How To Grow Onions From Seed & When To Start

Growing onions from seed sounds like it might be difficult, but it really isn’t. In this article, I’ll show you exactly when and how to plant onion seeds and give you tons of tips on properly caring for the seedlings.

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.

But you could also try the direct planting or winter sowing methods too, if you have a long enough season. Check out my post on the different seed starting methods for more info.

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.

Related Post: How To Can Onions

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.

Related Post: How To Make Onion Jam (Recipe & Instructions)

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.

Related Post: The Best Pickled White Onions Recipe

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.

Learn exactly how to care for onion plants in your garden here.

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.

If you want to learn exactly how to grow any type of plant you want from seed, then you should take my Seed Starting Course! It’s a self-paced online course that will walk you through everything step by step so you don’t have to struggle to learn by trial and error. Enroll and get started right now!

On the other hand, if you just need a quick refresher on planting them indoors, then grab a copy of my Starting Seeds Indoors eBook.

More About Growing Seeds

• How To Grow Seeds: The Ultimate Seed Starting Guide
• How To Grow Tomatoes From Seed & When To Start
• Growing Spinach From Seed – Everything You Need To Know

Share your tips for planting and growing onions from seed in the comments below.

Steps For Planting Onion Seeds

In order to enjoy the largest yield, planting your onion seeds with care is important. With a bit of planning and the right tools, it’s much easier than you might think.

How To Plant Onion Seeds Step-By-Step

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Once you know how to do it, planting onion seeds is actually very simple. Gather your supplies and follow the steps below to make the process easy and straightforward.

Materials

• Onion seeds
• Covered trays
• Seed starting soil
• OR Starter pellets
• Water

Tools

• Hand trowel
• Heat mat (optional)
• Soil thermometer (optional)
• Moisture meter (optional)

Instructions

1. Prepare the soil - If you’re using pellets, pre-moisten them before placing them in the trays. Otherwise, use your trowel to fill each cell with damp soil mix. Outdoors, mix a slow-release granular fertilizer and organic compost into the bed.
2. Decide how many seeds to plant - Onion seeds often do not have a 100% germination rate, especially older ones. They can be planted in clusters and are easy to separate or thin out later. So for best results, plant 4-8 seeds per hole.
3. Sow the seeds - They’re small, so plant them no deeper than ¼”. You can make a hole and drop them in, or place them on top and press them down gently into the soil. In garden beds, dig shallow furrows and sprinkle them across in groups spaced 4” apart.
4. Cover the seeds - Replace soil over the top and gently press it down so each seed has good contact. Take care not to push or compact the soil to avoid slowing germination.
5. Water until moist - Lightly water the soil until it's evenly moist, but avoid saturating it. It's best to water your trays from the bottom to avoid displacing the small onion seeds. A moisture gauge is a great tool to help you monitor the levels more easily.
6. Cover the trays - Tent a plastic bag or dome lid over the top of your trays to trap moisture and heat during germination.
7. Keep them warm - The optimal soil temperature for onion seed germination is around 75°F. Place them somewhere warm, like the top of your fridge or on a heat mat to speed things up. Use a soil thermometer to help you maintain an ideal temp.