Allium cepa is best known for the bulb onions, so widely used in almost all cultures and cuisines. This lesson is about growing them, and also their subtype, spring or salad onion, or scallion, used at an immature stage before they bulb.
Shallots are Allium ceps var. aggregatum, formerly called ascalonicum, from their origins in Ashkalon in Canaan. Shallots are closely related to onions and smaller in size. They have a softer cell structure, meaning they soften more readily when sautéed.
As with lettuce, this lesson is a visual feast for you, with so many of my photos from the last 14 years. Why 14? In 2006, I bought a digital camera!
I am not discussing perennial onions (Allium fistulosum) here, known colloquially as Welsh onion, Japanese bunching onion and many other names. Those plants do not make bulbs and keep growing new stems, a good way to have regular supplies of green onion.
- Seeds of Japanese bunching onion, such as the variety Ishikura, are sold as an alternative to Allium cepa salad onions. The selling point is that fistula onions do not bulb and make longer stems.
- Against that, I find them more prone to mildew – see ‘Diseases’ below.
- Days from seed to first harvest: for bulbs 140–160, overwintered 260
- Days from seed to first harvest: for salad onions 80–100, overwintered 190–240
Onion seeds sown or sets plantedOnions ready to harvestSeeds, sown late winter to early springBulbs, mid to late summerSets, planted early to mid-springBulbs, mid to late summerSeeds, sown late summer to very early autumnBulbs, by early summerSets, planted mid-autumnBulbs, by early summerSalad onion seeds, sown early spring to early summerSalad onions, summer to early autumnSalad onion seeds, sown late summer to very early autumnSalad onions, spring
Best climate is warm summers, continental or temperate, not too humid.
Why grow them
My customers, and friends helping in the kitchen to prepare food, remark on how my onions make them cry so much!
Why does chopping an onion make us cry? They release a substance, syn-propanethial-S-oxide, which irritates the eyes’ tear glands. Perhaps from high levels of sulphur, which is healthy.
- Most onions of commerce are grown with synthetic fertilisers which increase the water content, probably without a corresponding increase of micronutrients. They are therefore agreeably mild, with less flavour and pungency.
- Onions of all kinds have many health benefits, such as the vitamin C in raw onion, and chromium, which helps regulate blood sugars.
- With traditional digging and the wide spacing normally recommended, onions involve a lot of weeding and work per amount of harvest.
- With no dig and closer spacing, onions are easy to grow and harvests are so good!
I am diabetic type 1 and eat a lot of onion – it’s almost my staple vegetable ahead of potatoes. As a treat, I love them roasted, which heightens the sweetness.
Pattern of growth
Onions are hardy annual and biennial. For food, we grow them for their first-year stage of stems and bulbs. They overwinter as bulbs, then grow again and flower in the second summer. Or we can overwinter them as seedlings, to harvest in the following spring and summer.
Salad onions are bred to grow long, whiter stems, while others are bred to swell more at the base. There is a fine line between these two categories, a quality difference. Best results are from respecting the varietal descriptions, but it’s possible to grow salad onions for bulbs, and bulb onions for eating green – see the variety Lilia below.
- Leaf growth is healthiest through spring, especially late spring (May) when leaves suddenly grow tall and strong.
- They lose vigour by the summer solstice, when bulbs start to swell noticeably.
By late summer and autumn, green leaves are less healthy because they are out of season. Meanwhile, sow more onions in late summer, which overwinter as small seedlings with well-grown roots. They grow rapidly from early spring.
- Autumn-sown bulb onions swell for an early harvest from late spring, but do not store as well as spring-sown bulb onions.
Suitable for containers/shade?
The tidy leaves of onions make them ideal for containers. You will get the most value from growing the fistulosum types, for repeat harvest of green leaf and stem.
Be careful with growing onions in shade, because this may allow mildew to develop towards midsummer. They do grow in the shade, but are healthier and stronger in full sun.
Sturon is my go-to, for even growth and excellent storage.
Rose de Roscoff, ‘Keravel’, matures two weeks earlier than Sturon, is a beautiful pink colour and starts sprouting by late winter in storage.
