The Secret To Growing Onions From Seed Successfully

Ever wondered about growing onions from seed? Onions are very easy and cheap to grow. With just a little work and a couple of pounds, you can grow hundreds of onions that will last an entire year.

When it comes to growing onions, many people automatically assume that growing onions from sets is the way to go. But growing from seed gives a more reliable crop that is less likely to bolt and will store for at least 9 months. We’d always recommend growing onions from seeds rather than onion sets.

In this article we will show you the secret to growing onions from seed successfully.

Onion Varieties To Try Out

There are a lot of wonderful onion varieties that can be grown from seed with the range being far greater than sets. You can also find a range of seeds that are grown as giant onions, which are great for shows or just grown for bragging rights.

And no guide about growing onions from seed would be complete without a recommendation on what varieties to grow. So here are our best picks:

Red Baron: this is a red onion that gives you firm round bulbs and has a lovely red skin. They have a strong taste and are ideal for cooking.

Ailsa Craig: this is a white onion that has a golden brown skin and produces large bulbs. These onions have a milder taste and are great for both cooking and salads. It’s also a popular show variety, so if you want to grow them big, this is the variety for you.

Isobel Rose: this pink onion is worth a try, because of its delicate, mild flavour. You can cook with it or add it to your salad. The bulbs are round with white flesh and pink sin. 

We love onions and there is no better way to enjoy them than as a yummy onion tart

Growing Onions From Seed

growing onions

Sowing can begin as early as February indoors for large onions, with sowings in March producing average sized onions and those sown later in April giving small onions which are handy in the kitchen.

5 cm (2in) plugs, soil blocks or deep roottrainers are ideal for growing onions from seeds. A good multi-purpose compost should be used as this will have all the nutrients needed to get the plants growing strongly over the coming weeks.

Sow two seeds per pot and cover with a very fine dusting of compost. A light watering will make sure there is good contact between the seed and soil, which is essential for good germination.

Germination for onions can take up to two weeks, but once the onion seedlings are big enough to handle, they should be thinned to just one plant per container.

These plants need to be kept indoors for several weeks, a windowsill or unheated greenhouse are best suited to this.

Transplanting Onion Plants Outdoors

Once the plants are established in their pots and have reached height of 8 – 10 cm (about 3in), they can be transplanted outdoors.

If the onion seedlings are hardened off over a week, they are hardy enough to survive most cold weather. Planting out 4 weeks before the last frost date is fine.

General Article Opt-In

Enjoying This Article?

Get the latest gardening advice in your inbox

Download FREE helpful PDFs when you subscribe to our email newsletter with regular updates and tips not published anywhere else.

You can unsubscribe at any time. See our Privacy Policy.

Onions grow best in rich soil, as they are heavy feeders.

So make sure you prepare the soil of the bed they will end up in. In autumn, mix organic matter, such as well-rotted manure or green manure into the soil.

A handful of blood, fish and bone per square metre has been recommended by old gardeners for decades.

Westland Fish, Blood and Bone All Purpose Plant Food, 10 kg
2,124 Reviews
Westland Fish, Blood and Bone All Purpose Plant Food, 10 kg
  • Prolongs flowering & fruiting
  • Greater crop yield
  • Improves fruit colour & flavour

Plants should be spaced 15cm (6 in) apart in rows 30cm (12 in) apart. Firm the plants in well and water well.

Onions like moist soil, so make sure you water them in dry weather. But don’t overwater them in the winter months, as this could cause them to rot.

Looking After Your Onion Plants

Onions are tough plants and will grow away steady over several months.

The plants do not have a lot of leaves, so weed competition can be a problem. Weeding every two weeks is essential to ensure that the plants are not totally taken over by the weeds.

Watering when dry is essential to keep the onions growing strong bulbs, especially in late summer (July – August). Although watering at an allotment or veg garden can be difficult and time-consuming, with onions it is well worth the effort of giving the plants a very good watering every couple of days in really dry weather.

Top Tip: I do not recommend feeding with a liquid fertiliser as I’ve found this leads to the bulbs splitting or going to seed.

It’s easy to see when onions are ready for harvesting as the leaves will begin to fall – this is usually in August or September.

harvest your onions when the leaves have died off

Once the leaves begin to fall, they can be left in the ground for the foliage to go brown, or they can be removed somewhere indoors to dry off. Leaving the onions to dry and the tops to go brown is essential for good storage.

Onions that are put into storage are best hung in the traditional manner and kept somewhere cool. Stored in such a fashion, the onions can be expected to store for 9 months, with most good storing varieties still being suitable for eating an entire year later.

Growing onions from seed means you can store your onions for longer.

Pests And Diseases Affecting Onions

No guide about growing onions from seed would be complete without a section about pests and diseases.


Onion leave with sings of onion fly attack

Although onions help to repel many pests and protect other plants, they do have their own pests to content with.

One of these pests is the onion fly. They look like grey houseflies and like to lie their eggs on the stems of onions and other types of onions such as spring onions and shallots. 

Their larvae are the real pest though. These white maggots have no legs or head and will eat the onion roots and bulbs, making them unsuitable for consumption.

They are capable of killing onion seedlings.

This is bad news, because if you have put in so much effort into growing onions from seed, seeing them destroyed by a pest is disheartening.

The first sign you will see if your onions are suffering from an onion fly larvae infestation is the death of seedlings. More established plants will develop yellow leaves and start to wilt. 

So, once this pesky pest has infested your onions, there is nothing else to do than to dig them out and destroy them. Do not compost them though, as they can survive in the soil.

The only way to protect your onions from these larvae is by keeping the female flies from laying their eggs. This is best done by covering them with insect-proof mesh.

