How to Grow Onions: Your Complete Onion Growing Guide

Growing your own onions offers brilliant value. A single packet of sets costs less than a few shop-bought bulbs, yet can provide months of storage onions for your kitchen. I’ve found that home-grown varieties taste noticeably sweeter and store better than commercial ones, particularly when you choose cultivars suited to our variable weather. Whether you’re working with a proper vegetable plot in Kent or a few containers on a Manchester balcony, onions adapt remarkably well to different growing situations.

What makes onions particularly rewarding is their low-maintenance nature once established. They don’t need constant watering like tomatoes, they’re relatively pest-resistant compared to brassicas, and they fit neatly into crop rotation schemes. I’ll walk you through exactly how to grow onions successfully, from selecting the right type through to harvesting and storage. This isn’t complicated, but the details matter.

Another article you may also like: from seed to salad: growing chard that actually thrives.

How to Grow Onions

Essential Materials and Tools

The basic requirements for growing onions are refreshingly simple. You’ll need onion sets (small bulbs) or seeds, depending on your chosen method. I strongly recommend sets for beginners, as they’re more forgiving and give you a head start. You’ll also need a trowel or dibber for planting, a rake for preparing the soil, and a hoe for weeding between rows.

For soil preparation, well-rotted compost or manure is valuable, though not absolutely essential if your soil is in decent condition. A general-purpose fertiliser helps, particularly one with moderate nitrogen content. I keep a ball of string and some bamboo canes for marking out straight rows, which makes subsequent weeding much easier. If you’re growing in containers, you’ll need pots at least 20cm deep and a quality multi-purpose compost.

Choosing the Right Onion Variety

This decision matters more than many gardeners realise. Onions are day-length sensitive, meaning they form bulbs in response to the number of daylight hours. British growers need cultivars bred for our latitude. Autumn-planting sets like ‘Radar’ and ‘Electric’ go in during September or October and harvest the following June. Spring-planting varieties such as ‘Sturon’ or ‘Red Baron’ are planted March through April for late summer harvest.

A separate article you might also find helpful from seed to giant squash: growing pumpkins that actually thrive.

I’ve grown both types extensively across different sites. Autumn sets give you earlier crops and generally larger bulbs, but they’re more vulnerable to harsh winters and bolting if we get unseasonable warm spells. Spring sets are more reliable for beginners and still produce excellent yields. For storing through winter, choose varieties specifically labelled as good keepers. Salad onions (spring onions) are a completely different category, grown from seed and harvested young, which I’ll touch on separately.

Step-by-Step Instructions

Preparing Your Growing Site

Onions demand well-drained soil above everything else. I’ve seen perfect sets rot in heavy clay that stays waterlogged, whilst thriving in sandy loam that drains freely. If your soil is heavy, work in plenty of organic matter several weeks before planting, or consider raised beds. The site should receive full sun for at least six hours daily. Partial shade will give you small, disappointing bulbs.

Clear all weeds thoroughly before planting. Onions have shallow roots and weak foliage that can’t compete with vigorous weeds. I rake the surface to a fine tilth, removing stones and breaking up clods. The soil should be firm underneath but crumbly on top. Avoid freshly manured ground, which causes excessive leaf growth at the expense of bulb development. If you manured heavily for a previous crop like potatoes or courgettes, that residual fertility is perfect for onions.

Planting Onion Sets

Timing depends on your variety choice. Autumn sets go in when soil temperatures are dropping but before the ground becomes waterlogged, typically late September through October. Spring sets are planted from mid-March onwards, once soil conditions allow. I wait until the surface isn’t muddy and sticky. In colder regions like Scotland or northern England, delay spring planting until early April.

Push each set into the soil so just the tip shows above the surface. Space them 10cm apart in rows 25-30cm apart. This gives each plant room to develop without wasting space. I’ve experimented with closer spacing and it does reduce final bulb size noticeably. Plant sets the right way up, with the pointed end facing upwards. Birds sometimes pull newly planted sets out of curiosity. If this happens in your garden, cover rows with fleece or netting for the first week or two until roots anchor them properly.

Growing from Seed

Seed-grown onions require more effort but offer greater variety choice and cost savings. Sow indoors during January or February in modules or seed trays, keeping them at around 10-15°C. I use a windowsill in an unheated room rather than a warm propagator, as onions don’t need high temperatures. The seedlings look like thin grass initially and grow slowly.

