Growing Tutorials

How to grow onions

Red and white onions drying in the polytunnel

Onions are some of the easiest things to grow and require very little looking after. As our friend Dermot Carey (last head gardener of Lissadell House) says if you can’t grow onions you might as well hang up your gardening gloves and take up golf.
Now, Dermot is a professional vegetable gardener and while I agree onions are an easy crop there are a couple of things worth looking out for to make sure you get a successful harvest. Most of us will know you can either grow onions from ‘sets’ or seeds so here’s a little information on both:

Onion Sets
Onion sets are just immature or part grown onion bulbs from the previous year. They are produced by sowing onion seed very thickly which results in plants with very small bulbs. These little onions are sown the following spring to mature and produce full grown onions.

Sets are the easiest way for the beginner to grow this essential crop as they are so easy and pleasurable to set out. You won’t get as wide a choice of varieties but you will have a greater chance of success.

Click Here To View Our Range of Onion Sets

When to sow onion sets
The best time to sow onion sets is mid March to mid April. If you are sowing red onion sets is better to leave them till April as they are more prone to bolting and a later planting may help.

Onion sets growing in modulesSets can also be sown in Autumn from September to early October. If you have a wet garden or live in an area of high winter rainfall growing in raised beds is recommended as sets left in wet soil are likely to rot.

You can also plant sets in modular trays which can be kept in an unheated greenhouse or tunnel for planting out when the weather allows from mid March.

Site and Soil
Onions like an open sunny site. Your soil should be fertile with a good bucket or two of well rotted garden compost raked into the surface layer as onions have relatively small and shallow root systems. Avoid manure or high nitrogen feeds as this can result in thick necks which won’t try properly and will rot from the inside out.

If you are looking for a good soil amendment before planting onions I can happily recommend ‘Envirogrind‘ soil improver which I use every year, I just spread it on the surface of my beds and cover with the ‘Growgrid’ onion planting mat I mention below.

Sowing Onion Sets
Once you have your bag of little golden onion sets you’ll have to pick the best ones to sow. This is very important, if you’re doing your job properly you’ll end up throwing half of them out so bear that in mind when you’re ordering. Here’s a quick diagram to help you pick the best.

From left to right:

  1. You don’t want any shoots, you might think you’re getting a head start but you’ll just get a very poor quality bulb.
  2. Avoid any skinny looking ones.
  3. Discard anything with mould or brown patches on the skin.
  4. Any very big sets are more likely to run to seed.
  5. Now, that’s the fella. Nice, tidy and round, will produce a perfect onion.

Planting (If you have any left after sorting them…..)
Push the sets about 1 cm into loose soil approx 10cm apart, leave about half of the onion showing above the soil. Birds seem to love pulling your handiwork out of the ground and scattering them round the garden. You can solve this by placing enviromesh or cloches over them till they get going.

If you want to be cleveOnions planted in growgrid planting matr and save weeding later on you can plant through a sheet of black plastic. Spread the plastic over your prepared bed, make a hole just large enough for each set and just pop them in. We stock ‘Growgrid’ onion planting mats which are a ready made version with pre cut holes. I use them every year and find excellent.

The mats have a grid of heat sealed holes spaced at 15cm intervals which is perfect for growing medium to large sized onions. The heat sealing is the trick and prevents the mat from unravelling. I have been using the same mat for the last 4 years and it is still going strong, it is one of the best value items in my garden.

Sowing Onion Seeds
You have a much better choice of variety when you grow from seed but you need to start off much earlier in the year. You will need a greenhouse and heat bench to get them going as you’re sowing late January to February.

The best way is to sow 4 seeds per cell in a modular tray. You can move the tray from the heat bench around mid march but keep inside till early April when you can begin hardening off. Put your trays out on days without frost but back in again at night. You can plant them in their final positions in early May (Late April if not too cold) spacing them in a staggered pattern 30 x 30cm apart.

Spring Onion
Spring onion ishikura bunchingI never had much success with spring onions until I tried Ishikura. Most people, including me struggle with the popular ‘White Lisbon’ which produces a weedy looking result. You will get a scallion much more like the larger crunchy variety you’re used to in the shops with my new best friend, Ishikura Bunching.

You can sow them from late march in a greenhouse and continue outside until July. We find the easiest way of growing them is in modular trays. Sow 8-10 seeds in each module and plant the whole lot out 4 weeks later. There is no need to separate them making it easy to harvest a bunch at a time when you need them.

Your scallions will be ready to pick 8 to 10 weeks after sowing and don’t last long once ready. It’s best to sow a new lot every 2 weeks or so.

Onion Problems
The most common headache you’ll have in Ireland is downy mildew. This is because of our damp climate and why we recommend ‘Santero’ and ‘Golden Bear’ which have resistance to it. To try to avoid it keep your onion bed weed free with plenty of air circulation. If you have a wet garden it’s also worth growing on ridges or in raised beds to avoid wet, waterlogged conditions.

White rot can also be a problem but if you stick to a good rotation system you should be able to avoid it. Do please heed this advice as if you do get it you won’t be growing onions in your garden for 7 years. You’ve been warned!

Varieties for Spring Planting:
Santero F1
Resistant to downy mildew which can ruin gardeners onion crops. Santero produces an excellent quantity and quality of onion without any mildew problems, great news for onion growers. Stores extremely well.

An RHS Award of Garden Merit variety producing well sized semi-round bulbs with good keeping qualities. A standard, round globe shape with a sweet and mellow flavour, yellow flesh and light brown skin. Sturon is bolt resistant and stores very well.

