How and When to Plant Onions in the UK

In bolognese and burgers, stir-fries and soups, onions are an incredibly versatile crop that don’t need much space and tend to last well once harvested.

So if you want to know when to grow onions in the UK and when to plant onion sets in the UK, this article will let you know the best time of year to sow, how to look after your plants, and how to harvest them once they’re ready to be picked.

There are advantages and considerations to planting both onion seeds and onion sets but before we get to that, there are quite a few different types of onions you can choose to grow in the UK.

The most common types of onions are the yellow and red varieties you see in your local supermarket. Yellow onions are the most pungent and the red variety are slightly sweeter, but you can also grow spring onions which are great in stir-fries and salads, or shallots, which are a little harder to grow, but have a less astringent taste and can even be roasted whole.

Onion Sets

Onion sets are immature bulbs that offer a head start in the growing process, leading to an earlier harvest compared to seeds. This method is favoured for its simplicity and higher success rate, making it especially appealing to beginner gardeners, including understanding when to plant onion sets in the UK, or those seeking quick results. Sets are less sensitive to variable weather and soil conditions, as they bypass the delicate seedling stage. However, onion sets can be more expensive than seeds and may carry diseases from their previous growth period. Despite these concerns, their ease of cultivation and the ability to quickly produce a crop make them a popular choice for many UK gardeners.

Onion Seeds

Growing onions from seeds allows for a broader selection of varieties and can be more cost-effective than purchasing sets. This method appeals to those who enjoy the full gardening process, from germination to harvest. Seeds require more care in the early stages, including maintaining proper soil conditions, thinning, hardening off, and protecting young plants from pests and diseases, and knowing when to sow onion seeds in the UK is a vital part of the process. While seeds take longer to mature into harvestable onions, they generally pose less risk of introducing diseases into the garden. The additional effort and time commitment involved in growing onions from seeds are rewarded with a wider range of onion types and the satisfaction of nurturing the plants from their inception.

There are two main planting seasons for onion sets in the UK. Most varieties are planted between the middle of March and the middle of April – sometimes a little later if the last frosts linger – and will be ready by the late summer.

There are also hardier, more tolerant varieties that can be planted in the autumn and they will often mature slightly faster than the spring varieties.

In general though, most varieties need around 90 – 100 days to grow into decent-sized onions ready to be harvested.

Planting onions is relatively straightforward and can result in a generous harvest. Here’s how to grow onions in the UK in a handy step-by-step guide.

Choose Your Onions

Look for healthy, firm sets without any signs of mould or sprouting. Smaller sets are preferable as they’re less likely to bolt (go to seed prematurely).

Soil Preparation

Onions need well-drained, fertile soil in a sunny spot. Work the soil to a crumbly texture and incorporate some well-rotted compost or manure to improve soil quality. Remove any weeds, as onions don’t compete well with them.

Spacing & Planting

Make holes in the prepared bed with a dibber or the end of a stick, spacing them about 10 – 15 cm (4 – 6 inches) apart in rows. The rows should be about 30 cm (12 inches) apart. This allows for adequate air circulation and room for the onions to grow.

Place an onion set in each hole, with the pointy end facing upwards. Gently push the sets into the soil so that the tip is just showing above the surface. Firm the soil around them to secure in place and then ensure the soil remains moist but not waterlogged.

Plant Care

Keep the area around the onions weed-free. Be careful not to damage the onion bulbs when removing weeds. A light application of a general-purpose fertiliser can be beneficial a few weeks after planting.

Harvesting
Spring-planted onions are ready to harvest when the tops start to yellow and fall over, usually from late July until towards the end of August. Lift them gently with a fork and let them dry in a sunny, airy spot for a few days before storing.

Autumn-planted onions will overwinter – remain dormant in the coldest months – and resume growth in early spring.

As the days lengthen and temperatures rise, the onions will grow rapidly. It’s essential to keep an eye on their development as spring progresses.

They will typically be ready to harvest in early to mid-summer, around June or July in the UK. The exact timing can vary depending on the weather conditions and the variety of onion.

The best time to plant onion seeds in the UK is from late winter to early spring, typically around February to April. This allows the seeds to germinate and grow indoors or under cover during the colder months, ensuring they’re strong enough to be transplanted outdoors once the risk of frost has passed, usually by late March or April, depending on your local climate conditions.

Planting onion seeds requires a bit of planning and care to ensure a successful crop. Here’s a basic, step-by-step guide to get you started.

Prepare Trays or Pots

Fill seed trays or small pots with a good-quality, fine seed compost. Water the compost before sowing to ensure it’s moist.

Sowing the Seeds

Sprinkle onion seeds thinly across the surface of the compost. Cover the seeds lightly with a thin layer of compost. Label the trays or pots if you’re planting multiple varieties.

Heat, Light & Water

Place the seed trays or pots in a warm, bright spot. A temperature of around 15°C (59°F) is ideal for germination. Ensure they receive plenty of light but avoid direct sunlight that can scorch the young seedlings. Keep the compost moist but not waterlogged. Use a water sprayer to gently mist the surface if it begins to dry out.

Germination

Onion seeds typically germinate within 7-14 days. Once seedlings emerge, ensure they receive plenty of light to prevent them from becoming leggy.

Thinning Out

When seedlings are big enough to handle, usually when they have a couple of true leaves, thin them out to prevent overcrowding. If grown in trays, you may need to transplant them into individual pots.

Hardening Off

Before planting out, acclimatise the seedlings to outdoor conditions by placing them outside during the day and bringing them in at night. Do this for about a week to 10 days as the weather warms up, typically in late April or May.

Transplanting

Plant the seedlings into well-prepared beds, spacing them about 10 – 15 cm (4 – 6 inches) apart in rows that are 30cm (12 inches) apart. Ensure the soil is fertile, well-drained, and has been enriched with compost or well-rotted manure.

Plant Care

Water the onions regularly, especially during dry spells, to keep the soil moist. Weed carefully around the plants to prevent competition for nutrients and water.

Harvesting

Onions sown from seeds will be ready for harvesting when the tops begin to yellow and fall over, typically from late summer into autumn.

Knowing when to plant onions in the UK and how to grow onions is a rewarding and enjoyable experience for experienced gardeners and enthusiastic amateurs alike. If you time the planting correctly, and follow the steps for sowing, plant care, and harvesting, you’ll usually get a great crop of onions ready for salads, sandwiches and stir-fries!