growing onions
Why grow onions?
Onions are a relatively easy crop to become self-sufficient in, if that’s one of your goals. Most households eat relatively few of them to make it achievable to grow a year’s supply, or however many months’ worth you have the space to grow. Talking of space, they’re also fairly undemanding when it comes to space requirements.
When to sow
We grow our onions from seed, rather than sets (immature onions) because they are less prone to bolting (going to seed during adverse conditions) and we prefer not to take the risk of introducing white rot into the garden, which using sets can bring.
We start our seeds under cover and in module trays around mid February.
When to harvest/days to maturity
From a mid February sowing, we find that our onions are generally ready to harvest in mid to late August. Our favourite is a red onion called Red Baron, but time to maturity will vary dependent upon the variety you are growing, as well as the growing conditions. Seed packets should provide you with an indication of the likely harvest window for that specific variety.
Recommended varieties
We like to grow Red Baron as a red onion; we like the flavour and find it keeps well – 6 months plus, for us. We’re also growing Carmen for the first time in 2023 so we’ll see how that compares!
Sturon is well regarded as a white onion, and has good keeping/storage qualities.
Yellow Rynsburger is a yellow onion we’re trying for the first time in 2023, it is also supposed to be good for storing.
Garden Focused have created a good article comparing onion varieties if you would like to do some further reading.
How to grow
We like to sow our onion seeds in module trays and often have a mix of singularly sown seeds (i.e. one seed per module) and multi sown clumps (up to 6 seeds per module). Single onion plants produce larger onions, whereas multi sown clumps produce a bunch of medium sized onions. We fill our module trays with compost, firm it down, make a small depression in each module, place the seed(s) within these depressions, then top off with some compost to cover the seeds. Note that they don’t need to be planted very deeply, around half a centimetre of compost on top is about right.
Within a week or two you should see some signs of life, and once you can see some greenery you want to move them somewhere that they will have access to light – such as a south facing windowsill.
As they develop further by unfurling their light requirements will increase. At this stage, if you have access to a cold frame, greenhouse, polytunnel, polycrub, or grow light, then it’s a good idea to utilise one of them to maximise the quality of light they receive. If you don’t have access to any of these then keep them on the windowsill, but you may need to turn them around every so often to prevent them from growing in one direction (towards the light). If the weather is fine then you could put them outside in a bright, but ideally sheltered, spot during the day, then bring them back inside overnight.
After 6 weeks or so, they should big enough to handle (gently!), their roots should be visible at the bottom of the module tray, and they should also hold the compost together when you ease them out – helped also by the fact you firmed the compost well within the module tray. They are now ready to plant out into their final growing positions outside. If you have been growing them inside your house, or somewhere with supplementary heat, then it’s a good idea to harden them off before planting out, but if you’ve been growing them on in an unheated greenhouse, polytunnel or cold frame then you should be fine planting them straight outside at this stage.
To plant them out, you just need to make a small hole in your growing medium – ideally just slightly larger in diameter than the rootball, then pop it into the hole you’ve made. Push down gently on the top of the seedling’s root ball to ensure their roots are in contact with the earth below, then give them each some water directly into the planting hole. This helps to settle the compost/soil around their roots and removes any air pockets.
Once they’re all planted out, it’s. good idea to lay some horticultural fleece on top of them if it’s still early in the year, and weight it down around the edges to prevent it blowing away. This will provide the seedlings with some added protection from both the wind and cold temperatures to help them get growing. If you’re planting later in the season, or the climatic conditions are favourable, then fleece is an unnecessary step.
All that remains now is to keep an eye on them and provide them with water if there’s been no rain for a while. Remember it’s best to give them a good soaking less frequently than to give them water little and often, as this will encourage shallow roots and they will become dependent upon you watering them, rather than seeking out moisture themselves.
As the onions begin to form bulbs, it’s particularly important at this stage to ensure they have moisture available to them in order to form bulbs, it’s also best for the moisture level to be as consistent as possible.
Succession planting options
Oriental leaves (such as Pak Choi), salad rocket, spinach, lettuce, fennel, coriander, dill, spring onions, and spring cabbages are all possible succession plants for onions once they have been harvested in August.