RHS Agralan Enviromesh Fine Garden Insect Netting Mesh For Crop & Vegetable Pest Protection (Standard - 2.1m x 4.5m)
340 Reviews
RHS Agralan Enviromesh Fine Garden Insect Netting Mesh For Crop & Vegetable Pest Protection (Standard - 2.1m x 4.5m)
  • PROTECT: Protect your vegetables and soft fruit from a range of insect pests, birds, rabbits and the weather
  • Protects vegetables against carrot fly, cabbage root fly, caterpillars, birds, rabbits, wind and hail.
  • SPECIFICATIONS: Mesh size:1.35mm. Light passage = 90% and air passage = 95%. The weight is approx. 55gms/sq.m. Yarn Thickness is 0.24mm

It is also said, that onions grown from set are less susceptible to attacks.

One other way to keep the onion fly away is by confusing them. If they can’t find your onions, they can’t lay their eggs on them.

Companion planting can help with this. If you plant mint or parsley with your onions, the strong scent of these herbs will supposedly put off the fly.

Another fly that loves onions, is the allium leaf miner. It’s a relatively new pest that initially only occurred in the south and east of England, but that has moved further north. 

Unlike the onion fly, its larvae, again white headless and legless maggots, will burrow into the leaves, stems and bulbs to feed. 

You will spot the holes they leave on the surface. An infestation of this pest is often followed by bacterial and fungal infections, which will cause the plant to rot.

Once the allium leaf miner larvae have attacked an onion plant, there isn’t anything you can do apart from digging it up and destroy it.

General Article Opt-In

Enjoying This Article?

Get the latest gardening advice in your inbox

Download FREE helpful PDFs when you subscribe to our email newsletter with regular updates and tips not published anywhere else.

You can unsubscribe at any time. See our Privacy Policy.

So it’s best to prevent the female flies to laying their eggs. These are preventative measures you can take:

  • cover your onions with insect-proof mesh when the adult flies are about. This is in March and April and again September to November. 
  • Crop rotation is essential, as the pupae might survive in the soil.
  • Again, you can try companion planting. Carrots are said to repel this pest and, in turn, the onions repel the carrot fly. So, a win win situation!

There is another new pest, that is sometimes confused with the allium leaf miner. But the leek moth is a different animal. 

It is also making its way further north into England and tunnels into the leaves, stems and bulbs of onion plants. But it’s the moths caterpillar that does the damage. It’s white with a brown head.

The first sign you are likely to see is white areas on the leaves. And plants might also start to rot due to bacterial and fungal infections, which can follow an infestation of leek moth caterpillar.

To prevent these pests becoming a problem, you can take the following measures:

  • practice crop rotation
  • cover your onion plants with insect-proof mesh
  • encourage natural predators, such as the ground beetle, by creating a habitat for them. Log piles, compost heaps and leaf litter will all work.
  • Carrots also confuse the leek moth, so planting them with your onions will help keep this pest away.


heavy infestation of onion white rot

When you are growing onions from seed, or any other vegetable for that, you want to make sure you are keeping the plants safe from diseases.

One common disease that can infect onions is onion white rot. A soil-borne disease caused by a fungus.

The first sign you will spot is the leaves starting to go yellow and wilt. Below soil level, it will rot the roots and penetrate the bulb.

When you pull up an infected onion plant, you will see white fluffy mould on the bottom of the onion bulb. Over time, this mould will develop into black round spore structures.

There is no cure. Once a plant is infected, you can only destroy it. Importantly, the spores can survive in the soil for years, so it is wise not to plant any onions or other plants of the allium family in an infected bed for around 15 years.

If you have an infected bed, make sure you do the following

  • don’t move the soil of the infected bed to other beds
  • clean your tools and boots after you have worked on the bed to avoid contamination of other areas
  • grow onions, leeks, garlic, etc. in pots and always use fresh compost
  • never compost infected plants

Another disease that could affect your onion plants is leek rust

While leek rust mainly attacks leeks and garlic, it can also occur in onions and chives.

It’s a fungal disease, that is easily spotted, as it causes rust red/orange spots to appear on the leaves.

Onions don’t tend to suffer too much from an infection here in the UK, and the bulbs are still edible. Although a severe attack could impact on the health of the plant.

To avoid this disease becoming a problem, you can do the following:

  • don’t use fertiliser as soils that are rich in nitrogen make the infection worse
  • ensure good air circulation by spacing out the onion plants
  • Weed regularly and remove any plant debris from the bed

Onions can also become impacted by onion downy mildew. It’s caused by an organism that is very similar to a fungus.

It causes damage to the leaves, stems and bulbs and can reduce your yield or even stop the plants from forming onion bulbs. Not at all what you when you have been growing onions from seed.

It mostly hits in late spring and summer, especially when the weather is mild and humid. But the infections can become even worse if we have a cooler and wet summer.

The symptoms of an infection are yellowing leaves that will eventually die. You might also spot a white layer of mould on the leaves, which can turn into a darker purple.

Onion downy mildew can also impact on the bulbs, which can mean they won’t store well.

Once the plants are infected, there is nothing you can do. The only option is to destroy them.

You can try to prevent an infection by:

  • spacing out your onions to allow for good air circulation
  • weed regularly between the onions
  • practice crop rotation – a five-year cycle is best
  • remove all bulbs from the soil after the end of the cropping season
  • there are some onion varieties that are said to be resistant to the disease such as Hylander and Santero

Now that you know all about growing onions from seed, why not try it out for yourself. You will have healty and flavourful onions to cook with in no time. Happy Growing!

Share on: