Starting Onions from Seed: Simple Bulk Sowing Guide

Curious about starting onions from seed? Try this easy method of bulk sowing! Plus, get extra tips to help you grow, harvest, and store onions.

starting onions from seed by bulk sowing

When I first got into gardening, I noticed a trend. Many homesteaders bought started onion plants online and transplanted them into their gardens. This made me think that starting onions from seed must be really hard. After all, there had to be a reason these seasoned home gardeners were doing it this way, right?

Then, I watched a video by Luke from MI Gardener. He showed how to start onions from seed through bulk sowing. It looked so easy. This inspired me to give it a try myself. Now, after three years, I’ve learned something. Starting onions from seed is not just possible. It’s also incredibly easy and saves money. I get that not everyone has the time or wants to start from seed. But if you’re after an affordable and simple method for starting onions from seed, this is it.

Types of Onions

up close photo of onions

Before diving into starting seeds, it’s crucial to understand which type of onions will flourish in your location. Onions, scientifically known as allium cepa, vary in their daylight needs. They’re grouped into long-day onions, short-day onions, and day-neutral or intermediate-day onions.

Here’s a quick guide to help you choose the right type of onions for your climate:

Long Day Onions

  • Hours of Daylight Needed: About 14-16 hours.
  • Best for: Growing latitudes of 37-47 degrees.
  • Ideal Growing Zones: Suited for zones 6 and colder.
  • Includes sweet, specialty, and long-storage varieties.

Day Neutral or Intermediate-Day Onions

  • Hours of Daylight Needed: 12-14 hours.
  • Best for: Growing latitudes of 32-42 degrees.
  • Ideal Growing Zones: Best for zones 5 and 6, but will grow bulbs in any zone
  • Usually very sweet.

Short Day Onions

  • Hours of Daylight Needed: 10-12 hours.
  • Best for: Growing latitudes of 25-35 degrees.
  • Ideal Growing Zones: Optimal for zones 7 and warmer.
  • Usually planted in fall and mature the following spring.

Living in zone 5a, previously 4b, I opt for long-day onion varieties, focusing on yellow and red storage onions. This way, I have fresh onions in the summer and fall and stored ones to use during the winter and into spring.

When to Start Onion Seeds 

Onion seeds should be planted indoors about 10-12 weeks before your area’s expected last frost date. Enter your zip code here to find your last frost date. Mark this date on your calendar and count backward 10-12 weeks to find your seed starting date.

My last frost date falls around the end of May, so I started my onions indoors in early March. 

How to Start Onions from Seed

seed starting supplies on a table

Now that you’ve figured out which onion varieties suit your garden and the ideal time to start them let’s move on to starting onions from seed using bulk sowing.

Choosing Containers

drainage holes on the bottom of a solo cup

Use at least 3-4″ containers for bulk sowing onion seeds. I generally use solo cups. They’re cheap and available at virtually any grocery store. I slice four drainage holes on the bottom. 

Preparing the Soil

mixing seed starting mix with water

Use a seed-starting mix to ensure proper moisture, air circulation, and a disease-free start for your seedlings. The goal is to find that sweet spot with moisture — you want the mix to be moist enough to hold together but not so wet that it clumps.

Start by gradually mixing water into your soil until it reaches a consistency that’s neither powdery nor soggy. This step is crucial because the right moisture level will ensure your seeds have the best environment to start germinating.

Sowing the Onion Seeds

woman holding onion seeds in her hand

When your containers are ready, fill them with the moist seed starting mix. Make sure it’s packed lightly to avoid any air pockets. Then, sprinkle 30-50 onion seeds on top of the soil in each container. There’s no need for precise spacing since you’re bulk sowing. 

onion seeds in solo cups

After the seeds are down, gently cover them with a thin layer of soil. This is just enough to hide the seeds from view but not so much that they struggle to break through.

planting onion seeds indoors

Watering comes next, and it’s essential to be thorough here. You want the soil to be evenly moist to kickstart germination without drowning your seeds. Use a spray bottle so you don’t disturb the seeds. 