Stuttgarter Giant is a great keeper, with a flat shape and strong flavour.
Hylander F1 and Santero F1 both resist mildew, are yellow and round, and store well.
Tropea Rossa Lunga is a long red onion, close to being a ‘banana shallot’. There is only a small difference between some onion and shallot varieties – it can be confusing.
Rosa Lunga di Firenze is even longer, thinner and more pink than red. Not for long storage.
Robelja red onion is a little paler than Red Baron, with a flatter shape and stores well.
Walla Walla grows a white bulb of mild and sweet flavour; it needs eating before it shoots in early winter.
Senshyu Yellow Express is an excellent Japanese bulb onion for sowing late summer, of fistulosum type.
Red Baron grows lovely firm onions that store well, with an intense dark colour.
Zebrune F1 is a good shallot to grow from seed, with long brown to pink bulbs.
Golden Gourmet is a round, yellow shallot that stores well.
Longor are true to their name and have a lovely pink colour – see the photo above.
Eschalotte Grise or Griselle or Gray Shallot are all the French grey shallot, quite small and more long than round, and renowned for their top flavour.
White Lisbon is my favourite, for its long white stem, dark green leaves and also a tasty white bulb when allowed to mature.
Apache grows lovely red-bottomed salad onions.
Lilia is called ‘dual-purpose’: 1) for lovely red salad onions, and 2) for bulbs in midsummer, although they are prone to bolting by solstice time.
When I started gardening in 1981, most people were growing onions from sets, not seeds. I heard of one grower who had some volunteers plant his 10,000 sets, and they put them all in the wrong way up! They do grow from that position, but the bulbs are less shapely.
I also knew growers who were using multisown module plants which they had bought – this was in 1983. I don’t know how long multisowing had been around before that, but it’s not a new practice. Eliot Coleman at Four Seasons Farm was multisowing in the 1970s.
I bought some transplants in 1983 and appreciated the benefit of using them, then adopted and developed the method. The more I do it, the more it intrigues me – plants growing alongside their mates!
Multisowing works for all vegetables as I describe here, but in many cases we simply don’t want clumps, because fruits are smaller and can be more difficult to pick, tomatoes for example.
- Seeds germinate in five to ten days.
- Sets show new leaves within seven to fifteen days, depending on spring temperatures.
Sowing time – SEEDS
Onion seeds germinate most strongly at temperatures around 16–21 °C/61–70° F, and not much more. This is probably achieved most easily in your house. After about one week you should see some tiny, spiky leaves.
The best sowing time is late winter, so that you have strong seedlings early in spring when the first warm afternoons are possible.
- A rule of thumb is that the more growth onions can make before summer solstice, the larger your harvest.
- To sow seeds outdoors, in a drill for transplanting, wait until the middle of March/early spring. After sowing, lay fleece over the bed. Sow about two seeds per centimetre/four per inch, in rows 15 cm/6 in apart. You can transplant them within six weeks.
Make your first sowing of salad onions at the same time as above, and then a second sowing in late spring. This interval is long but should assure continuity of picking lovely green onions. The later harvests, in early summer from your first sowing, will be larger and with some bulb, though still great for eating raw and ‘green’.
You can sow salad onions during the summer; however, that means they mature in autumn and with more risk of mildew on the leaves.
- The year’s last sowing of onions for salad is on the cusp of summer and autumn, for seedlings to overwinter and harvest through spring.
- If you have a few spare onions or shallots in early spring, plant them out closely, then use their new stem and leaves as salad onions before the end of spring.
Sowing time – SETS
Sets are very immature onions, sown in late summer in order for them to stop growing as baby onions before winter, then to continue next spring after planting. Over the years I have used sets from time to time to grow onions, and encountered enough problems to put me off using them again.
- Often a few onions from sets are diseased or have soft spots.
- There is more risk of bolting from planting too early, mainly for red onions.
- Sets cost more than seed and you can’t keep them for more than a few weeks, let alone a year.
One positive is that sets are quick and easy, with no need for propagating kit.
Ignore what it may say on the packet of sets and what you may read elsewhere. Best results come from putting sets in the ground later rather than sooner.
