Grow Your Own: Onions

This care guide is intended for beginner gardeners who want to grow their own onions. Onions are a popular and straightforward crop that can be grown in small spaces and yield reliable harvests. The guide explains the two ways to grow onions, from sets or seeds, and provides helpful tips on selecting onion varieties, preparing the ground, and sowing onions. The guide emphasises the importance of selecting a sunny location with well-drained soil, adding organic matter to the soil, and avoiding the use of fresh manure. It also explains how to sow onions from sets or seeds and provides useful tips on caring for seedlings.

Getting Started

Growing onions is a straightforward process that can be done in two ways: from immature bulbs called sets or from seeds. Onions are a popular and uncomplicated crop that can be grown in a small space and yield reliable harvests. Additionally, onions can be stored for a few months, so you’ll always have a supply on hand.

To grow onions, sets are typically used, and they can be planted in the autumn or spring. They grow quickly and reliably with minimal maintenance. Although they can also be grown from seed, it requires more time and care.

Onions prefer a warm, sunny location with well-drained soil. They require watering during dry spells and regular weeding. The crop should be harvested in the summer or early fall, and then either used immediately or stored for use throughout the following months.

Onion harvesting schedule
Onion harvesting schedule

Choosing Onions

You have a wide array of options to choose from when it comes to growing your own onions. These options include various varieties that produce yellow, white, or red onions, each with a unique flavour profile ranging from tangy and spicy to mild and sweet. These varieties mature at different times, with some ready for harvest as early as summer while others can be harvested in early autumn. Some onion varieties are more resistant to disease or bolting and store better than others.

To ensure a reliable harvest, consider selecting onion varieties that have received the RHS Award of Garden Merit (AGM). These varieties have been recognised for their ability to produce a bountiful crop.

Growing onions from sets, which are young bulbs, is the simplest and most efficient method. This approach results in a faster harvest and provides a better chance of healthy growth because plants grown from sets are less susceptible to disease. It is important to note, however, that sets are more likely to bolt, producing a flower instead of a bulb. To minimise the possibility of bolting, it is recommended to select heat-treated sets.

Prepare the Ground

To successfully grow your own onions, it’s important to select a planting site that is sunny and has soil that drains well. Before planting, be sure to thoroughly weed the area. Keep in mind that onions have a shallow root system, so it’s helpful to enrich the soil with organic matter. Add a bucket of garden compost or well-rotted manure to improve the soil structure, add nutrients, and retain moisture. It’s important to avoid using fresh manure.

To further support the growth of your onions, scatter a high-potassium general fertiliser over the soil. Use one handful per square meter. Onions prefer soil with a pH level of above 6.5. If your soil is too acidic, you can reduce acidity by adding lime during autumn or winter.

Sowing Onions

Growing from sets

Onion sets can be planted either in spring or autumn, depending on the cultivar and soil type. Spring planting is recommended from mid-March to mid-April for most sets, while those less sensitive to cold can be planted from October to mid-March. However, autumn planting is not suitable for heavy soils that are prone to waterlogging, as the crop may become vulnerable to disease.

To prepare the planting site, follow the instructions given above. Plant the sets by either gently pushing them into loose soil or placing them 2cm deep in drills. Ensure that the tip is just visible at the surface. Space them out 5-10cm apart, in rows 25-30cm apart. Firm the soil around the sets and water them thoroughly. To prevent birds from pecking at the newly planted sets, cover them with fleece until they have rooted.

Covering the ground with a black weed-control membrane before planting the sets through slits is another option. This method eliminates the need for weeding, which saves time and prevents any accidental damage to the bulbs when hoeing.

Growing from seeds

Onions are typically grown from sets, but it’s also possible to grow them from seeds indoors or outdoors. While seed is a more affordable option, it requires more attention and takes longer to grow. However, seed-grown plants are generally less prone to bolting (flowering).

For a successful harvest, it’s essential to ensure that seed-raised plants are growing vigorously by late spring. As the days get longer, bulb formation is triggered, and the more leaves the plants have at this stage, the better the resulting bulb will be.

Sowing indoors

To grow your own onions, you can start by sowing onion seeds in modular trays filled with seed compost. It’s best to sow the seeds in mid-to-late winter. You can either plant one onion plant per module or grow three to four plants per module to save space. However, keep in mind that the resulting bulbs may be smaller if you choose the latter option. Sow two seeds per module if you opt for one plant per module, or five or six seeds per module if you choose to grow multiple plants per module. Later, thin out the seedlings if necessary to achieve the desired number of plants.

After sowing the seeds, place the tray in a heated propagator in a greenhouse, or on a warm sunny windowsill with a temperature of 10-16°C. Within a couple of weeks, seedlings should appear. Once the seedlings emerge, remove the tray from the propagator and place it in bright light while keeping the compost moist.

