It's easier to grow onions from seed than most people think, and you'll have the best selection of varieties to choose from.
All you need to get started is some basic equipment and know-how, and I've shared it all in this guide.
Below you'll find detailed step by step instructions for how to plant and grow onion seeds.
Plus I give you essential seedling care tips so you can keep them healthy from germination all the way through transplanting them into your garden.
Growing Onions From Seed
You can grow any type of onion you want from seeds, so you'll have a much bigger selection to choose from than you would if you bought sets.
Once you know the right types for your region, it's fun to experiment and find the ones you love.
Types Of Onion Seeds To Plant
The most common way we think about onions is by color: white, red, and yellow. But when it comes to starting the seeds it's important to find the right variety for your climate.
They're divided into three categories based on the daylight hours they need.
- Long-day – These require 14+ hours of sunlight per day. Some popular ones are Yellow Sweet Spanish, Red Wethersfield, Walla Walla, Yellow Gold, and Copra.
- Day-neutral – They need 12-14 hours of daylight, and include Candy, Red Stockton, Sierra Blanca, Cabernet, and Super Star.
- Short-day – Best for regions with 10-12 hours of sun. Choose Granex Yellow, Red Burgundy, Red Creole, Southern Belle, White Bermuda, or Texas Supersweet.
Best Way To Grow Onions From Seed
Many gardeners choose to get a head start on the growing season by starting onion seeds indoors.
Some can take months to reach maturity, so it's best to plant them early if you live anywhere that has long and freezing winters.
How Long Does It Take To Grow Onions From Seed?
The number of days it takes to grow onions from seed to harvest can vary widely between the different varieties. But on average, the time from sowing to maturity falls somewhere between 3-5 months.
You can always enjoy some of your crop earlier however, they are edible at any size. You can pull up young ones and use both the tops and the bulbs in your recipes.
When To Plant Onion Seeds
Exactly when you should plant onion seeds depends on the method you want to use. Indoors, start them 10-12 weeks before your average last spring frost date.
To direct sow outside, wait until the ground is workable in the early spring, and the temperature is above 40°F. This is usually a few weeks before your last frost date.
In warmer climates, you can plant them directly in the garden in the fall or early winter.
Onion Germination Time
Though it can vary depending on the environment, you can expect to see your onion seeds germinating sometime between 5-10 days after sowing them.
If it's too cold or wet, the process can be slowed. Keep the soil evenly moist but never soggy, and add bottom heat to help speed things up.
What Do Onion Seedlings Look Like?
Right after they sprout, onion seedlings look like thin green stalks that closely resemble chives. The leaves will get taller and thicker as they mature.
How To Care For Indoor Onion Seedlings
Properly caring for onion seedlings makes all the difference. Give them the best light, water, and occasional pinching to encourage strong roots.
If you encounter any issues, read my guide on how to fix common problems here.
Light
Once sprouted, onion seedlings will need 12 hours of light a day. It's hard to achieve this indoors, even with a sunny window. To avoid legginess, I recommend using a grow light set 1" above the tops.
Water
They need to be kept evenly moist at all times, but never in water-logged or soggy soil. Use a moisture meter to help you get it just right.
Fertilizer
After three weeks you can begin using a half-strength organic fertilizer once a week. Compost tea or fish emulsion are also two great options.
Air Circulation
As soon as they have sprouted, remove the plastic to encourage air flow around them. You can also use an oscillating fan set on low nearby to simulate wind and strengthen them.
Thinning
Though you can plant onion seeds in clusters, it's best to thin them so there's only one plant every 4-6" in the garden, or one per cell or pellet indoors.
Potting Up
When seedlings have at least three leaves each indoors, pot them up into larger containers to encourage stronger root development. They transplant very well, so you could use either plastic or plantable pots for them.
Transplanting Onion Seedlings Into The Garden
You can transplant your onion seedlings into the garden when the soil is workable in early spring, and nighttime temps are above freezing.
Although they're cold tolerant, it's important to harden them off so they can adjust to the elements, and survive the transition outdoors.
They can tolerate a light frost, but it's best to cover them if a late freeze is in the forecast.
FAQs
Here I've answered some of the most commonly asked questions about growing onions from seed. If yours isn't on the list, please add it to the comments section below.
How many onion seeds do you plant per hole?
You should plant 4-8 onion seeds per hole for the best results. Newer seeds have higher germination rates, so you can use less if you want.
How deep do you plant onion seeds?
Plant onion seeds no deeper than ¼". They're small and may not germinate if they're too far down.
Do onion seeds need light to germinate?
Onion seeds don't need light to germinate, but the seedlings will require 12 hours a day as soon as they begin to emerge.
Are onions hard to grow from seed?
Onions are not hard to grow from seed once you understand their light, water, and temperature needs.
Can I direct sow onion seeds?
Yes, you can direct sow onion seeds in early spring, or as soon as the soil is workable if you live in a cold climate, or in the fall in warmer areas.
What month do you plant onion seeds?
The ideal month to plant onion seeds varies based on where you live. In cold areas, start them indoors 10-12 weeks before your average last frost date. To direct sow, plant them 1-2 weeks before the last frost.
Now that you know all the steps, supplies, and tips on growing onions from seed, you'll be able to enjoy any type you like, and tackle it with confidence.
