how to plant, grow & care for onions
complete growing guide
Onions are the basis for so many of our recipes and are easy to grow from sets or seed. Onion sets are easy to grow, with a small set turning into a full-sized bulb without much attention from you during the summer. Seeds are the way to go if you like a juicy salad onion, or have your sights set on a prize at the flower and produce show. The thinnings are also brilliant to garnish a soup or add to a stir fry. If you grow them onto maturity you can show off your skills with a handsome string or two, to keep you going through the winter months.
- Common name Onion
- Latin name Allium cepa
- Type Hardy annual vegetable
- Height & spread 30cm
- TLC rating Easy
- Aspect Full sun
- Spacing 10-15cm
- Yield 200g per plant
- Suitable for pots No
- Grow in a greenhouse? No
how to grow onions
where to grow onions
Soil type: Plant your onions in a well-drained neutral or slightly alkaline soil.
Aspect & Position: Choose a sunny open position in the kitchen garden for your onions. The risk of bolting is reduced if you avoid planting in cold, wet soil. Move the onion bed around every year to prevent the build-up of diseases like onion white rot.
when to plant onions
If you are starting off your onion seed indoors, they can be sown in January through to March. If you are direct sowing seed for either cooking or spring onions, it is better to wait for the soil to warm up in April. If you are planting out onion sets, mid-March to mid-April is also a good time. Overwintering varieties can be planted out in autumn to give you an earlier crop the following year.
how to plant onions
Onions can be grown in a variety of ways, depending on the amount you want to grow and the end result you require.
growing onions from sets
To grow full size mature onions that you can store over the winter, planting sets is the easiest method. Sets are the immature plants that are raised from seed the previous summer and stored over the winter months. Because they were sown at a very high density, they do not reach sufficient size to bolt. They just carry on growing instead.
Plant sets in early to mid-spring, pushing them gently into the soil so the tips are level with the surface. It’s a good idea to cover them with pea sticks or fleece to prevent birds pulling them out, it also helps to snip off the dried shoot at the top before you plant. For lots of small bulbs about 5cm in diameter, plant the sets 10cm apart in rows 15cm apart. For larger bulbs 10cm across, widen the spacing of sets to 15cm.
growing onions from seed
If you need to grow a lot of onions it is more economical to grow them from seed, and you can end up with much larger showing onions, should you want them. To win the prize you will need to start the seed off under cover in January or February. Try to sow individually into modules, in a fine seed compost. Place in a greenhouse (10-15°C) or on a bright windowsill. Once germinated, grow on in cooler conditions until all risk of frost has passed and they’re large enough to plant outdoors at the same spacing as the sets above.
You can also direct sow into the open ground in spring. Ensure the soil is firm and not too wet and avoid recently manured soil. Sow seed thinly 1cm deep in rows 30cm apart. Thin seedlings to 10cm apart for medium sized bulbs, or slightly further apart if you want to let them grow a little larger. The thinnings make perfect salad onions. The chance of bolting is decreased if you avoid planting in cold, wet soil. They thrive in a sunny, well-drained situation.
growing spring onions from seed
There are varieties of onion that are bred specifically for use in salads, often known as spring or bunching onions. They come in all shapes and sizes, depending on whether you favour more or less green on your shoots, and the bulbs white, red, bulbous or slim. But the principle is the same, direct sow in the kitchen garden in April, into firm finely raked soil. You can sow more thickly than the cooking onions, as you will be pulling them at a very immature stage, so aim for one seed every centimetre or so.
how to care for onions
watering & general care
Water during dry spells. Hoe between plants regularly to keep on top of weeds (we even sell a tool specifically designed for the purpose – the onion hoe). Remove any flowers that appear to prevent bolting. Pull the thinnings carefully (after watering in dry weather) and remove from the site to avoid attracting onion fly. Thinnings can be used as salad onions. It is important to move the onion bed around every year to prevent the build-up of diseases like onion white rot.
The cooking onions will be ready to harvest in mid-summer to early autumn, depending on the variety. The foliage will yellow and fall over naturally. Lift the bulbs with a fork to break the roots and leave them on the soil surface to ripen fully in the sun. In a wet summer, move them to a greenhouse bench to ripen.
Once they are completely ripe and the skins a lovely golden brown (red or white for the coloured varieties) it is fun to have a go at stringing them, French onion style. There is an art to this, which is hard to explain in words, but once mastered is very satisfying and ensures your onions stay fresh until the following year.
First, sort the onions into different sizes. It is useful to have strings of small, medium and large onions available for different recipes in the kitchen. Peel off the muddy outer skin to the glossy layer underneath, making sure you do not peel too far into the edible part. Trim off any excess muddy roots too. Hang a strong jute double string from a high hook or beam of the polytunnel and start to plait in the onion stems, making sure you space them evenly around the string, adding more onions one by one as you go down. If you plan to move the completed string do not make it too long as they can get extremely heavy and unstable. As long as it is a cool airy spot they can stay put, and just remove from the bottom as and when you need one.
