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Why planting onion seeds is better than planting sets

Are you thinking about planting onions in your garden this year? Good choice – they’re an absolute staple in so many meals in our house and last year I discovered just how easy they are to grow. Last year, however, I grew using something called sets. This year I’m growing from seeds.

So let me explain. Onion seeds are, well, the seeds of the onion plant. Onion sets are essentially baby onions that have been grown from seed and then harvested and their growth halted until you replant them. So, both options will eventually give you a full-grown onion, but they start at different points in the process. There is a third option, although it’s not something I’ve ever seen in the U.K. and that’s starts, basically, this is onion seedlings and it’s popular in North America.

So why would you choose seeds over sets or vice versa? Well, to be honest, there are pros and cons for each way of doing things.

Growing from seed


  • You have a wider variety of onion types to choose from when starting from seed. If you’re a fan of heirloom varieties or just want to try out something a little different, seeds are the way to go.
  • It’s much cheaper to buy seeds than sets or starts.
  • You can start onions from seed indoors in the late winter or early spring, which can give them a head start on the growing season. Just be sure to transplant them outside when the weather warms up.


  • It takes longer for onions to mature when starting from seed. You’ll probably have to wait an extra month or so before you can harvest your onions.
  • There’s a higher risk of failure when starting from seed. Onion seeds are tiny and can be difficult to handle, so there’s a chance that some of them won’t germinate or will succumb to pests or diseases.

Growing from sets


  • It’s much faster to grow onions from sets. You can plant them in Autumn and be harvesting onions the next Summer.
  • There’s a lower risk of initial failure when starting from sets. Since they’re already little baby onions, they have a better chance of surviving and thriving in your garden.


  • You have a limited selection of onion types when starting from sets. Most nurseries only carry a few types, so if you’re looking for something specific, you might have to start from seed.
  • Sets are more expensive than seeds.
  • Sets are more likely to bolt in the warmer months.

Growing from starts


  • These are essentially a seedling that someone else has germinated and grown on, so will be faster to grow and mature than you sowing seeds.
  • You don’t have to account for germination failure so higher success rate.


  • More expensive than buying seeds.
  • The same as with sets, you have a limited selection of onion types when starting from starts.

So for me, I found quite a lot of my onions bolted last year, which was a real disappointment given that I have to give over an entire bed to growing them, hence this year I’ve decided to grow from seed.

Choosing your variety

This is the bit where some people get themselves in a stooshie. The best thing about being a gardener these days is that the internet has brought so much new information. The worst thing is also that the internet has brought so much new information. Let me explain, before we had access to all this info, we all pretty much grew varieties that were suited to our growing regions because we got advice locally and generally bought seeds, etc locally. Now however with the ability to buy endless varieties from all over the world, and be presented with information from other countries that isn’t always relevant to you… we hear terms and worries from other gardeners that can confuse us. Not to mention the plethora of social media misinformation we have to sort through.

One of these terms that I’m hearing a lot more this last few years is about long or short-day varieties. Not something that really gets mentioned in the U.K. as much, but we hear it from our American gardening buddies, youtube channels, and blogs. So let me explain.

Onions are classified as either long-day or short-day varieties based on how they react to the period of daylight they have. The fancy term for this is “photoperiodic”. Long-day onions need at least 14 hours of daylight to initiate bulb formation. So those types are generally best for areas that have long days with plenty of sunlight, such as the northern Regions.

Short-day onions, on the other hand, are varieties that require a shorter photoperiod of fewer than 12 hours of daylight to initiate bulb formation. Hence these types are generally grown in areas with shorter days, such as the southern regions. As you don’t want the onion to start focussing on “bulbing up” before the plant can cope.

It’s also worth noting that some varieties of onions are “intermediate” varieties, which means they can grow and form bulbs under a range of photoperiods. Just to make things confusing.

Now I mentioned it’s not really something you hear discussed in the U.K. but theoretically, it is a thing. If you think of the difference in daylight hours in summer between say Shetland (in the very very northern most part of Scotland) and say Cornwall (in the very south of England) can be as much as 4 hours more in Shetland. So if you are trying to relate this to yourself, imagine where you are between those two regions.

However, don’t let this become a worry, you’ll see lots of successful onion harvests from growers down south (England) who grow varieties like Ailsa Craig, which is a long-day onion. The U.K. is a relatively small country land mass wise so there is a lot more middle ground than extremes.

So what varieties am I growing this year?

Well, I’ve got a white, brown (yellow) and a red variety.

My brown variety is a Scottish variety that hails from an area where my family spent holidays when I was a kid. It’s called Alisa Craig. It’s named after a rocky island on the west coast of Scotland and pronounced ail-sah craig . This is a long-day onion that does well here and can grow really big if you have the perfect conditions.

The white variety I’ve chosen is Sweet Spanish, another long-day variety. I’ve not tried these before so I’ll keep you updated.

Lastly, the red onion is Red Baron, another long-day variety. 3 types are maybe more than I have room for but I love red onion in salads and they are less reliably available in our supermarket.

When to sow and how

This is the big question. Onions take a long time to grow and reach full maturity, from 90 to 140 days. So in order to steal as much time as possible, and hopefully get fully mature onions before our temperatures drop at the end of the season, I’m starting mine on Boxing Day. Now, this is not a set day, generally, late December or early January is when people here start their onion seeds. Boxing Day is just a nice easy date to remember.

One thing to note, I’m trying to ensure I get the best germination rates possible, so I’m striving to achieve the optimum temperatures for my seeds (around 10c to 16c. so I’m starting mine in my heated propagator. I’ll keep them in the warming propagator or greenhouse until a few weeks before my last frost when I’ll start hardening them off to then transplant outdoors once the soil warms up in late spring.

When growing onions, it’s good to remember that onions don’t have a long root structure, so plant them in good nutritious soil to make it easier for them and keep on top of watering.

So there we go… onions in a nutshell.

The seeds I’ve sown

Sweet Spanish onion seeds: https://amzn.to/3Wm1h5p

Alisa Craig onion seeds: https://amzn.to/3GbcsIC

Red Baron onion seeds: https://amzn.to/3jouvSP

Catch up with me sowing my onion seeds on boxing day this year

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