Centurion is an early maturing variety that keeps well. Uniformly globe shaped bulbs with even skin colour – excellent eating onions which are good enough to show.

Rumba Onion sets are a top performing crop variety that’s ideally suited to growing in the our climate. Producing high yields of large round onions with a sweet flavour. A good all rounder.

Stuttgart onion sets produce firm semi flat shaped onions with a vibrant golden skin colour. They have a deliciously crisp and mild flavour, are slow to bolt and store very well.

Varieties for Autumn Planting:
Radar onion sets for autumn plantering and overwintering. They are extremely frost hardy and produce nice round bulbs.

Shakespeare is a brown skinned variety with excellent storage potential. Has good resistance to bolting.

Senshyu Yellow
Senshyu Yellow onions are a very popular overwintering Japanese variety that have a high yield. These yellow globe onions are a very hardy variety with golden skin and white flesh.

Red Winter
Red Winter onions are a heavy yielding variety of autumn onions that store very well. They are a mid to late maturing variety that are great for cooking with or using as an addition to any salad.

Onion Seed: Golden Bear F1
This is the one to grow if you have ever had White Rot on your onions as this variety is resistant to it. It also has good resistance to grey mould and some tolerance to downy mildew. Matures early and has a very high yield of perfect onions.

Spring Onion: Ishikura Bunching
An outstanding variety, perfect for bunching. Leaves are upright and dark green with straight long white stem. If you’ve been disappointed by spindly spring onions in the past, try this you’ll never turn back! Perfect.

  1. Mandy Morton

    We planted red and white onion sets in the Autumn on the allotment, they showed quite quickly and then all the shoots died back into the soil, should we lift them and replant new sets or will they still be OK.
    Our allotment is near the coast in Cornwall.

    1. admin

      Hi Mandy. That sounds odd. You might try digging up one or two sets to see if they have root growth or if they have died. If they have strong roots they should be OK though I would be a little concerned about them being prone to bolting. If it was my garden I think I would leave half of them (providing they have good root systems) and replant the other half with new sets in March in case bolting is an issue, at least you will get half a bed on crop!

      I hope this helps


  2. Tony Norris

    Hi, Autumn 2018, planting Senshyu onions, 5 to a 30ltr pot.
    Should I leave the pots outside or put them in my polytunnel through the worst of the winter?


  3. Tony Norris

    Thank for helping
    Have you tried growing Mellon’s in your polytunnel, I am in NW UK and grew three types last year (in heated soil).

  4. Steve b

    I’ve just bought some snowball ,electric , and some senshyu onion sets
    All of them state can be planted outside September October and November
    Hope this helps
    Steve b.

  5. Tony Hoskinson

    Hi guys, I grew Red Karmen sets from Wilkins (Wilko) last year. I knew nothing about onions and didn’t plant until early June in a spare raised bed. Having previously had a go a couple of years ago that were systematically attacked by “Cut Worms” last year I had just one onion munched. Tip! If you get this problem, as soon as you see the plant chipped down carefully tease out of the soil and the ugly brown caterpillar will be found. Scuff it on the path before it destroys any more….show no mercy!!! Anyhow last years Red Karmen were sensational even though planted so late

    1. Andrew

      Hi Tony. Thank you for getting in touch. That is very interesting about onions, I am amazed you were above to get sets that hadn’t sprouted in June, did you have them growing in trays and then plan t out in June? Either way that is all very interesting, thanks for sharing your experience. Andrew

  6. John Cooper

    I have grown some winter onions and they getting tall and look untidy now, should i support them by tying them to a cane

  7. richard roberts

    Hi im to late to plant any onion sets, so will plant some around September soon as I have some free space.
    When would you expect to harvest them if planted in September.

  8. shezyia kadzitche

    hi am new in onion farming ,, i planted the red onion in march and april i transplanted them after a 4 days of transplanting i added fertilizer 23:21 ,, i think the fertiliser was too much and after 2 days i notted that my onions are drying out then i started applying more water to it so that the fertiliser would dissolve and allow my onion to grow and which is happening now my question is in other beds it has dried out so i want to find out if i can sow another onion just straight to the bed without planting them on the nursery ? . and can it work in this month of june and when will i harvest this ? ,.

    1. Andrew

      Hi there, onions need a long season to ripen. I’m not sure what part of the world you are in but here in the British Isles June would be too late. You could grow some spring onion instead. I’d recommend using a slow release organic feed which releases nutrient over a longer period of time. Happy growing!

  9. Jane Walters

    Hi All
    This is my third year of growing onions on the West Sussex coast.
    The first year I didn’t know what I was doing and planted the onions in June, as I had just got an allotment. My crop was brilliant, the onions stored well and they lasted through to the following Spring.
    The next two years I planted sets in the Autumn and in both cases the onions bolted in the Spring and flowered. Breaking off the seed heads didn’t help and the usable onions were minimal. Having read about why onions bolt the third year I ensured I watered during dry spells in the Spring, but this didn’t help. The same has happened on the other allotments, even where people planted the sets in the Spring.
    Next year I think I will plant onion sets in June!

    1. Andrew

      Hi Jane, sorry to hear you are having bother with onions. It really all depends on how the season pans out weatherwise. This season, because of the late cold snap, a lot of gardeners have seen their onions bolt. In general, for spring planting, it’s best to wait until after the spring equinox to plant sets. The later the better but, as with everything in gardening, it’s a balancing act. The longer you wait to plant the lower quality of the sets available as some will start to sprout. Here’s to next season!

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