Here in the UK, we typically grow onions from either seeds or sets, with sets being the easier choice. And while seeds must be started early in the year, sets can be planted throughout spring.
So with that in mind, let me show you exactly how I plant my onion sets in springtime!
Why Onion Sets Are Perfect For Spring Planting
Sets are immature onion bulbs that have been grown and then stopped prematurely. When you plant them, they begin regrowing and reach a large size quickly.
Because of this, they are perfect for planting in the spring. They already have a massive headstart on onions grown from seed, so they don’t need to be started so early.
With a few growing tips, you can plant them out as well-established plants, meaning they are much more likely to survive the rigour of growing outdoors.
Planting Onion Sets
With onion sets, you can plant them directly into the soil or start them off in trays first for a little added protection.
Starting in Trays
If you are starting your sets later in spring, then I would definitely advise starting them this way in a greenhouse or polytunnel.
The extra warmth and protection from growing under cover will help your later plantings quickly catch up to where they should be.
Use a tray and regular multipurpose compost. I like to have each set in an individual pot or module so you don’t need to damage the roots when transplanting them.
I will set up a seed tray with lots of small pots and then plant one set per pot. Leave the top of the set just poking up above the surface of the compost.
Grow them on this way for roughly a month (maybe two, depending on how early you start them) before planting them out into their final spot.
When it comes to direct outdoor planting, there are two main methods for planting your sets. Either individually or in long trenches.
As I try and do as little digging in the garden as possible, I plant them individually, although rows work just as well.
To plant individually, push them into the ground, leaving just the tip of the set exposed. If you have excellent soil then this is really easy.
If your soil is a little more solid, it may be trickier to just push them in. Instead, use a dipper or wooden handle on one of your garden tools to make a hole before popping the set into it.
The main concern for your sets until they are established will be birds. They like to pick up the sets and fly off with them!
If birds are a particular concern in your area, then I would net my onions until they are established. I don’t need to do this, and yes, I do lose one or two bulbs, but I just keep a few spares to replace any missing onions.
How far apart you plant your onions will depend on the expected size of the final bulb. You want to keep most onions roughly 10cm apart from the next bulb in all directions. If growing large bulb onions, maybe make this 15cm.
Planting In Rows
Planting in a trench is a very common way of sowing onion sets.
The general idea is to make one long trench, pop all of your onion sets in it, and then come back and cover them with soil, leaving the tips exposed.
Improving Your Soil Before Planting
Onions like a nice, well-draining, but nutrient-packed soil to grow to their best. Luckily compost is the perfect growing medium.
If your soil is a little heavy or overly compacted, adding a layer of compost will drastically improve things and lead to a much more successful crop.
This can be homemade compost or shop bought if you don’t have your own compost production line set up just yet.
Add a layer 3-5cm deep and plant your sets into this. If your soil is poor draining and prone to waterlogging, consider putting in a raised bed or growing your onions in pots.
Further Growing Tips
Once established, onions are straightforward to grow. The only thing you must do is keep them well-weeded. Onions are very poor at competing with weeds for nutrients, so any weeds around them will be very harmful.
I use a handheld onion hoe for this and just run it in between my rows every couple of weeks to kill off any weeds before they get the chance to become established.
If you have any further questions or any tips of tricks of your own then please let me know in the comments below!