Top Of The Crops - Onion

Onions are one of the most versatile and delicious vegetables that you can grow in your garden. Revered the world over for their ability to add flavour, depth and complexity to almost all savoury dishes, the many different varieties of onions are a welcome addition to any harvest. 

Thankfully, onions (and indeed everything in the allium family, including leeks, garlic and shallots) are refreshingly easy to grow. Spring onions, red onions, Spanish onions and brown onions – they all have their unique flavour profiles and purposes in different cuisines. 

Read ahead to learn all about how to grow onions in your garden. 

How To Grow Onions

Here are some of the main phases and instructions for sowing seeds, planting sets, caring for your plants, and dealing with everyday problems. 

Sowing onion seeds

It is simple to sow onion seeds, making this a perfect crop for novice gardeners. Remember – onions like fertile, well-draining soil, and they need organic matter (such as compost). While someone people start their plants in pots indoors in late October to mid-November, the ideal time to sow the seeds outdoors is often around March or April. 

Sow your seeds approximately 3 cm deep in the soil, and 5 to 10 cm apart. Your rows should then be a third to half a metre apart. The germination will most likely occur within 7-10 days of planting, but the temperature of the soil will affect this process. The cooler the soil, the longer to germinate. 

Planting sets or seedlings

If you have decided to grow your onions from sets (immature onions) or seedlings, you need to ensure that you have a sunny, sheltered site in mind. It should also have fertile, well-draining soil that has a PH balance above 6.5 (onions do not like acidic soil). 

Add a few buckets of well-rotted manure or garden compost to each square metre in your garden, as well as 35 grams of general purpose fertiliser.

The best time of year to plant onions is mid-March to mid-April. Plant your onion sets approximately 5 to 10 cm apart in rows of approximately 25-30 cm apart. Gently plant the onion sets into the soil, and ensure that the tip is poking through the surface. Birds are sure to be interested in your new onions! Cover them with fleece until the roots have a chance to establish themselves. 

Caring for your onion plants

Once your onions plants have developed a stable root system and have sprouted, you should water them if the weather is dry (but be careful not to flood the soil). You can also give them the occasional treat of liquid fertiliser, and a small amount of sulphate of potash around June can help the bulbs to ripen. Once your onions are round and bulbous in mid-summer, you can stop feeding and watering them. 

Dealing with common problems

Onion white rot: This is a fungus that lives in the soil, and it can cause your onions to become yellow and wilt, all the while rotting the roots. You will see a fluffy white fungus appear on the base of your bulbs – these will soon transform into round black structures. Sadly, there is no cure for onion white rot, and so you must take a lot of care to avoid cross-contamination and introduction. 

Leek rust – Leek rust is a fungus that creates yellow spots all over the leaves, and can be exacerbated by wet periods. While this is usually a cosmetic issue, a severe infection can cause problems. Ensure you don’t crowd your plants, and prevent cross-contamination.

Onion downy mildew – This is a fungus that will damage both your foliage and your onion bulbs, and is made worse in damp weather. Make sure your plants have a lot of light and air and don’t water from above. 

Growing Onions in Polytunnels

If you have chosen to plant onion sets rather than seeds, it can be a great idea to utilise a polytunnel. A polytunnel is a polythene-covered frame that has specifically been designed to house seedlings so that they can more easily be grown outdoors. Onions are the perfect crop for a polytunnel, as the polytunnel will protect them from pests and too much moisture. 

It can also be a wise idea to mix other crops that are more vulnerable to insects with the onions, as the onions’ strong smell can deter them. 

How to Harvest Onions

As soon as you see the foliage start to turn yellow and teeter onto their side, it’s time to harvest them. This usually takes place in late summer/early autumn. Lift the bulbs out of the soil, and place them on a rack in a dry, warm position (outside in the sun, or in a well-ventilated greenhouse) for two weeks. Once the onion skins are dry, you can store in a cool, dry place. 

The Sowing and Harvesting Calendar


February to April


July to September


Growing onions in your garden yield a cost-effective and satisfying crop, as you will have plenty of opportunities to use your bounty, and they store wonderfully in a cool and dry place. If you want your onions to last even longer than is possible in a cupboard or closet, you can store them in a single layer in a few inches of moist, cool sand. Do this, and your onions will last the whole year – you’ll be able to enjoy their pungent flavour until next year’s harvest!

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