Saving seed (optional)
Onions are biennials so if you would like to save seed your own seed for use in subsequent years, you will need to leave some of your onion plants (perhaps around 5 of them) untouched the first year. Note that you should only save the seed of non-hybrid (non-F1) varieties, otherwise the resulting onion will not be the same as the parent. Within this first year of growth they should have produced a bulb, they will then die back over winter and re-emerge next spring. They will continue to grow through their second year and will flower over summer, producing a seed head in autumn. Once the seed head has turned brown and is brittle/dry, the seedheads can be removed from the plants and the small black seeds can be extracted from their husks. We snip the seedheads off and the bring them in to dry for an extra few days in the house, then you can either:
- Put the seedheads in a bag then shake and crumple them which will separate the seeds from their husks. The seeds will be heavier than the chaff so you can then use a process of winnowing to separate the seeds and place them in a paper envelope for storing somewhere cool and dry.
- Shake the seedhead upside down into a bag or onto a flat surface and collect the seeds that fall out on their own, then roll each husk that still contains a seed between your finger and thumb to remove, then collect the seeds up and store within an envelope somewhere cool and dry.
Harvesting
If you have multisown your onions then you can harvest some of them from the clumps at any stage to make more room for the others growing within the clump, or you can leave them all to mature into fully grown bulbs for harvest in August. In terms of clump size, 1 or 2 onions growing together should produce large onions, 3 to 5 in a clump should produce medium sized onions, and 6 or more will produce smaller onions. Note that if you choose to harvest some of the onions within a clump throughout the growing season, immature onions can be used in the same way as you would use a spring onion.
To harvest the onions, whatever the stage, you just need to simultaneously twist and pull them out from the growing medium. If you’re removing some of the onions from clumps during the growing season then try to disturb the others in the clump as little as possible, then gently firm the compost/soil back down after you’ve successfully removed the ones you’re taking. Tip: you can also try giving them a bit of a wiggle whilst you’re pulling them out to help lessen the disturbance to the other plants’ roots.
For harvesting mature bulbs (which will also store the best), you want to wait until at least half of your onion plants are showing signs that their leaves are bending over – pointing their tops down towards the ground. By this stage you should be able to see the bulbs protruding from the surface of the growing medium. We tend to leave them for a week or so after seeing this, and then choose a dry day to then pull them up. Note that when you’re pulling them out, you want to keep as much of the roots intact as possible as they’re attached to the basal plate of the onion, and if this plate is damaged then they won’t store well. Dependent on how soft your growing medium is, you may want to use a small hand held fork to help loosen the surface just beneath the roots as you harvest them.
Once removed from the ground, they want to remain dry for around a month in order to cure – you want all of the green leaves to turn completely brown, ready for long term storage. If it’s a dry day and you can leave them on the surface of the soil to start this curing process then great, then you will need somewhere undercover to put them to complete this drying process. Try to lay them out in such a way that there’s airflow between the bulbs to promote drying and reduce the risk of any rot or mould forming. If it’s more convenient for you to hang them to dry, rather than laying them out horizontally, then you could plait them and hang vertically somewhere dry, but it’s still best to lay them out for a while first if you can find the space – even if only for a few days.
Storing/Preserving
Once fully dried, your onions will be ready to store for use through until the following year (if you’ve grown enough, and a good storing variety). They can be stored either loose (leaves and roots trimmed off – being careful not to damage the basal plate, and leave bit of a stub when removing the leaves) or in plaits/braids.
If you’re going to braid them for long term storage, then it’s easier to do so before the leaves have turned fully brown because they start to become more brittle at this stage, and you want them to still be plenty pliable enough to braid.
Note that you should only try to store onions that have no damage, and that haven’t bolted (gone to seed/sent up a flower stalk) – you should eat any of these ‘imperfect ones’ first and not attempt to store them long term.
Remember also to check on your stored onions regularly to ensure that none are rotting/going mouldy as this will quickly spread to the others unless the affected onion(s) are removed. A very strong smell will likely be the main giveaway if this does occur. If you cook with onions fairly frequently then you will most likely be checking on them with more than enough regularity to need to think about checking on them specifically!