How to Plant & Grow Onions

How to Plant and Grow OnionsOnions are a beautifully versatile vegetable. Making the base for all your favourite recipes, including soups, sauces, curries and stews, onions can be enjoyed in so many ways. If you grow your own, you’ll always have an organic selection to hand. In this guide, we’ll cover everything you need to know about growing onions in your garden, polytunnel, or for a homegrown start-up.

Even if you’re a seasoned gardener and already know a thing or two about how to grow onions, it’s worth seeing if you can benefit from our handy tips, including the perks of growing onions in a polytunnel. Whether you’d like to start making your own delicious soups or you’re hoping to double your harvest next year, see below for our comprehensive onion growing guide.

How long do spring onions take to grow?

Onions are part of the allium family, which also includes garlic, leeks, and scallions – more widely known as spring onions. While red and white onions take a few months to fully develop, spring onions can usually be harvested just eight weeks after sowing.

As such, spring onions could be suitable if you fancy a fast-growing crop. If you’re looking for an introduction before learning how to grow onions on a larger scale, these versatile vegetables could make a great taster, requiring mostly the same maintenance steps as other onion varieties.

How to plant your onions

If you’re new to the onion-growing game, your first thought might be to go and buy some seeds. Even though seeds are available, onions are generally much easier and quicker to grow from immature bulbs called sets, also known as seedlings.

Growing onions from sets

You need to have a sunny, sheltered site in mind if you’ve decided to grow onions from sets. The best time to plant is between March and April to be ready for an autumn harvest. Onions thrive in fertile, well-drained soil, and need organic materials like compost to promote growth.

Plant your sets gently in rows approximately 25-30cm apart, with each set at least 5cm from the next. Purpose-built raised beds could provide the perfect growing environment for your new onion project.

Sowing onion seeds

For gardeners who choose to start from scratch, seeds should be sown in modules in mid to late winter, ideally at temperatures between 10 and 16 degrees Celsius. Growing three to four plants per module will save space, with seeds sown approximately 3cm deep in the soil, and each between 5 to 10cm apart.

How to grow onions

Knowing how to grow onions comes with having an idea of the time it takes them to mature, potentially giving you two harvests a year. It’s possible to sow onion seeds outdoors from late winter through to the middle of spring and, with the right maintenance, your crop should be ready by the end of the following season.

Watering and feeding

It’s best to water your onions once every two weeks, stopping once the onions have swollen during mid-summer. An occasional feed with liquid nitrogen-rich fertiliser could also benefit your crop, but any extra nutrition should be minimal to make sure your onions store as much water as possible.


Since onions are a shallow-rooting plant, they don’t thrive if they have to compete with other plants growing nearby. Weed carefully by hand instead of hoeing between rows, which could potentially damage the bulbs or foliage.

Keep an eye on your onion plants with regular checks – they’re susceptible to being swamped with weeds due to their minimal foliage.

Onion-neck rot can be a problematic disease, particularly in wet summers, and is often only apparent after they’ve been harvested and stored. Look out for strange brown marks and fluffy grey mould. You can also minimise the risk in advance by ensuring you source your seeds properly and harvest only in dry conditions.

Protecting your onions

Onion plants might need to be covered to prevent birds and other pests from pulling them up. Growing under a horticultural fleece could be one solution, and companion-planting parsley works well against onion fly larvae feasting on your bounty.

If you’re planning on growing onions in a polytunnel or another protected environment like a garden cloche or greenhouse, you could yield more crop thanks to more protection from natural hazards and pests.

Can you grow onions in a polytunnel?

Yes! You could produce an impressive yield from growing onions in a polytunnel, especially if you’ve chosen to plant onion sets instead of seeds. Using a polytunnel could help your seedlings reach their full potential, since you’ll have much more control over adjusting the growing environment for your onions.

A polytunnel warms the soil beneath, not only promoting faster germination but allowing you to plant them earlier in the season, naturally encouraging an earlier harvest. Polytunnels provide protection from the elements without blocking sunlight and prevent pests from trampling on your seedlings or even digging up the bulbs.

Overwintered onion sets, such as Radar and Senshu Yellow can be planted in a polytunnel from mid-September to October, making reasonable-sized onions in March or April.  

How to harvest your onions

You can harvest your onions as soon as they look big enough to eat! Once you see the foliage starting to turn yellow and lean to one side, it’s certainly time to fetch your onions from the soil. This usually takes place naturally in the late summer or early autumn.

Gently lift your bulbs out of the soil and place them on a sheltered rack in a dry, warm position, like a windowsill or in your polytunnel. Leave them for two weeks until their skins are dry, after which you should store them in a cool, dry place – or get cooking up your favourite recipe in the kitchen!

Start growing onions in a polytunnel with our help

For more advice on equipment to help you produce your biggest onion yield yet, don’t hesitate to contact our team on 01282 811250 or enquire via email at

With over 30 years of experience in the industry, we’re here to answer any questions you might have on how you can grow onions in a polytunnel.

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