Just How Big Do Onions Have To Be Before Transplanting?

I’ve mentioned before that onions are among my favourite crops to grow in small spaces. You can sow sets indoors before transplanting and they don’t tend to be high maintenance. But how big do onions have to be before transplanting?

I tend to plant my sets in trays or small pots before moving them over to larger containers and placing them outside. However, it can be tricky for beginners to know exactly when to transplant onion sets.

A key sign your onions are big enough for transplanting is that multiple shoots show and that they are at least a few inches long. The shoots should be strong and green and not spindly or weak-looking.

Read on to learn more about just how big your onions need to be before transplanting – as well as the benefits of using sets over seeds, tips on how to care for your onions in small spaces and more…

Table of Contents: How Big do Onions have to be Before Transplanting?

How do You Start Onion Sets in Seed Trays?

As someone who has transplanted onion sets in small spaces in the past, I can attest to the effectiveness of starting onion sets in trays.

The process is fairly simple, and it’s a great way to get your onion sets started in a controlled environment before transplanting them into larger pots.

To start onion sets in trays, you’ll first need to fill the trays with a good quality potting soil or mixed compost.

Once the trays are filled, press the onion sets into the soil, making sure that the pointed end is facing upwards.

It’s important to space the onion sets apart in the trays, as overcrowding can lead to stunted growth. This will usually be easy as your trays should come with modules you can easily place your sets into – I use these seed starter trays – complete with drainage holes and ventilation useful lid so you can control the

The ideal time to start onion sets in trays indoors is typically in early spring, around four to six weeks weeks before the last expected frost date – this will vary according to your region, but where I am in the UK it is around February/March or even April.

This will give your onion sets enough time to grow and establish their root systems before being transplanted into larger pots.

A word of warning: you can start off your sets in trays indoors or outdoors – but if you are going to go for outdoors, be aware of the rainfall in your area. I kept most of mine inside, but left one batch outside and they got pretty soaked in a heavy downpour.

The Benefits of Planting Onion Sets

I’m a real fan of onion sets over seeds. This is a topic of debate among gardeners – but personally, I believe that onion sets have the upper hand. In fact, I have only ever sowed onion seeds once and may never bother again!

In a nutshell, onion sets are essentially immature onion bulbs that have been partially grown in the previous year before being pulled up and stored.

With a two-year lifecycle, onion sets produce a storage bulb in the first year and flower in the second. They are smaller versions of bulbs with a pointed end and a flatter end.

When planting them, it’s crucial to have the pointed end pointing upwards, as demonstrated in my video on planting onion sets in the spring.

Onion sets are also easily available and less expensive than onion seeds. All in all, onion sets offer a simpler and less complicated choice for small-space cultivation, making them an ideal option for novice gardeners living in urban areas.

Where to Keep Your Onion Sets

To ensure optimal conditions for onion sets, make sure you keep the trays in a warm, sunny location, such as a windowsill or under grow lights if you don’t have space on a windowsill, or if you start out planting at a dark time of year.

Onion sets require consistent moisture to germinate, so make sure to water the trays regularly, but not so much that the soil becomes waterlogged. And, remember to fertilise your onions – but take it easy and don’t over-fertilise, which can damage your plants.

How Big do Onions have to be Before Transplanting?

With proper care and attention, your onion sets should germinate within a week or so and be ready for transplanting into larger pots in a few weeks.

When assessing the size of your onion sets, you will need to look at the shoots. This is a sign that they are ready to be moved to larger pots.

Ideally, the shoots should be a few inches tall before transplanting, as this indicates that they have developed a good root system and are ready for more growing space.

The shoots also need to be strong, with a healthy green colour. If the shoots are thin or spindly, it may be an indication that the onion sets need more time to develop before being transplanted.

You’ll also need to check that there are several shoots sprouting up, which is a sign they are actively growing and need more space.

In terms of the roots, at this stage there should be a lot of thin roots that should be visible through the tray holes.

But in reality, sometimes it may be hard to tell if you are using trays, so use the signs above for indication that your onions are ready to transplant.

How to Transplant Onion Sets to Bigger Pots

When transplanting onion sets to bigger pots, it’s important to handle your sets carefully and avoid damaging the shoots or the roots, as this can slow down growth and potentially harm the plants.

Here are the key steps for transplantation:

• Try to remove your onion sets like ‘plugs’ from the trays so they slot out easily.
• Place them into larger containers by filling those containers with the right soil and then adding a hole in the centre where you can place the sets.
• You can then backfill them so the sets are fully covered with soil with the shoots showing.
• If you are going to move them outdoors, you will need to harden them off by leaving them outside for an hour a day for a week beforehand, so they can get used to the weather.

And remember that if you find the transplanting process too tricky to manage, you can always grow your onion sets in containers that are big enough to see them from germination to full growth. Such containers would need to be at least eight to ten inches deep to allow your onion sets to fully grow.

Tips On Growing Onions In Small Spaces

You can look after your onions by pruning and thinning techniques to encourage healthy growth and maximise space. Pruning involves removing the top portion of the onion plant, which can help to divert more energy towards the development of the bulb.

Thinning involves removing some of the smaller onion bulbs from the plant when they get bigger, which allows the remaining bulbs to grow larger and healthier.

By combining these techniques, it’s possible to grow larger, healthier onions in smaller spaces.

How Big do Onions have to be Before Transplanting: Final Thoughts

Determining the optimal size of onion sets for transplanting can be a bit of a challenge, but keeping an eye on the size and growth of the shoots can be a reliable indicator.

With the right techniques and a bit of attention, even those with limited growing space can successfully grow onions that are both flavourful and nutritious!