Transplant outdoors in April when seedlings are pencil-thick, spacing them as you would sets. Alternatively, direct sow outdoors in March or early April where plants will grow. This avoids transplant shock but requires perfect weed control, as the tiny seedlings are easily overwhelmed. I thin direct-sown onions to the correct spacing once they’re established. For salad onions, sow seed direct every three weeks from March through August for continuous supplies.

Watering and Feeding Requirements

Young onion plants need consistent moisture to establish their root systems. I water regularly during dry spells in spring, particularly for spring-planted sets that are growing rapidly. However, onions hate sitting in wet soil, so gauge your watering by conditions. In typical British springs with regular rainfall, I rarely water established plants at all.

Reduce watering as bulbs begin to swell, typically from June onwards. Excessive moisture at this stage can delay ripening and reduce storage quality. Stop watering entirely once foliage begins to yellow and topple over naturally. Feeding requirements are modest. I apply a balanced liquid feed once in May if growth seems slow, but avoid high-nitrogen fertilisers after early June, as these promote leaf growth when you want energy going into bulb development.

Ongoing Maintenance

Weeding is your main task throughout the growing season. Onions cannot tolerate weed competition, and their upright foliage makes hand weeding between plants necessary. I hoe between rows regularly when weeds are tiny, which takes minutes rather than the hours required if you let them establish. Mulching between rows with compost helps suppress weeds whilst improving soil structure, though I keep mulch away from the bulbs themselves.

Watch for bolting, particularly with autumn-planted sets. If a plant sends up a flower spike, remove it immediately by snapping it off at the base. Use bolted onions promptly in the kitchen rather than trying to store them. They won’t develop proper bulbs once they’ve decided to flower. I’ve found that inconsistent temperatures in spring trigger most bolting, but you can’t control British weather, so some years are simply worse than others.

Harvesting and Curing

Onions are ready when foliage yellows and falls over naturally, typically July through September depending on planting time. Don’t be tempted to bend the tops over yourself, which can introduce disease. I wait until at least half the foliage in a row has collapsed before harvesting. Choose a dry day and lift bulbs gently with a fork, taking care not to bruise them.

Curing is absolutely critical for storage. Spread harvested onions somewhere dry and well-ventilated for at least two weeks. I use a greenhouse staging, but a shed, garage, or covered porch works equally well. The skins should become papery and the necks completely dry. Trim off roots and dead foliage, leaving about 5cm of dried stem. Properly cured onions store for months in a cool, dry place. I keep mine in net bags or wooden crates in the garage, checking periodically and removing any that show signs of rot.

Common Mistakes to Avoid

Planting in Poor Drainage

This is the single biggest cause of onion failure I’ve witnessed. Onions genuinely will not tolerate waterlogged conditions, even temporarily. If your soil puddles after rain, don’t plant onions in-ground. Use raised beds or containers instead. I’ve seen entire rows rot in heavy clay during wet summers, whilst onions in raised beds three metres away thrived. The difference is dramatic and entirely preventable.

Similarly, avoid low-lying areas where cold air or water accumulates. Onions need good air circulation around their foliage to prevent fungal diseases. If you’re working with challenging soil, improving drainage takes priority over everything else. Adding sharp sand and organic matter helps, but sometimes you simply need to choose a different growing location or method.

Planting Sets Too Deep

I see this constantly, particularly with new gardeners who assume deeper planting means better anchorage. Onion sets planted too deep struggle to develop properly sized bulbs. The tip should be just visible above the soil surface. That’s it. Yes, birds may pull some out initially, but the answer is temporary netting, not deeper planting. Sets buried completely often produce stunted bulbs or excessive leaf growth.

Conversely, sets that work their way upwards as they grow are perfectly normal. Don’t push them back down. The developing bulb naturally sits partially above the soil surface. This actually helps with ripening and reduces the risk of basal rot.

Over-Fertilising with Nitrogen

More isn’t better with onion feeding. Excessive nitrogen, particularly later in the season, produces lush green foliage at the expense of bulb development. I’ve made this mistake myself, applying a nitrogen-heavy lawn feed residue near onion beds and getting enormous tops with disappointing bulbs. Onions need balanced nutrition early on, then virtually nothing from midsummer onwards.