- January-March: Sow seed undercover.
- April: plant out sets or seedlings.
- Direct sow seed outside for salad onions.
- Keep rows weed free.
- Water during dry spells.
- Thin seedlings for salad use.
- Harvest and dry off mature onions for storage.
- Plant over-wintering sets.
- Decide which varieties of onion you want to plant in the spring.
pests, diseases & common issues
There are a few pests and diseases that affect onions which is why moving them around the garden each year makes sense.
onion white rot
Onion white rot is a fluffy white fungal growth that attacks the base plate of the onion, attacking the roots and spreading into the bulb. The leaves turn yellow and wilt as the root dies off. Remove and destroy affected plants and do not grow any of the onion family in the same patch of ground for at least eight years. Make sure when you are planting onion sets that you only plant the best quality firm bulbs with no sign of rot on them.
onion rust disease
Onion rust disease is commonly found on leek foliage, but also can affect onions. Yellow raised patches appear on the leaves in moist weather. Remove infected leaves to stop the spread, and usually plants will recover. Improve air circulation, and ensure the seedlings are thinned and not growing too closely.
onion neck rot
The onion neck rot fungus attacks the neck of the onion, causing the stem to bend over prematurely and the top section of the bulb starts to rot. Avoid applying high nitrogen fertilisers, and make sure that plants are not watered too heavily. Never grow onions in the same area of ground two years running.
onion downy mildew
Downy mildew goes for the leaves of your onions and turns them blotchy grey and slimy. Air circulation is the key, so make sure you weed well between the rows, and thin the plants in good time. There are some varieties that show resistance to all these fungal attacks, Norstar being one of them.
Onion eelworm are microscopic nematodes that attack the foliage and bulbs, leaving them stunted and prone to rot if they get as far as harvest stage. Remove and destroy infested plants, and make sure another type of crop (e.g. any of the brassica family) is grown on that patch of land to break the life cycle of this pest.
Onion fly are much easier to spot with the naked eye, as the maggots are up to 8mm long, although they do live underground. You will spot them when you pull up a poor looking wilted onion and see it has been eaten off at the root. Try to pull up all the affected onions gently, maggots and all, and that will stop them pupating in the ground to affect future crops. Companion planting with a stronger smelling plant (e.g. carrots or coriander) can deter the adult flies, and enviromesh will offer physical protection.
Tiny flies and nymphs leave a tell-tale white mottling on the leaves of onions, but rarely kill the plant. They love hot dry weather, so at least you shouldn’t have the fungal infestations if you have the thrips!
why are my onions so small?
This is usually caused by them being grown too close together, so that they are competing with their neighbours for light, food and water. If you have grown them from seed thin them out in good time, having watered the row first to avoid root disturbance. Make sure the ground is fertile but not too rich in nitrogen, as this can exacerbate fungal attacks.
frequently asked questions
why do onions bolt?
A healthy onion should not flower until its second year, but if it is under stress it may decide to set seed in year one in case it doesn’t survive to year two. The stress can be caused by cold weather early in the spring, which is why it is important to wait until the weather warms up in April before you plant out your onions. Excessive heat and drought can also be the cause, which is why it is not a good idea to try to grow onions in a greenhouse or pot where conditions can get very hot and dry. Once an onion has decided to bolt there is little you can do to stop it, so pull it up and eat what bulb there is, and make sure the rest of the row get enough water if the weather is dry.
how often should you water onions?
Onions are not desperately thirsty plants, so just a weekly water if the weather is dry should be sufficient.
do onions need a lot of sun?
Yes, they do. A shaded onion will not have enough energy to store in its bulb for next year, resulting in a skinny leggy specimen with very little to eat. If you are growing salad onions for their leaves alone, they should be able to cope with a bit of partial shade.
how many onions will one plant produce?
If you are growing a standard cooking onion from a set or seed, one set or seed makes one onion. If you are growing shallots, however, one shallot bulb will have up to six shallots at the end of the summer. Shallots grown from seed only produce a single bulb.
do onions come back every year?
Most onions are strictly biennial, in that they usually flower in their second year and then die. However, there is the perennial Welsh Onion (Allium fistulosum) that clumps up each year, providing salad onions very early in the spring right through to the following winter.
how long do onions take to grow?
From germination to harvest, onions take around 6-9 months. You can shorten this considerably by using onion sets, but of course they have been growing elsewhere for a few months before you get to plant them!
what to grow with onions
Growing carrots near to onions should help confuse the onion fly (and of course the carrot fly). Brassicas, especially from the mustard family help deter the onion eelworm.