Growing Onions from Seed to Harvest

Up there with the tomato, potato, chilli, pepper, carrot and other popular veg, is the humble onion. Onions are one of the most popular vegetables in the UK both to eat and grow. It is a sweet and tasty versatile staple ingredient that pops up everywhere from soups to salads, curries, pizzas, dips, crisps, and, let’s not forget how essential a good pickled onion is to accompany our fish and chip suppers the nation over. Onions can be baked, caramelised, fried, grilled, roasted, picked and more. So many dishes start off with an onion, think of the stocks, stews, and sauces we love that just would not be as flavoursome without it.

Gardeners across the UK love to grow onions in their gardens and allotments because all varieties are so easy to grow, and properly stored will keep reliably for up to six months. Onions are also great because you can get good cropping and yields from small growing areas. They’re a great one for beginners to try if it’s a first foray into growing in the garden at home. This guide is for all onions grown in the ground at home – be they brown, red or white – it doesn’t matter because in terms of growing, there’s no major difference. We’ll also cover the differences in growing from seed which is a bit more involved but still easy to do, or from set, which is the easiest method and saves you time.
Ready? Let’s get into it.

As mentioned, onions are unbelievably versatile vegetables and growing your own means you will have a supply nearby when you need them for your dishes. But onions form part of a balanced and healthy diet providing us a range of health benefits, vitamins, and minerals like vitamin C, B6, folic acid, as well as a good source of fibre. They are an immune system booster, they help to regulate blood sugar levels, and links have been found in studies that show onions can help regulate the body’s cholesterol levels.

Onions have the added beneficial characteristic that they generally store well over winter. They’re also super-easy to grow, and all varieties are grown the same way, meaning you don’t need to worry about any nuances in the growing process. Plus, fresher is always better, hands down.

There are many varieties of onion across the brown, red or white types. Consider what types of onions you most enjoy, or specific use cases. Maybe you would like to pickle your own small white onions, or maybe you want to grow your own brown onions for sauces or red onions for a nice addition (and talking point) to those summer BBQ burgers and skewers. Or maybe you want to be a little fancier and grow deliciously sweet shallots. Whatever the use case and variety you’ll follow the same process of growing.

If you’re a beginner, up until now you probably thought a brown or red onion was just a brown or red onion. UK supermarkets aren’t known for offering diversity across vegetables. Whether it be onions, potatoes, tomatoes, carrots – most if not all veg – you get one or two choices and that’s it. But with all vegetables, there are so many more varieties, and therefore choices, out there that you can only really experience if you maybe buy from a farm shop, or better yet, grown by yourself! From mild to medium to strong, sweet tasting, huge or small, there are so many more choices out there. This list below is just a small selection of the many varieties you can choose:

Globe Onions (brown)

A fine-looking variety that has a great smooth shape and rich brown skin. A reliable cropper and can grow exceptionally large (for an onion). It has a typical onion flavour and should keep for a long time if stored correctly. 

Electric (red)

Lovely glossy red skins with an excellent flavour. Great eaten raw in salads and if you want longevity after harvesting it will last most of winter when stored correctly.

Golden Gourmet (shallot)

A lovely shallot variety with a white flesh that will generally produce a heavy crop. Good, sweet flavour and keeps well when stored.

Red Gourmet (shallot)

A typical round red onion with white and red lined flesh. Its flavour is mild which makes it suitable for salads and cooking. It was specifically bred from early red onions which means it will be ready for harvest a bit sooner than other onion varieties.

Red Sun (shallot)

A lovely shallot variety with a white flesh that will generally produce a heavy crop. Good, sweet flavour and keeps well when stored.

Zebrune (shallot)

A typical round red onion with white and red lined flesh. Its flavour is mild which makes it suitable for salads and cooking. It was specifically bred from early red onions which means it will be ready for harvest a bit sooner than other onion varieties.

Unlike some other store-bought vegetables which wouldn’t be suitable to use to start your crop, such as potatoes, onions can be used in this way. However, we don’t really advise it. Firstly, it’s boring. You get one choice at supermarkets so do you really want to grow more of the same? Heck no! Choose something new from the varieties above. Secondly, these types of onions are grown for their longevity at the expense of other qualities like flavour, texture and size. This is the same for any store-bought vegetables. Opt instead for organically grown sets or seeds from a good garden centre, use natural, good quality composts and fertilisers, and these, combined with the fact you will be able to pick and eat when fresh will make for a night and day experience.