If you’re growing in soil that was heavily manured for a previous crop, you probably don’t need to feed at all. Watch the foliage colour. Healthy growth should be blue-green, not the dark emerald associated with excess nitrogen. Pale, yellowing leaves suggest nutrient deficiency, but this is relatively uncommon in established garden soil.

Expert Tips

Maximising Storage Life

The secret to long-storing onions starts during growth. Varieties matter enormously. Some cultivars are bred for immediate use and deteriorate within weeks, whilst proper storage varieties last until spring if handled correctly. I always check descriptions when buying sets. Beyond variety, growing conditions affect storage potential. Onions grown in rich, moisture-retentive soil don’t store as well as those from leaner, drier conditions.

Curing cannot be rushed. I’ve tried shortcutting this process and regretted it every time. Those extra days of drying make the difference between onions that last months and those that rot within weeks. Once cured, storage location matters. Cool temperatures (just above freezing) and low humidity are ideal. My garage stays around 5-8°C through winter, which is perfect. Avoid anywhere that freezes or anywhere warm and humid like kitchens.

Dealing with Pests and Diseases

Onion white rot is the most serious disease, causing yellowing foliage and white fungal growth around the bulb base. Unfortunately, there’s no cure, and the spores persist in soil for years. If you encounter it, grow onions elsewhere and avoid replanting that area for at least eight years. I’ve never had it in my main plot but saw it devastate a community garden plot in Berkshire. Prevention through crop rotation and buying certified disease-free sets is your only defence.

Onion fly larvae tunnel into young bulbs, causing them to rot. The adult flies are attracted to the smell of damaged onion tissue, so minimise disturbance when thinning seedlings. Growing onions from sets rather than seed largely avoids this problem, as sets establish quickly with minimal thinning. I’ve experimented with companion planting carrots near onions, and whilst some swear by this, I’ve not noticed significant pest reduction. Physical barriers like fleece are more reliable if onion fly is problematic in your area.

Getting Earlier Harvests

Autumn-planted sets give you harvestable onions by late June, sometimes even early June in southern regions. This is brilliant for kitchen use when shop prices are highest and your other crops aren’t ready yet. However, success depends on choosing specifically bred autumn varieties. Standard spring sets planted in autumn will almost certainly bolt.

Another approach is starting spring onions under cover. Sow seeds in modules in a greenhouse or cold frame during February, then transplant outside in April. This gives you usable salad onions by May rather than waiting until June from direct-sown crops. I do this routinely and find it doubles my salad onion season.

Frequently Asked Questions

Can I grow onions in containers?

Absolutely, and they perform surprisingly well in pots. Use containers at least 20cm deep and wide enough to space sets or transplants 10cm apart. Drainage is even more critical in containers than in-ground, so ensure adequate drainage holes and use a good-quality multi-purpose compost rather than garden soil. I’ve successfully grown both storage onions and salad onions in large containers on a patio. The main challenge is preventing the compost from drying out completely during hot spells, as containers dry faster than beds. Water regularly but don’t allow the compost to become waterlogged. Container-grown onions may be slightly smaller than those grown in open ground, but the quality is identical and the convenience can’t be beaten for gardeners without much space.

Why are my onions bolting and producing flower stalks?

Bolting happens when onions experience stress that triggers their natural flowering response. The most common cause is temperature fluctuations, particularly unseasonably warm periods followed by cold snaps during spring. Autumn-planted sets are more prone to this because they experience a longer period of variable temperatures. Using the wrong variety for your planting time also causes bolting. Spring sets planted in autumn or vice versa will almost always bolt because they’re not adapted to the day-length patterns they’re experiencing. Unfortunately, once an onion bolts, you can’t reverse it. Remove the flower stalk immediately to salvage a small usable bulb, but understand it won’t store and should be used within days. Some years are simply worse for bolting than others, and there’s little you can do beyond choosing resistant varieties and planting at the correct times.

How do I know when onions are ready to harvest?