- Plant after mid-March, and preferably after the spring equinox.
- Dib holes about twice the length of your sets, and push them in with the broad end downwards – you will probably notice a little twist of onion skin at the top end.
Bulb onions grow brilliantly in clumps, and six seeds is a good number for sowing in each cell. This makes the ideal four plants per clump most likely, because a few seeds never germinate and a few grow weakly, which you can pull out after three weeks.
Use the photos as a guide to sowing depth and don’t sow any deeper than you see in them! I sometimes notice that my onions grow more strongly around the edge of module trays, where seeds are usually covered in less compost.
- When comparing module size, I find the best results are from smaller 3 cm/1.2 in cells. Or, measuring the distance from centre to centre of each cell, it’s 4 cm/1.6 in.
- This is the size of cells in my CD60 trays, and 60 cells per tray means you can germinate and grow a lot of onions in a small space.
- If the compost is of good quality, seedlings can grow large enough in a 60 cell tray, for transplanting outside without any potting on.
Onion seed does not keep brilliantly, and I have had several experiences of sowing newly bought seed with poor germination. One time, when I complained to the seller, he told me that I had germinated the seeds too cold or too hot, with never any acknowledgement that the seeds might be at fault. I knew that they were because I had sown some others at the same time and in the same conditions, all of which grew.
As far as possible, buy new packets every winter. Just hope the seeds in them are fresh, or quite possibly one year old, but not two. Seed packets do not tell you when seeds were harvested, only the date when they were packed.
Onion leaves are thin and look so fragile, yet they are resilient too. You can see in the photos below how they manage to push a fairly tight fleece cover upwards.
Transplant size and time
Small is good, but you can also plant onions seedlings larger. Occasionally we plant them at 5 cm/2 in tall, and sometimes at 10 cm/4 in, but more normally at 7.5 cm/3 in.
In the spring, unless the weather is lashing rain and very cold, I find that seedlings do better in the ground than waiting in trays. You can pot them to larger modules, or small pots if bad weather dictates that.
- Onion seedlings survive freezing. In the photo series below, there were three sub-zero nights from 12th to 15th May.
With the rounded end of a wooden dibber, make holes that are 2.5 cm/1 in deeper than the rootball of a module cell. Push the module in firmly and then water in as well, unless it’s about to rain.
Onions are great for interplanting. It’s more worthwhile than intersowing because of the time you save from using transplants. The photos below give you ideas, which are almost limitless.
I would not worry about the old-fashioned sayings of which plants are good companions. I have not come across any of these issues, as long as one respects each plant’s need for space and light. Onions’ upright growth and non-spreading habit give lots of possibilities.
- Transplant salad onions in early autumn under tomato cordons which are soon to finish.
- Transplant salad onions in late spring between newly planted celeriac, and between soon to finish carrots.
- Onions are a good interplant between newly planted asparagus crowns, up to the end of asparagus’ year two.
- Transplant early summer sowings of beetroot, kale and herbs between maturing onion bulbs.
Clumps with four seedlings can be as close as 30 cm/12 in, in all directions. This gives medium-sized onions, as long as your soil has reasonable fertility. Larger total harvests are from planting on the square or equidistantly, as opposed to in rows.
- Close spacing works well with no dig beds, because they grow few weeds.
- If you till, dig or rotovate, then many more weeds grow and a wider spacing makes it possible to access more easily with hands or a hoe; it would then also be better to transplant in rows rather than equidistantly.
- Mildew is a factor. If your climate is damp in late spring through summer, I would space a little wider. In wet summers mildew is difficult to avoid, see below.
Two different spacings are:
SALAD onions – either space more closely for clumps of four to six, at 20 cm/8 in rather than 30 cm/12 in, or space at 30 cm/12 in if your clumps each have eight to ten onions.
SETS can be as close as 7 cm/3 in apart, in rows 30 cm/12 in apart. This close spacing is equivalent to multisowing, but there is no point in planting sets in clumps when you can space them close anyway.
Onion seedlings grow slowly at first and, if early spring is dry, you may need to water new plantings. Give water about every five days, during the first two or three weeks.