Onions are easily grown in two ways: seeds or sets.  Seeds are fairly obvious. Sets however are just baby onions. While growing from seed isn’t particularly difficult, set growing is the easiest and quickest way to grow them. If you want less time or hassle opt for sets. Sets are typically heat treated to kill their flower embryos which makes them far less likely to run to seed or bolt. This basically means for whatever reason the onion plant decides to try to produce and spread seeds instead of focusing on producing onions. Usually this happens because of the conditions or timings when growing. Say it is too cold, then they might think winter is coming and it’s time to produce seeds. Likewise, if the plants are suddenly stressed and could die, they’ll try to produce seeds quickly.

Sets are also sold as spring or autumn planting varieties. Generally though, most gardeners plant in the spring time and this is the focus of our guide too. But it’s something to be aware of when purchasing. However, there’s a small trade-off to consider. While it may take a bit more time, growing from seeds allows for more choice of variety. You may find lesser choice when shopping around for sets alone as not all varieties are grown to be sold as sets. Growing from seed is also a good bit cheaper. If all you are really seeking is fresh onions without too much fuss about any specific variety, you’ll be fine with sets.

When it comes to planting your onions in the UK generally it will be around late March or April time. All going well, this will allow for harvesting around August. Please note though that some varieties will crop sooner so just be aware of the specific growing timeline of your chosen onion variety. Weather and growing conditions will also be factors to consider. If you are in a colder region of the UK on average, it might pay to wait a bit longer to plant which will influence when you can harvest. Likewise, if you have a greenhouse, then you will likely be able to plant sooner, and achieve a faster harvest. Consider all the factors and get a good idea of your plan ahead of time. As mentioned, some sets are sold as autumn sets, but we will be focusing on spring sets and spring planting in this guide. Please be aware though, if planting from seed you would begin the whole process while still in winter. We will detail what to do to prepare your seeds for planting in the ground just below.

Now you know some more about onions you’re ready to get started growing your own at home. It pays to know what to do in advance so read through our guide, get up to speed, and then when you’re ready to go, get started! Remember, you’ll be starting off from seed or sets. Seeds are available year-round, but sets are generally available in spring.

If you are planting from sets, jump down to the next section. Now, when growing your onions from seed there are a few things to do before planting in the ground. For the best start, get a tray or container, and fill it with a good quality compost. Then sow your chosen onion seeds into the tray or container for transplanting later when they become seedlings. Don’t overcrowd the seeds, but plant a few more than you think you may need to choose the best seedlings later. Top them off with a 1cm layer of potting soil. Then place the container on a bright windowsill, where the temperature does not drop below 15°C. Water with a fine spray until the soil becomes damp. In the coming weeks be sure to keep the soil damp but not overly wet. You will do this as many as 12 weeks before planting, so if planting in April start it off in January.

When the seedings appear and the temperature is not only warm enough, but the nightly frosts have passed it will be time to transplant the seedlings into the soil in the garden. Depending on where you are in the UK this will generally be sometime in April, maybe May if your area has slightly cooler average temperatures. At this stage you can remove any visually weaker looking seedlings to focus on the ones that will grow best and have the best chance of yielding a good crop. To avoid shocking them – which can happen with any plant due to a change in environment – because they’ve been used to your comfortable windowsill for weeks now, harden them off for a week or two before permanently transplanting in the garden. To do this, simply sit them in a sheltered position outside when the weather is fair. Naturally, you don’t want to do it when there’s heavy rain or high winds as this will more than likely scupper your good work to this point. Start off doing it for a couple of hours at first, increasing each the length of time each day, until they’ve been out for a full day.

When the time does come to transplant your seedlings moisten the soil beforehand, this will cause less damage to the root system when moving. Turn the plants out of their pots or delicately dig them out of their tray/container avoiding the roots as much as possible. You will then place them into pre-made holes in prepared soil, but we’ll detail how to do that next in the ‘Preparing the Ground & Soil’ section. A final note, while you can plant seeds directly in a prepared site, you’ll have to wait to do this when the weather meets the right conditions. Starting the seeds off in your house helps you make a good start on them.

We’re perhaps being a little cheeky giving this its own section, but if you opt to grow onions from sets you will simply purchase them when available in spring, then plant in your prepared site. Easy.

