How many onions grow from one bulb? This is a question you need to know the answer to so you can work out how many onions to plant.

Here’s the answer: you’ll get one onion from each onion bulb (or onion set) you plant.

Unlike potatoes which produce lots of new potatoes from each seed potato you plant, an onion bulb simply grows into a larger onion.

(To get more than one onion per seed or bulb you need to grow bunching onions.)

While onion bulbs make planting easy, they’re not always the best way to get a big crop of onions, and I don’t often grow onions from bulbs these days.

To get more onions from the same amount of growing space, I find it’s better to sow onion seed and use a growing technique called multi-sowing.

In this post I’ll tell you how to plant onion sets, and I’ll explain why growing onions from seed is often a better option.

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Why Do Onion Bulbs Only Produce One Onion?

Onion bulbs are more commonly known as onion sets. These immature bulbs are grown from seed started in the summer, then the bulbs are harvested in the fall while the bulbs are still small.

The onion bulbs remain dormant over the winter months, then they’re shipped to stores in the spring ready for planting.

When you plant a bulb in your garden, it puts down roots and begins to grow again. And this time, because its growth isn’t interrupted, it reaches full size.

One onion bulb becomes one full size onion.

A bulb is simply an immature onion taking a rest.The only thing it can do once it’s planted is resume growing into a mature onion.

Contrast that with a potato. One small seed potato will grow into a new plant; and then the plant grows more potatoes. At harvest time, the original seed potato is a shriveled husk, but the plant has given you a big return by producing a couple of pounds of potatoes.

Learn: How to sprout potatoes faster.

With onions, one bulb goes in, one onion comes out. With potatoes, one potato goes in, lots of potatoes come out.

For every onion you want to harvest, you need to plant one set or sow one (or more) onion seed.

Onions are biennial, which means they have a 2 year growing cycle. They set seed in their second year.

How does that work?

Onions grow into mature bulbs during the first year, whether they’re planted from sets or sown as seed.

Then, unharvested onions lie dormant and overwinter in the ground. The following spring, they send up a “bloom” or flower stalk, and the flower eventually produces hundreds of tiny black seeds in the summer.

The flowers are really pretty, by the way, and bees love them.

A photo showing a lilac colored onion flower beginning to open next to a small pile of black onion seeds.

If you leave 2 or three of your onions to overwinter, you’ll be able to harvest your own onion seed the following summer.

Now you know how many onions grow from one bulb, you can work out how many onions you need to sow or plant.

5 Reasons Growers Choose Onion Bulbs

While I don’t grow onions from onion bulbs these days, for many gardeners, they’re the perfect choice.

  • Bags of onion sets are widely available in springtime.
  • Onion sets are ready to plant, avoiding the extra work needed to raise seedlings.
  • The small bulbs are super easy to plant.
  • Sets may be the only option if you’ve got a short growing season.
  • These onions mature in about 3 to 4 months from planting compared to 5 months for onions grown from seed.

5 Reasons Growers Choose Onion Seeds

  • There’s more variety.
  • Onion seed is a lot cheaper than onion sets.
  • You get bigger onions if you sow single onions.
  • You can multi-sow onions in clumps to get more onions overall.
  • Onions won’t bolt.

There are so many delicious varieties of onions you can grow from seed, but if you choose to plant sets, you’re limited to a very small selection of onion varieties.

With onion sets, you’ll get the best results from the smallest bulbs. Larger bulbs often don’t grow as well and produce smaller onions.

Onion sets are also more likely to bolt (send up a flower stalk) unless they’re heat treated bulbs. Heat treated sets are available in the UK, but they’re not common in the US.

Often, the bags of sets you buy have a mixture of small and larger sets, meaning you’ll get mixed results at harvest time.

If you decide to grow onions from seed, you get to choose whichever varieties you like best. You can make sure the onion seed you buy is suitable for your growing conditions, and because the onions are in their first year, they won’t bolt.

When you grow onions from seed, the trick is to start early. Don’t wait for spring, that’s too late.

How to Grow Onions From Bulbs

The traditional time to plant onion sets is after the spring equinox, which falls around March 20th.

Onions prefer a free draining, slightly acidic to neutral soil. The Ohio State University Extension service advises that a soil pH of 6.0 to 6.8 is ideal.

Suboptimal pH reduces a plant’s ability to use the nutrients in the soil effectively and can make plants more susceptible to pests and diseases.

It’s always a good idea to test your soil with a pH testing kit so you know which vegetables will thrive in your native soil, and which will need soil amendments.

Ideally, your onion bed will be in a spot that gets full sun, but onions can tolerate partial shade too.

Add a sprinkling of blood and bonemeal fertilizer to each row before you plant.