The plants tell you clearly when they’re ready. The foliage yellows and topples over naturally as the bulb reaches maturity and the neck weakens. Wait until at least half, ideally two-thirds, of the plants in a row have fallen over before harvesting. Don’t be tempted to bend the tops over yourself, which can introduce disease organisms through the damaged tissue. Timing varies with variety and planting time. Autumn-planted sets typically mature by late June or July, whilst spring-planted varieties are ready from August through September. Weather affects this, with hot, dry summers speeding maturity. Once foliage has collapsed, don’t leave bulbs in the ground for weeks, as they may start growing again if autumn rains arrive. Harvest within a fortnight of the tops falling and begin the curing process immediately for best storage results.

What’s the difference between onion sets and seeds?

Sets are small onion bulbs grown the previous year, then harvested and stored specifically for replanting. They’re essentially a head start, producing mature bulbs faster and with less effort than seeds. Sets are more expensive per plant but far easier for beginners and more reliable in variable conditions. They establish quickly with minimal care and are largely immune to early pest problems that affect tiny seedlings. Seeds offer much greater variety choice and are more economical if you’re growing large quantities. However, they require earlier sowing (typically January or February indoors), more space for propagation, and careful transplanting. Seed-grown onions take longer to mature and need more attention to weeding when young. I use sets for my main crop and save seed-growing for specific varieties I can’t get as sets, or for salad onions which are always grown from seed.

Can I eat onions that have started sprouting in storage?

Yes, sprouted onions are perfectly safe to eat, though the quality deteriorates as sprouting progresses. The green shoots are edible and can be used like spring onions. However, sprouting indicates storage conditions aren’t ideal, typically too warm or too much light. The bulb itself becomes softer and less flavourful as energy goes into producing new growth rather than remaining dormant. I use sprouted onions promptly rather than trying to store them longer. If multiple onions start sprouting, check your storage conditions. Temperature should be cool (ideally below 10°C) and light levels low. Interestingly, you can plant sprouted onions back in the garden during spring. They’ll grow, but you’re better off buying fresh sets for consistent results. Some gardeners deliberately plant stored onions to produce seed, though this is quite advanced and the resulting seed may not grow true to type with hybrid varieties.

Do onions need full sun or will they grow in partial shade?

Onions absolutely need full sun to develop properly sized bulbs. They’ll survive in partial shade and produce foliage, but bulb development will be severely compromised. I’ve compared onions grown in a bed receiving morning sun only against those in full-day sun, and the difference was stark. The partially shaded plants produced bulbs perhaps half the size despite identical soil and care. Onions are day-length sensitive plants that respond to both the duration and intensity of light. Six hours of direct sunlight is the absolute minimum, and eight-plus hours is ideal for British conditions. If your garden is heavily shaded, onions simply aren’t the best crop choice. Focus on shade-tolerant vegetables like lettuce, spinach, or some herbs instead. However, if you have one sunny spot, onions are perfect for it because they don’t shade out other plants with their upright, minimal foliage.

Conclusion

Growing onions successfully comes down to understanding their fundamental needs: free-draining soil, full sun, the right variety for your planting time, and proper curing after harvest. I’ve grown onions in heavy Oxfordshire clay (with amendments), sandy Suffolk soil, and various containers across different sites. The principles remain consistent, though you’ll need to adapt your approach to your specific conditions.

Start with spring-planted sets if you’re new to onion growing. They’re forgiving, reliable, and will build your confidence. Once you’ve succeeded with these, experiment with autumn sets for earlier harvests or try seed-grown varieties for specific cultivars you can’t get as sets. Keep records of what works in your particular garden. Some varieties thrive in certain regions whilst struggling in others, and your own observations are worth more than general advice.

The satisfaction of lifting your own onions, properly cured and stored, then using them through winter is genuinely rewarding. You’ll taste the difference, save money, and gain valuable experience with a staple crop that fits into almost any vegetable growing scheme. Don’t be discouraged by setbacks like the occasional bolted plant or smaller-than-expected bulbs. Every season teaches you something, and onions are remarkably consistent performers once you understand what they need.

Author: Megan Walker

Megan focuses on seasonal food, kitchen garden growing, and how households can reconnect with where their food comes from. Her writing blends practical growing advice with ideas for cooking and eating in season. With a passion for fresh ingredients and sustainable living, Megan’s articles help readers make the most of local produce while supporting British farms.