In the spring, harden off the indoor-sown plants before planting them into the ground. Prepare the site as described in the guide, then space the plants 5-10cm apart, in rows 25-30cm apart. Firm the soil around the plants and water them well. Assuming you’ve developed various plants per module, don’t separate them into individual plants; To create an onion bulb clump, plant them as is.

Sowing outdoors

To develop your own onions, you can begin planting them outside from pre-spring until mid-spring when the dirt is drying out and starting to heat up. Make sure you have prepared the ground properly, following the steps mentioned earlier.

When sowing the onion seeds, ensure they are planted 1.3cm deep in rows spaced 20cm apart. It’s essential to thin out the seedlings first to 5cm apart and later to 10cm. Although a closer spacing will result in a larger overall crop with more bulbs, the individual bulbs will be smaller.

Onion Plant Care


In times of extended dryness, it’s recommended to water your onions every 14 days. However, it’s important to halt watering once the bulbs have swelled up in mid-summer. Overwatering after this period may result in poor storage.

It’s advisable to refrain from using overhead watering techniques since they can promote the development of fungal diseases.


During late winter, it’s recommended to fertilise your autumn-planted onions with a nitrogen-rich fertiliser, like sulphate of ammonia, at a rate of 35g per square meter. This will help promote growth and prevent premature flowering. If you prefer, you can also use dry poultry manure as a substitute.

Additionally, give your onions an occasional feeding of general liquid fertiliser from spring to mid-summer. This will help ensure they have the necessary nutrients to thrive throughout the growing season.

Weeding and removing

To ensure optimal growth of your onion plants, it is important to minimise competition from other plants. Regularly weed between your onions, being careful not to harm the bulbs or foliage if using a hoe. It is best to weed by hand to avoid any accidental damage. Due to the minimal shade cast by onion foliage, weeds can easily overtake the crop, hampering growth and reducing bulb size.

To minimise the need for weeding, consider planting through a weed-suppressing membrane.

To encourage healthy bulb growth, promptly remove any flower stems as soon as they appear. Allowing the plant to divert energy into producing flowers will hinder bulb development and reduce its storage capability.

Harvesting Onions

Onions that are planted in autumn can be harvested by early to mid-summer, while onions that are sown or planted in the spring are typically ready for harvesting in late summer to early autumn.

It was previously recommended to bend over the foliage or gently lift the bulbs to break the roots, but this is no longer advised. Instead, look for yellowing and toppling of the foliage as a sign that the crop is reaching maturity. Harvest the onions before the foliage dies down completely.

To harvest, use a fork to lift the bulbs, making a point not to harm or injure them, as this could make them decay away. Any onions that are damaged should be used immediately.


If you choose to plant onions in autumn, keep in mind that they will only last until early winter. However, if you opt for spring planting, your onions can last well into the following season.

To properly ripen your onions, arrange them in a single layer on a wire drying rack to allow air to circulate beneath them. You can leave them to ripen in full sun outdoors for approximately two weeks, or in a greenhouse or well-ventilated shed in case of wet weather.

It is essential to wait until all the foliage turns papery and dry before storing your onions. You can store them in net bags or trays, making sure to place them in a single layer. Alternatively, you can tie the bulbs into plaits and hang them up. Store them in a well-ventilated, cool, dry, and light place, but avoid storing them in the dark, as this can lead to sprouting.


Several fungal diseases can affect onions, including onion neck rot, leek rust, onion white rot, and onion downy mildew. Although growing onions is generally simple, these diseases can occur. These diseases can become a problem in damp growing conditions or if onions are poorly stored.

In addition to fungal diseases, onion flies, slugs, and snails may cause damage to the foliage. If you face such a situation, take a look at our suggestions on how to prevent slug and snail attacks. To avoid birds from pulling up newly planted onion sets, it is advisable to cover them with fleece until they are well-rooted.

Bolting, which is caused by low temperatures during spring, can result in flowering, and bulbs won’t store well under such circumstances.


In conclusion, growing your own onions is a simple and rewarding experience that can be achieved by beginners. You can choose to grow onions from either sets or seeds, with sets being the easiest option. Choosing the right onion variety, preparing the planting site, and sowing correctly will help you to have a successful harvest. Remember to keep the soil well-drained, add organic matter, and fertilise with potassium-rich fertiliser. Whether you are growing in a small space or a larger garden, with the right care, you can enjoy a plentiful supply of onions with unique flavour profiles. With some practice, you’ll soon be able to create your own delicious meals using fresh, home-grown onions.

Updated on September 7, 2023

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