Onions are arguably one of the most indispensable vegetables in the kitchen, forming the aromatic base for countless dishes. For UK gardeners, growing your own onions, whether from tiny seeds or ready-to-plant sets, offers a profound sense of satisfaction and a superior flavour to anything store-bought.
The key to a successful onion harvest, free from bolting or pest issues, lies in precise timing. With options for spring and autumn planting, and the choice between seeds and sets, understanding the optimal calendar for onions in the British climate is crucial for continuous availability.
In this comprehensive guide, we'll walk you through the entire onion growing season in the UK, from the earliest indoor sowings to the strategic autumn plantings, ensuring you enjoy a year-round supply of your own delicious, home-grown onions.
Quick Answer: When Can You Grow Onions?
In the UK, onions can be started from seed indoors (January-March) or direct-sown outdoors (March-April). Onion sets are planted in spring (March-April) for summer harvest, or in autumn (September-November) for overwintering and early summer harvest.
By staggering your approach, you can ensure a nearly year-round supply of onions.
Onion Growing Timeline in the UK
January – March: Indoor Sowing from Seed
For large, well-formed onions with a wider choice of varieties, starting from seed indoors is highly recommended. This is a longer process but offers greater control.
- Conditions: Sow in trays or modules in a warm (15-20°C / 59-68°F), bright spot. A windowsill or heated propagator is suitable.
- Varieties: Any onion variety can be started from seed. Consider those known for good storage.
- Method: Sow seeds thinly, barely covering them with compost. Keep moist. Prick out carefully into individual modules once seedlings are large enough to handle.
These indoor-started plants will be ready to transplant outdoors in April or early May after hardening off.
March – April: Outdoor Direct Sowing from Seed
Once the soil has warmed and is workable, direct sowing onion seeds outdoors is a popular method, particularly for spring onions (scallions) or smaller bulb onions.
- Conditions: Soil should be fine, raked free of stones, and not waterlogged.
- Varieties: 'Ailsa Craig', 'Sturon' for bulbs; 'White Lisbon' for spring onions.
- Method: Sow thinly in shallow drills (1-2cm deep), in rows 20-30cm apart. Thin seedlings to 5-10cm apart for bulbs, or leave denser for spring onions.
Direct-sown bulb onions typically mature from late July to September.
March – April: Planting Spring Onion Sets
Onion sets (small, immature bulbs) are the easiest and fastest way to grow bulb onions. Spring-planted sets typically yield earlier than those grown from seed.
- Conditions: Plant once the soil is workable and not frozen or waterlogged.
- Varieties: 'Sturon', 'Stuttgarter Riesen', 'Red Baron'.
- Method: Plant sets gently, ensuring only the very tip is visible above the soil, 10-15cm apart in rows 30cm apart. Protect from birds.
Harvests can begin from July, continuing through August.
September – November: Planting Autumn Onion Sets
For the earliest possible onion harvest the following summer, plant specific overwintering onion sets in autumn. These establish roots over winter and grow rapidly in spring.
- Conditions: Choose a well-drained site. Early autumn is best for establishment before deep cold.
- Varieties: 'Radar', 'Senshyu Yellow', 'Electric Red'.
- Method: Plant as with spring sets, just the tip showing, 10-15cm apart.
These will be ready to harvest from June onwards, often a month or more before spring-planted varieties.
Harvesting Onions (June – September)
Onions are ready for harvest when their foliage starts to turn yellow and flop over. This usually happens from late July to September for spring-planted crops, and from June for overwintered sets.
- Method: Gently lift the bulbs with a fork on a dry, sunny day. Leave them on the soil surface to dry for a few days, then move to a well-ventilated, dry spot to cure for 2-3 weeks before storing.
MyGrowPal helps you track your onion calendar. Set your UK postcode, and the app adjusts sowing/planting dates to your local conditions, sends reminders for different batches (seeds vs. sets), and flags crucial care tasks – so you always know what your onions need.
Key Factors for Successful UK Onion Growing
Site and Soil
- Sunny Location: Onions need plenty of sunshine (at least 6-8 hours daily) to develop large, healthy bulbs.
- Well-Drained Soil: They hate 'wet feet' and are prone to rot in waterlogged conditions. Improve heavy clay with plenty of well-rotted organic matter.
- Fertility: While onions are hungry, avoid fresh manure or high-nitrogen fertilisers immediately before planting, especially for sets, as this can encourage leafy growth at the expense of bulbs, or lead to bolting. Well-rotted compost worked in the previous autumn is ideal.