watering onion seeds

Onion Germination and Care

Once sown and watered, your seeds need a warm spot to germinate, around 68-77 degrees. Place them on a warm windowsill or on top of a heat mat. After you see those first sprouts, put them under a grow light. This ensures they get enough light to grow strong and healthy, especially in the early stages when natural sunlight is insufficient.

onion seedlings under grow lights

Monitor the soil’s moisture level as your onion seedlings grow. Check on them daily. Consistent watering is critical — not too much to cause waterlogging but enough to keep the soil from drying out. As your onions grow, trim them to stay around 5″ to help them grow nicely and straight.

Transplanting Onion Seedlings Outdoors

Before your seedlings move to the garden, they need to get used to the outdoor environment. This process called hardening off, involves taking them outside for a few hours each day and gradually increasing their exposure over a week. This gradual introduction helps them adjust to the sunlight, temperature differences, and wind they’ll face outside.

Planting Your Onion Seedlings 

onion starts being planted

When it’s time to move your onion seedlings outdoors, remember that onions need full sun to thrive. Plant your seedlings in well-drained soil where they’ll get at least 6-8 hours of direct sunlight every day.

Space your transplants 4-5 inches apart and arrange rows 12-18 inches apart to give each plant enough room to grow. To keep the weeds at bay and maintain soil moisture, mulch between the rows with straw. 

Caring for Onions

onion plants growing in garden
  • Keep immature bulbs covered with mulch to help protect them. 
  • To get large bulbs, fertilize with nitrogen every few weeks. Stop fertilizing when bulbing starts, and onions start pushing away the soil. 
  • Water about 1″ per week, and more during hot spells.
  • Use a row cover to protect against onion maggots. 
  • Rotate crops every three years to protect against basal rot and other soil-borne diseases.
  • Consider companion planting with carrots and tomatoes to deter thrips. 

Harvesting and Storage

braided onions

The right time to harvest your onions comes when the onion tops start to yellow and fall over, usually in late summer or early fall. After picking, let your onions dry in a spot with plenty of air circulation for a few weeks until the outer skin turns papery.

After they’re dry, cut off the roots, trim the onion tops back to about 1-2″, and store them in a cool, dry place. You can hang them braided or in mesh bags or store them in a box or basket in a cool, well-ventilated area (around 40-60 degrees) to extend their shelf life.

I can’t use my root cellar right now, so I’ve been storing mine in baskets in my basement, and they’ve stored just fine with very minimal sprouting.

onions in a basket

FAQs

Should I soak onion seeds before planting?

There’s no need to soak onion seeds before planting. I’ve never soaked them before sowing and have always had successful germination rates. The seeds are designed to sprout without the need for pre-soaking.

Is it better to start onions from seed or sets?

Starting onions from seeds is cheaper and offers more variety of choices, while onion sets can save time and effort. Seeds are ideal for specific varieties or organic gardening, whereas sets are great for a straightforward and quicker harvest.

Does it take 2 years to grow onions from seed?

No. Onions grown from seed typically mature and are ready to harvest within the same growing season, usually taking about 3-6 months, depending on the variety.

Which is the best month to sow onion seeds?

The best time to sow onion seeds indoors is about 10-12 weeks before the last expected frost, usually between January and March, depending on your climate zone.

Do onion seeds need cold to germinate?

No, onion seeds don’t need cold to germinate. They prefer warmer temperatures, typically 68-77 degrees), for optimal germination.

Can I direct sow onion seeds?

If you live in a warmer climate and your growing season is long enough for onions to mature, you can direct sow seeds in the garden. But in colder climates, you need to start onion seeds indoors for a head start. 

Conclusion

Starting my own onions from seed has convinced me that it’s the best approach. It’s a great way to garden, save money, and enhance skills. I’ve found bulk-sowing onions to be incredibly effective. For me, there’s nothing more satisfying than enjoying the onions I’ve grown 100% myself. So, I hope this guide has motivated you to give it a try; I promise you won’t regret it!

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