How to Grow Onions from Seed to Harvest: Complete Growing Guide

Onions are a must in any vegetable garden. And honestly, you can’t beat the taste of homegrown ones. In this guide, I’ll show you exactly what works. You’ll learn whether to use seeds or sets, how to choose the right type, and how to grow big, beautiful bulbs.

How to Grow Onions

Why Early Spring Matters

Early spring is the perfect time to start onions. This is when everything begins. There is always a debate. Should you use seeds, sets, or transplants?

I prefer seeds. They are cheaper. They are less likely to bolt. Bolting means the plant flowers too early. When that happens, a hard stem forms inside the bulb. That ruins the texture. Also, onions grown from seed are usually bigger.

How to Sow Onion Seeds

I started my seedlings a few weeks ago. They are doing great. Here is how I sow them.

  • Use a plug tray
  • Fill it with sieved all-purpose potting mix
  • Make small depressions
How to Sow Onion Seed

Now comes the easy part. Use the multi-sow method. This means adding small pinches of seeds into each hole. Aim for 5 to 8 seeds per plug. No need to be exact. Cover them lightly with soil.

Where and When to Grow Seedlings

Late winter is the best time to sow. I started a bit late, but it still works. If your weather is mild:

  • Nights above freezing
  • Days above 10°C (50°F)

You can leave them outside. If not, start them indoors. This speeds things up. Once they grow to about 2 inches (5–6 cm), plant them outside.

Seedling

What About Onion Sets?

If you want an easier start, use sets. Sets are small onions grown last year. They are stored and sold for planting. But there is a downside.

What are Sets

Onions are biennial. They flower in their second year. Sets are already in year two. So they may bolt more. Still, most sets grow fine. To reduce bolting:

  • Choose heat-treated sets
  • Do not plant too early in cold soil

Plant them in spring. Around the spring equinox works well in mild climates.

Preparing the Soil

Onions love loose, well-drained soil. Before planting:

  • Add compost or well-rotted manure
  • About 1 inch (3 cm) deep
Preparing the soil

They also like nitrogen. It helps grow strong leaves. Strong leaves mean bigger bulbs. I also add chicken manure pellets at planting time.

Planting Onion Sets

Plant sets like this:

  • Push them into the soil
  • Keep the tip just above the surface
  • Roots go down

Spacing matters.

  • 6 inches (15 cm) apart for medium bulbs
  • 10 inches (25 cm) apart for bigger bulbs

Check each set before planting. If it feels soft or rotten, throw it away.

sowing sets one

Starting Sets in Trays

If the soil is too cold or wet:

  • Plant sets in plug trays
  • Use potting mix
  • One set per plug

Let them grow until conditions improve. By then, they will have strong roots and shoots.

Sowing Sets

Choosing the Right Onion Type

There are many varieties:

  • Sweet white
  • Red
  • Yellow

You can also choose award-winning types. But the most important factor is day length.

Understanding Day Length

Onions grow based on daylight hours.

  • Short-day onions: bulb at 10–12 hours
  • Long-day onions: bulb at 14–16 hours

Choose based on your location. At 52° latitude, long-day onions work best. They also store well.

Spring Onions Bobs

Planting Seedlings

My seedlings are almost ready. Wait until:

  • Soil is workable
  • Soil is slightly warm

Plant clusters as they are. Do not separate them. Space them about 10 inches (25 cm) apart. You will get slightly smaller bulbs, but the total yield stays the same.

Seedling

Using Transplants

If you buy transplants:

  • Plant them individually
  • Space 6 inches (15 cm) apart

Increase spacing if you want bigger bulbs.

Using Transplant

After Planting: What to Do

Now comes the waiting part. The plants will grow leaves first. Bulbs form later when day length is right.

Watering

Keep Weeds Away

Onions have thin leaves. Weeds can easily take over. Remove weeds regularly. Be careful. Onion roots are shallow. Use hands or a gentle hoe.

Remove weadings Regularly

Watering and Moisture

Onions need:

  • Full sun
  • Moist soil

Water during dry weather. This helps bulbs grow bigger. Drip irrigation works best. It keeps leaves dry and reduces disease risk.

Mulching

Mulch helps a lot.

It:

  • Keeps moisture in
  • Reduces weeds

Use:

  • Dry grass clippings
  • Straw
  • Shredded leaves
Mulching

Feeding the Plants

If your soil is not rich:

After 1–2 months, add fertilizer.

Use something rich in nitrogen like blood meal. Sprinkle it beside plants and mix it lightly into the soil.

Fertilizer

Dealing with Flowering

If onions start to flower:

  • Harvest them soon

They will stop growing. They will not store well.

Dealing with flowering

Harvesting Onions

You can harvest anytime they reach usable size.

For storage, wait longer.

Harvesting and saving

Look for this sign:

  • Neck becomes soft
  • Leaves fall over

When half to two-thirds of leaves flop, they are ready. This usually happens 4–5 weeks after the summer solstice.

Drying and Curing

Harvest on a sunny day.

Then:

  • Move onions to a dry, airy place
  • Keep them out of rain
  • Dry for 2–3 weeks

Use racks for better airflow. This creates a papery skin. It helps them store longer.

Hanging Onion

Storing Onions

Store onions in:

  • Breathable nets
  • Or woven onion strings

Hang them up and use as needed. Try to grow at least one variety that stores well through winter.

drying onions

Final Thought How to Grow Onions

Growing your own onions is simple. Follow these steps, and you’ll get a great harvest. Once you taste homegrown onions, you won’t go back.