Can You Start Out Planting Onion Sets Indoors?

When I began growing my own food, initially, my focus was on planting low-maintenance and space-efficient vegetables. Being a resident of a flat in a metropolitan area, my aim was to maximise both space and time. Onions sets were among my top choices, given their ease of growth from sets instead of seeds and suitability for container gardening. But, one of the key questions I initially asked was can you start out planting onion sets indoors?

The good news is that you can start out planting onion sets indoors using trays or small containers filled with multi-purpose compost. After hardening off, you can transplant them to bigger pots and move them outside a few weeks later when spring starts. It’s best to plant your sets around February and it won’t be long before you see the green shoots start sprouting.

When I first grew onion sets indoors, I was a bit worried they wouldn’t have enough access to light and that transplanting them to larger containers would be a problem, as I’d only transplanted seedlings outdoors before. But after going through the process for the first time, I realised that planting my onion sets on windowsills was a viable way to free up extra room in my backyard.

So, for all of those looking to maximise self-reliance by growing their own food, but who don’t have a ton of space available, I highly recommend getting started with onion sets. In this blog I am going to discuss can you start out growing onion sets indoors, the benefit of growing them indoors, best containers to use, onion sets versus seeds and more.

The Benefits Of Growing Onion Sets Indoors

The reason it’s good to plant your onion sets indoors, is because onions need to get as much growth in as early as they can because they are photothermoperiodic. This is a rather complicated word that means onions are sensitive to light and temperature and need to get enough hours of daylight at the right temperature for optimum growth, so bigger onions need to be planted early. When the bulb grows bigger, it can benefit from those long summer days in the UK (the few we’re lucky enough to get!).

Another reason growing onion sets indoors can be a good idea is because if there is a cold snap in spring, the plants will see this as winter occurring and could bolt – that’s when they run to seed suddenly. This is a problem because in such a case, the bulb stops growing. So it is important to keep conditions as constant as possible because onions are susceptible to weather changes.

By keeping your sets indoors until spring starts, you can avoid this problem. Just remember to harden off your onion sets and get them used to the elements, by leaving them outside for an hour a day for a week or so before you permanently locate them outside.

Can You Start Onion Sets in Trays?

In the past, I started off growing sets indoors in containers, but these days I tend to used trays like in the picture below, as they are much more efficient for smaller spaces.

For those of you who have not used trays for onion sets before, at first it may seem a little strange. At least in my case, I used trays for sowing seeds not sets. With seeds, you add a few per module to thin out once they start to sprout. With sets, you can see a lot more of the bulb and your trays can look more crowded. But the process is the same, you harden them off and transplant them to bigger pots with roots intact. Just be careful to only grow as many as you can keep outside – it’s tempting to get carried away and then you’ll be overwhelmed with onion sets and won’t have room for anything else.

Below is a video I made last year about growing onion sets indoors. I was using containers at this time, but as I said earlier, I wish I’d started out with trays. Containers need substantial root room so have to be relatively big. I did get some ok results, but it was really inefficient having such large pots on my windowsill and I could only plant three per pot!

Onion Sets Versus Onion Seeds

There’s a bit of a debate on the advantages of onion sets versus onion seeds. Personally, I think onion sets have the advantage.

Onion sets are immature onion bulbs that were partially grown in the previous year, then pulled up and stored. Because they have a two-year lifecycle, in the first year they produce a storage bulb and they flower in the second. They are a tiny version of a bulb, with a pointed end and a flatter end.

When you plant them, it’s important to have the pointed end sticking upwards as demonstrated in my video about planting onion sets in springtime. Onion sets are also very popular because they are easy to purchase – almost every gardening centre has them, even some local stores.

In comparison, onion seeds have more variety than onion sets which typically just have a few varieties available. Onion sets are also more expensive than onion seeds. When I first ventured into growing onion sets, I appreciated the limited options available as it made the process less complex. With an abundance of seed varieties and insufficient knowledge, I felt inundated by the options that depended on multiple factors such as weather conditions and garden type. All I wanted was a simple and uncomplicated choice that was easy to cultivate.

Also, if you are looking to plant onion seeds indoors and get them outdoors by spring, you generally have to do so under grow lights – artificial lights that help stimulate photosynthesis. This is much more complex than growing sets and you’d have to start them earlier in the year like January, because seedlings take longer to grow.

When Do You Harvest Onion Sets?

If you plant onion sets indoors in February and move them outside in the spring, the logical time to harvest would be around June or July, possibly extending to August. The indications that onions are ripe for harvest are yellowing foliage and drooping stems.

Ensure that you pick them during a sunny period so that they can dry out in the sun, which would enable long-term storage. However, it can be challenging to determine the exact harvesting period due to the impact of weather conditions. Last year (2022) in the UK, we experienced extreme weather with scorching heat, strong winds, and what seemed like incessant rain towards the end of summer/autumn, making it difficult to identify the optimal time to harvest.

Can You Start Out Planting Onion Sets Indoors? Final Thoughts

If it’s your first time starting onion sets indoors, good luck. Remember that things can go wrong – but that it’s a learning process so you can hone your skills for the next year. And, if you’ve grown them outside before, but are thinking about growing them indoors this time, I hope you feel confident enough to give it a go. Either way, learning how to grow vegetables in different circumstances is a key part of your self-sufficiency journey. And for me, whether indoors or outdoors, seeing those early shoots pop up never gets boring!