Seeds vs. Sets
- Seeds: Cheaper, wider variety choice, less prone to bolting (especially overwintered onions), but take longer and require an indoor start.
- Sets: Easier, faster to mature, generally larger bulbs, less prone to pest damage in early stages, but more expensive and limited variety. More prone to bolting if spring-planted sets experience a cold snap.
Weeding is Critical
Onions are poor competitors. Weeds can quickly outgrow and smother young onion plants, severely reducing your harvest. Keep beds meticulously weed-free, especially in the early stages, by hand-weeding regularly and shallowly to avoid disturbing the shallow roots.
Watering and Feeding
- Consistent Moisture: Water consistently during dry spells, particularly when bulbs are swelling (June/July). Reduce watering as leaves start to yellow, to aid ripening.
- Feeding: Once bulbing begins, a balanced feed or one slightly higher in potash can be beneficial, but generally, good soil preparation is enough.
Pest and Disease Prevention
- Onion Fly: A major pest whose larvae tunnel into bulbs. Best prevented by rotating crops, covering young plants with fine insect-proof netting, or planting 'Sacred Valley' onions which are naturally resistant.
- White Rot: A fungal disease that causes leaves to yellow and bulbs to rot. No chemical cure; avoid planting alliums in affected soil for many years.
Common Onion Growing Mistakes in the UK
Planting Sets Too Deep
Planting onion sets too deeply restricts bulb development. Ensure just the tip of the set is visible above the soil for optimal bulb formation.
Not Weeding Regularly
Onions compete poorly with weeds. Allowing weeds to grow significantly reduces light, water, and nutrient availability for your onion bulbs, leading to a smaller, poorer harvest.
Inconsistent Watering
During the crucial bulbing phase, dry spells followed by sudden heavy watering can lead to splitting or uneven growth. Consistent, deep watering is best.
Ignoring Crop Rotation
Repeatedly planting onions in the same spot builds up soil-borne pests (like onion fly larvae) and diseases (like white rot). Rotate your crops annually to break these cycles.
Planting the Wrong Type of Set for the Season
Using spring sets in autumn or autumn sets in spring can lead to premature bolting (flowering) rather than bulb formation, as they are bred for specific day lengths and temperature fluctuations.
Using MyGrowPal to Master Your Onion Season
With multiple planting methods and seasonal timings, growing onions can be a nuanced process. MyGrowPal simplifies this, transforming your onion ambitions into a clear, actionable plan tailored to your UK garden.
Frequently Asked Questions
What is the best month to plant onion sets in the UK?
For a main summer harvest, plant spring onion sets from March to early April. For an earlier harvest the following year, plant specific overwintering (autumn) sets from September to November.
Is it better to grow onions from seed or sets?
Both methods have merits. Sets are generally easier, faster to establish, and less prone to bird damage, producing larger bulbs. Seeds offer a much wider variety choice, are cheaper, and less prone to bolting, but require an earlier indoor start (January-March) and more initial care.
Can I grow onions in winter in the UK?
You can 'grow' onions over winter by planting hardy autumn onion sets or hardy onion seeds in September-November. These will establish roots through the cold months and then grow rapidly in spring for an early summer harvest.
Why are my onions flowering instead of bulbing?
Onions 'bolt' (flower) when they experience stress, most commonly due to sudden temperature fluctuations (like a cold snap after warm weather) or if overwintered sets don't receive enough consistent cold. Bolting diverts energy from bulb development. Remove flower stalks immediately to try and save the bulb, though it won't store well.
How deep should I plant onion sets?
Plant onion sets so that just the very tip of the bulb is visible above the soil surface. Planting too deep can restrict proper bulb development, while planting too shallow can make them prone to being pulled out by birds or drying out.
Final Thoughts
Growing onions in the UK is a deeply satisfying endeavour, offering the promise of fresh, flavourful bulbs for your culinary adventures. Whether you opt for the convenience of sets or the vast choice of seeds, understanding the nuances of timing is paramount for success.
By carefully planning your indoor and outdoor sowings, selecting varieties suited to the season, and providing consistent care, you can overcome common challenges and enjoy a continuous supply of this essential ingredient. The humble onion, when homegrown, truly shines!
And with MyGrowPal as your dedicated garden companion, you can navigate these seasonal strategies with confidence, transforming guesswork into a reliable system for a bumper onion harvest, year after year.