist soil into your container.
Using your finger, make a 1/4″ deep spot for the seeds to go. Do not go too deep or the onions will not germinate.
Place seeds in spot.
Sprinkle the seeds lightly with soil.
Water soil slightly with a spray bottle. Do NOT over water here, just want the soil to get a bit damp.
Cover with plastic wrap or put the lid on if re-using a clamshell container.
Place in a warm area. A heat mat works great, but also on top of the fridge or in front of another heat source will work. Keep the onion seeds there until they have germinated. Do not let the soil dry out, especially during the germination phase.
Once the seeds have germinated, remove the plastic covering and place under a grow light, with the light being about 1″ above seedlings. The seeds will need 12 hours of light per day.
When there are 3 leaves on onions, transplant to larger container to encourage root development if needed. Also thin out the seedlings at this time.
When the onion tops get to about 6 inches tall, trim them with scissors, down to 4 inches. Use the trimmings to eat just like you would green onions. Top your eggs or put into some homemade butter.
Transplant onion seeds in early spring. To learn more about transplanting and harvesting onions, check out this blog post here.
Winter Sowing Onion Seeds
If you don’t want to grow seeds under grow lights, you have the option of winter sowing.
- Using a plastic container, such as a clamshell container, poke a few drainage holes in the bottom.
- Make a few ventilation holes in the top of the container.
- Fill the container with at least 3″ of soil.
- Sow the onion seeds, basically right on top of the soil, not any deeper than 1/2″. Using your finger to just push the soil down a tiny bit before placing the seed is usually enough.
- Lightly sprinkle soil on top.
- Water seeds with a spray bottle so the soil is damp, not soaking.
- Place the lid on top of the container and tape shut.
- Place container outside. The seeds will stay dormant until nature tells them it is time to sprout.
- When the seeds sprout, make sure to water them when necessary.
- Open the lid on warmer days, but make sure to close it before night!
While it is possible to directly sow onion seeds into the ground, I have never done this as I live in the upper mid-west. The growing season here does not provide a long enough season for the bulb formation to get large. If you are going to direct sow onion seeds, just wait until the soil temperature is 40 degrees F and keep watered. Generally 1-2 weeks before last frost date.
Transplanting to Garden
Transplant onions when the soil is workable and the night time temperatures are above freezing. Before transplanting however, make sure you harden the onions off. Onions can handle a light frost, but a hard freeze will kill them. Therefore, if you do transplant too early, make sure to cover the onions if there are freezing temperatures.
Plant onions next to things like like beets, tomatoes, lettuce, or strawberries. Do not plant next to peas or beans. Planting lettuce or some kind of salad green in between rows of onions is a great option.
Plant onions in a place where they get direct sunlight for 12 hours. The full sun should be on them for most of the day length.
Onion maggots can travel to other onions so scattering onion plants throughout the garden can help keep those down.
How long does it take to grow onions from seed?
How long does it take to grow onions from seeds? – It varies based on type and variety but 3-5 months.
Are onions hard to grow from seed?
Onions are very easy to grow from seed. It is better than buying onion sets as you can get the exact onion varieties you choose. Sow the seeds, wait until sprouting, and then place in a spot where they can get 12 hours of daylight or grow light a day until ready to transplant! Growing onions from seed is also a lot more economical.
What is the best month to grow onion seeds?
This varies based on what your garden zone is. I’m in zone 5 and the best time to sow onion seeds is in late January. This will ensure the bulbs have enough time to turn into large bulbs.
How many years do onion seeds last?
Onion seeds last one year. While you can plant older seeds, the germination rate will be much lower.
Do onion seeds need heat to germinate?
Yes, onion seeds need heat to germinate. A good option is placing them on a heating pad or on top of the fridge is one way to speed up the germination process. Seeds should emerge in 1-2 weeks.