Onions require a sunny but sheltered site with fertile and well-draining soil. Choose a suitable area. A good tip mention here if you are not growing for the first time is to reduce the chances of disease, avoid growing onions in the same plot for more than two years running because anything untoward can linger. When a suitable site has been chosen, start to prepare the soil by digging thoroughly and deeply, loosening it well, and, digging in a good vegetable compost and a little fertiliser at the same time. Be sure to remove any weeds you encounter too as these will compete for nutrients. Onions have a limited root system so it does pay to use some organic matter too if you can get it such as well-rotted manure.

To give you an idea of how large your area will have to be, you will plant each seedling or set approximately 10 – 15cm apart in long rows, and each row in turn should be about 30cm apart from one another. So, consider how many seedlings/sets you have or wish to grow, and this will tell you how large your planting area should roughly be. You really want to do all this well ahead of time before transplanting your seedlings (if you opted to grow from seed) or planting your sets. Ideally, the winter before planting.

When your soil is prepared and the weather conditions are right, transplant the seedlings carefully by making holes into prepared ground deep enough for the roots. Then plant each seedling about 10cm apart before gently firming in and watering. If you would like them to grow them a bit larger, try spacing slightly further apart. It’s just the same process for sets, except here instead of seedlings you will simply, and gently, press your sets into the prepared soil ensuring the tip is just about showing. Firm the soil around them and water in well without over watering.

Water regularly as the plants grow, especially when the weather is particularly hot or dry. Be careful not to overwater though. Onions are not a particularly thirsty vegetable and will not like being over watered. Keep on top of any weeds that try to grow in or between your rows because they really do sap nutrients from onions. If there’s heavy rainfall a protective cover might be an idea to protect them and keep the soil from waterlogging. Onions like a regular supply of nutrients but at a low level, so you don’t have to give them regular feeds of fertiliser. If you did do this, chances are you’ll just encourage them to grow more foliage and potentially flower at the expense of producing bulbs. Instead, fertilise sparingly once per month if at all. Cut off watering and fertilising when you see the onions have formed well. This will encourage them to mature further by developing thinner necks and firmer bulbs, which will help them to store better. Rest assured that most of the work has been done at this stage, and we think you’ll agree it’s quite straightforward. Keep an eye on your onions until it’s time to harvest.

Store onions in nets, preferably suspended, tied into bundles or if you have the time and inclination woven into long strings. Depending on the variety and amount grown your onions should keep until at least midwinter, and some will keep as long as spring. Be sure to only store perfect undamaged bulbs. Grown well and stored correctly, they should last for months.

Birds

Birds don’t eat onions, but they do enjoy yanking the protruding tips of onions up. Cheeky. They think it is potential material for their nests or to check what goodies might be underneath. Prevent this by using protective netting.

Onion Neck Rot

This is visible in the form of a dense white fungal growth around the roots and base of the bulb. Unfortunately, there is no cure or remedy, and the only real option is to dig up all traces then destroy them by burning. This site should not be planted on again.

Onion White Rot

This is visible in the form of a dense white fungal growth around the roots and base of the bulb. Unfortunately, there is no cure or remedy, and the only real option is to dig up all traces then destroy them by burning. This site should not be planted on again.

Onions can suffer from bug infestations, slugs will eat them, but they are particularly enjoyed by root fly. The larvae of the fly eat the roots of the bulbs and may also burrow into them in late summer. Growing onions from sets reduces the problem, as does interplanting. If you discover an infestation, remove infested bulbs promptly before the larvae move into the soil to pupate. If it gets too far the onions will have to be destroyed by burning and this site should not be used again. They can also suffer from infestation by thrips. If there’s a fine white mottling on the foliage it indicates their presence. They are tiny yellow or black bodied insects about 2mm long and are particularly troublesome in hotter weather. The damage they do to leaves results in smaller crops for harvest. There are a couple of decent treatments and remedies for thrips if caught early enough.

Splitting

If the soil is overly dry as they are growing onions can split or double which will reduce their ability to store well.

Soil Position

You should never grow onions in the same soil year after year as this could lead to a build-up of pests and diseases. Be sure to dig out all traces of plants and onions to avoid rot that will attract disease and pests to the area.

And we’re done! You’ll be growing onions or shallots in no time now you know how straightforward it is to take them all the way from seed to harvest. What varieties will you grow? Maybe you’ll learn to pickle them for homemade fish and chips, or for all the lovely salads enjoyed in the garden through summer. As you get better maybe you’ll even exhibit them in competitions – yes, it’s a thing to exhibit vegetables in a horticultural show. Whatever your aim, we hope you’ll give it a go.