Use your finger to make small holes 10 cm (4″) apart along the row, and place an onion set into each hole. The root end goes into the soil and the tassel end points up.

Next, push some soil into the hole, making sure that the tip of the onion set is still showing.

Leave 30 cm (12″) between rows.

Water the sets to settle them in, then cover with a layer of horticultural fleece to give a little protection from the cold and to stop birds pecking the onions out of the ground.

You can remove the fleece after one month.

If the weather is dry, you’ll need to water your onions.They prefer less frequent deep watering over regular light watering.

Towards the end of July, stop watering the onions so they can ripen ready for harvest.

Why Multi-Sowing Gives You More Onions

The traditional method of growing onion from seed is to sow seed in rows, then thin the seedlings to give the onions room to grow. Growing in this way will give you the biggest onions.

But with multi-sowing, you grow a clump of onions together. The onions will be smaller, but the total weight of onions harvested from an area will be higher.

If you don’t mind using medium-sized onions, then multi-sowing is the way to go.

Related: How to grow bigger onions.

How To Grow Multi-Sown Onions

It’s super simple to multi-sow onions. All you need are some pots or seed modules, compost, and your onion seed. I use yogurt pots with holes poked in the bottom as seed pots.

Sow In Pots

In mid to late winter, fill your pots or modules with compost or starter mix and firm the compost down with your fingers.

Place 5 to 8 seeds in the pot, then cover with a thin layer of compost and water well.

I put mine on a heat mat to speed up seed germination, and I lay a piece of bubble wrap over the top of the pots to keep the compost moist. But as long as you set your seeds where the temperature will stay between 13°C – 24°C (55°F – 75°F) they’ll germinate.

Keep the seeds indoors while they germinate and make sure they get plenty of daylight once they sprout.

Harden Off

After 4 to 6 weeks, when the onions are about 4 inches (10 cm) long, you can harden them off, and then plant your clumps of onions outdoors in a prepared bed.

Hardening off gets transplants used to outdoor conditions so they aren’t shocked when they move from a warm house to the chilly outdoors.

Take your onion seedlings outdoors and place them in partial shade in a sheltered spot for a couple of hours, then bring them back inside.

Increase their time outdoors by 1 hour each day, until day 7, then you can move them out into full sun during the day and leave them out overnight.

Plant Outdoors

In zone 9 where I live, the middle of March is the perfect time to plant onion seedlings.

Choose a sheltered growing site that gets full sun.

Onions don’t mind a little bit of cold weather and the seedlings can withstand light frosts. However, they’ll appreciate being covered with a layer of horticultural fleece for the first month.

A fleece layer also protects onions from birds. Once the onions are a little bigger, the birds won’t bother them.

Make a hole with a trowel or wide dibber and plant each clump. Leave 36 cm – 40 cm (14″ – 16″) between each clump in a row and leave the same distance between rows.

Conventional spacing is 10 cm (4″) between onions in a row, and 30 cm (12″) between rows.

The multi-sown clumps are spaced further apart in their rows than single onions, but you’ll get 4 or 5 medium onions and 2 or 3 green onions from that space instead of 3 onions.

Give the onions a good watering to settle them in.

As they grow, the onions will elbow each other aside and get themselves the room they need.

Harvest First Green Onions

In about 10 weeks, you can start to harvest a few green onions (spring onions) from each clump.

To harvest these onions without damaging the others, choose an onion on the outside of the clump, and gently pull and twist to ease the onion out of the soil.

This first harvest gives you onions to use in the kitchen for salads, sandwiches, stir-fry, and so on. And it gives the remaining 4 or 5 onions in the clump more room to grow so you get bigger onions.

Harvest Your Main Crop Onions

Wait another 6 to 8 weeks and your main crop will almost be ready to harvest.

You’ll see the green tops of the onions begin to fall over.

Leave the onions like that for one week, then as long as the weather has been dry for 3 or 4 days, you can harvest them.

Spring planted onions are usually harvested at the end of July or early August.

Then all you need to do is let them cure, and you’ll have plenty of onions to use during fall and winter.

Wrapping Up

You’ll only get one onion from each bulb or seed you plant.

Onion bulbs (sets) are easy to grow, but you need to watch out for bolting, and the varieties on offer are fairly limited.

Growing onions from seed eliminates the bolting problem and you get to choose from a wide variety of onions.

You need to start early if you plan to grow onions from seed, but if it’s too late for seed this year, then you’ll do fine with bulbs.

Thanks for reading: How many onions grow from one bulb!

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Kate Prince

Hey there! I’m a small scale homesteader sharing what I know about the off-grid life. I grow fruits and vegetables, raise chickens and goats, and produce my own power, heat, and clean water.   Feel free to send me a message.