Once you see onion leaves looking strong and upright, you probably won’t need to water again until early summer. By then, bulbs are swelling noticeably and, if it’s not raining at all, give some water to increase your harvest. Just once a week is worthwhile, a decent amount.
In Britain, it’s rare that you need to water maturing onion bulbs, except at transplant time.
- I find that even in dry summers, such as 2018, although you lose a bit of yield by not watering, this is compensated by superb quality thanks to extra sunshine.
- A result is no fungal damage or losses in storage.
Salad onions need more watering than bulb onions, to have more growth of green leaf.
Extra mulch to retain moisture?
In temperate climates, this is unlikely to be worthwhile. In arid and hot climates, it may be.
There is much confusion about timings and methods. Often there is no wrong or right time, it just depends what you want. One reader commented how his grandfather had always left the onion stems to fall over, on every onion, before harvest. He concluded ‘this must be correct’.
However, this delays harvest and reduces your opportunity to grow more vegetables. Furthermore, storage quality depends on whether your summers are dry or damp. In the latter case, it’s more successful to harvest onions sooner rather than later, so they have time to fully dry before winter.
How to judge readiness of onions
For fresh and small onion bulbs in very early summer, twist out a few as thinnings from your clumps of bulb onions. You have a harvest, while the remaining onions can grow more.
Harvest time refers to the moment you pull onions to detach their roots from the soil. Many will have some green leaf and stem – don’t wait for them to go yellow.
Firstly watch for some onion stems falling from upright to horizontal. Once about a quarter to a third have done this, it’s good to push over all the others. Some will have noticeably thick necks and need a firmer bend.
- The idea is to have a thin neck by harvest, rather than a strong stem still coming out of the bulb, which may not dry and store well.
For salad onions, your range of harvest times from one sowing may span a whole three months, from thin seedlings to juicy and bulbous.
How to pick
A week to ten days after the first stems fall over, leaves will be yellowing more, as energy flows from them back to the bulb. It’s a good time to pull your onions! Usually they ease out with some root attached, but for large ones you may just need to slip a trowel underneath.
- Pull gently in order to disturb soil only a little. Also not to snap roots from the bulbs – if any do that, use those bulbs first, because the bulb base now has an open wound.
- In dry weather, leave the pulled onions outside for an initial phase of drying, for a week or so. A combination of air, sun and even some rain is fine!
If you want the bed immediately for new plantings, move the onions to crates or a bench, or a wire undercover, as in the photo below. Otherwise clear the onions in ten days or so, by which time they are already noticeably drier.
There is one more stage of harvest, three to four weeks after pulling the bulbs. Leaves and stem are now mostly, but not completely dry.
- Either cut off most of the stem, to within 5 cm/2 in of each bulb. You can now fit more onions in a smaller box or crate.
- Or tie stems of, say, ten onions in a tight bunch, to hang anywhere dry.
- Or plait stems to look really nice – another level of enjoyment from your harvest.
Onions need to be kept warm and dry, especially if you want to store them until spring. In the house is good.
I tried this only once and was unlucky enough to encounter a wet summer, when the onion leaves suffered mildew while seed was developing. This took most of the quality out of my seed, which had used a fair amount of time and space to grow. The British climate is not brilliant for seed saving, but it is possible.
Save ten or so of your best onions, to plant in early spring at 30 cm/12 in apart. Or leave some good onions in the ground. In spring they soon grow new leaves and then long stems, which are best supported with four corner stakes and a strong string around, about 60 cm/24 in high.
The critical period is late summer, for cutting off heads once the blue flowers have died off. With one cluster, check that you can rub out the black seeds. Cut them all with some stem and put or hang somewhere dry and warm for two weeks at least. You can rub out the seed any time in autumn.
- In December 2019, Max of Krautgaart no dig market garden in Luxembourg (@Krautgaart, worth a look) gave me some of his homesaved Rose de Roscoff seed. Within a week of sowing it, in February 2020, I noticed the amazing difference compared to freshly bought seed, which grew at half the speed. Harvests were excellent too!
- Onions cross-pollinate, so save only one variety.
For once, slugs are not a problem, as long as soil is healthy and sowings are at the best times.
Rabbits mostly eat new and tender shoots. If present, your new plantings will need a mesh or fleece cover until plants are well established.
Allium leaf miner (Phytomyza gymnostoma)
They have been in the UK since about 2002, mostly in the south eastern third. A horrible pest and damage is significant if they are present.
A cover of mesh from February to early June can help, and again from September to November, if you grow onion seedlings then. These timings protect onions from the two generations of egg-laying flies, which are present mainly in early spring, and again, more importantly, during mid to late autumn. However this won’t protect them from any pupae which may already be in the soil.
The white maggots look similar to leek moth and bore into foliage, stems and bulbs for two weeks or so, before turning into brown pupae. Pupation occurs within stems and bulbs, in both late spring and autumn after the respective hatchings. From decayed leaves and plants, the 3 mm/1.2 in long pupae can end up in soil, until hatching as flies again.
- I would feel safe putting all damaged leaves and plants in the middle of a compost heap, with heat of more than 50 °C /122 °F.
- Rotation is worthwhile. For example, don’t plant onion after there has been garlic, onion, or leek in the last three years, approximately.
Onion downy mildew (Peronospora destructor)
This occurs in wet early summer weather, when your options are limited.
The first signs of infection are the tips of leaves turning yellow with some signs of white, and then very soon you see purple mildew. It’s difficult to spot, and onions can be infected for a while before you realise.
During any spell of rainy weather, from late spring through to midsummer, check your onion leaves closely. If you see this mildew and it stays wet, you may not enjoy a good harvest of onion bulbs or of salad onions. Infected plants are best pulled sooner rather than later, and the bulbs of infected onion plants will not store well.
- If your climate often has wet conditions in early summer, grow resistant hybrids such as Hylander and Santero.
- Allow more space between onion plantings for better air circulation; however, in favourable conditions the mildew spreads anyway.
- If you suffer this mildew, do not grow overwintering fistulosum for salad onions or onion bulbs in early summer. In winter their leaves can give the mildew a host to live on. This does not apply to A. cepa salad onions, such as White Lisbon.
White rot (Stromatinia cepivora)
Stromatinia exists in soil for 5–15 years, as dormant sclerotia. They spring to life in temperatures of about 15–20C/60 °s F, when they detect allium roots nearby.
Onions may suddenly fall over in late spring to early summer, even with bright yellow leaves. Their roots are dying fast, with a ball of white fungus fluff around them.
- Grow onions from seed not sets.
- Dispose of the fungus rather than compost it, but you can eat any unaffected part of the onion.
- Do not spread soil from infected areas and wash tools where appropriate. No dig reduces the spread of white rot.
- Leave a gap of three to five years before growing alliums in a bed where there is infected soil. When taking over an allotment, you may have to discover where there are spores the hard way.
Rust (Puccinnia allii)
Allium rust affects onions far less than garlic and leeks. The first signs are in spring – orange and slightly raised spots or pustules on lower and older leaves, rarely the new middle ones. If you see it on onion leaves, I would not worry. You can compost affected leaves.
After removing bulbs from bed to shed, there may be a few weeds to pull, then rake lightly. You now have clear ground and are ready to plant again, perhaps after some watering if it has been dry.
- You don’t need to apply any compost or amendments at this stage, wait until late autumn or winter, after the next planting has finished.
After salad onions in spring, you can plant almost any vegetable. You may want to spread some compost, if none was applied in autumn, before transplanting the onions.
After bulb onions finish in summer, your choices depend a little on the exact timing of the space coming clear. They include, as transplants, not sowing direct:
- Kale at close spacing, for medium leaves in autumn.
- Multisown beetroot, for small to medium roots by late autumn.
- Bulb fennel.
- Celery for small heads by mid-autumn.
- Salads such as endive, lettuce and radicchio.
- Chinese cabbage and other brassicas, such as pak choi and salad rocket, but with no rush to transplant them – see Lesson 13, Course 3A.
See also ‘Interplanting’ above.