Spring onions are one of the easiest crops to fit into a UK garden or allotment. They grow quickly, take up very little space and work well in raised beds, containers or gaps between slower-growing crops.
The main difficulty is getting them started well. Cold, wet soil, old seed, overcrowding and dry compost can all lead to patchy germination or weak seedlings. Get those early conditions right and they need very little attention.
Below, I’ll cover when to sow spring onions, the easiest sowing methods, spacing, container growing, basic care and when to harvest.
When to Sow Spring Onions in the UK
You can sow spring onions outdoors in the UK from March to August, but the calendar is only half the story. Soil conditions matter just as much as the month.
If the ground is still cold, claggy or sitting wet after rain, germination can be slow and patchy. I’d rather sow a week or two later into better soil than waste a row in cold mud.
For an earlier crop, start spring onions under cover from February or March in modules or trays, then plant them out in small clumps once the seedlings are sturdy and conditions improve.
For direct sowing outdoors, wait until the soil is:
- Workable
- Starting to warm
- Not sitting wet after rain
- Clear enough that weeds won’t swamp the seedlings
Sow small batches every 2–3 weeks for a steady supply rather than sowing the whole packet at once.
| Sowing time | Best method | What to expect |
|---|---|---|
| February–March | Sow under cover in modules or trays | Useful for early crops and cold springs |
| March–August | Direct sow outdoors or transplant modules | Main UK sowing window |
| Every 2–3 weeks | Successional sowing | Gives a steady supply rather than one glut |
| Late summer–early autumn | Sow hardy varieties | Can provide an overwintered crop for spring |
The main mistake is sowing too early outdoors and expecting quick results. Spring onions are fairly tough once established, but germination can be unreliable in cold, wet soil. If spring weather is miserable, start them in modules or wait a little longer — a later sowing in better conditions can easily catch up.
Where to Grow Spring Onions
Spring onions grow best in a sunny, open spot with free-draining soil. They do not need a heavily fed bed, but they appreciate loose soil improved with a little compost.
The main thing to avoid is cold, compacted or waterlogged ground. Their fine roots do not cope well with sitting wet for long, so on heavy clay you will usually get better results in:
- Raised beds
- Containers or troughs
- Improved allotment rows
- Better-drained patches of the garden
Spring onions are particularly useful in small spaces. They can be grown along bed edges, in containers or in gaps between slower-growing crops, provided they still get enough light.
Full sun gives the strongest growth, although they will tolerate some light shade, particularly in summer. Avoid crowded spots where weeds or larger crops can shade them out — spring onions are easily outcompeted while young.
Give them a clean, reasonably well-drained patch and keep on top of weeds, and they usually need very little attention once established.
How to Sow Spring Onions
Spring onions are usually grown from seed, and there are two good ways to start them:
- Direct sow outdoors where they are going to grow
- Start them in modules and plant them out as small clumps
Direct sowing is the simplest method once the soil has warmed. Modules are useful in a cold, wet spring, or where the ground is heavy, weedy or prone to slug damage.
Direct Sowing
Rake the soil level, remove any weeds and make a shallow drill about 1cm deep. If the soil is dry, water the drill before sowing.
Then:
- Sow the seed thinly along the row.
- Cover lightly with soil or compost.
- Water gently.
- Keep the row evenly moist while the seed germinates.
- Thin the seedlings if they come up too crowded.
Avoid sowing the whole packet in one thick line. Overcrowded seedlings quickly become thin and weak, and you can always sow another short row a few weeks later.
It is also worth using fresh seed. Onion seed can become less reliable as it ages, so an old packet may be partly responsible for poor germination, particularly during cooler weather.
Sowing Spring Onions in Modules
Modules give spring onions a more reliable start when outdoor conditions are poor.
Sow around 4–6 seeds per module, cover lightly and keep the compost moist. Allow the seedlings to grow together as a small clump rather than separating them individually.
Once they are sturdy and outdoor conditions have improved, plant the whole clump into the bed or container.
Direct sowing is usually easiest in good conditions, but if cold soil, weeds or slugs repeatedly cause problems, starting spring onions in modules is a simple alternative.
Spring Onion Spacing
Spring onions do not need much room, but there is a difference between close spacing and overcrowding. If seedlings are packed too tightly, they tend to stay thin, floppy and slow to bulk up.
For direct-sown spring onions, thin seedlings to around 2–3cm apart, with rows roughly 15–20cm apart. Leave a little more room if you want thicker stems, or grow them closer and harvest young for slimmer salad onions.
If you start spring onions in modules, plant them out as small clumps rather than separating every seedling.
| Growing method | Spacing guide | Best use |
|---|---|---|
| Direct-sown rows | Thin to 2–3cm apart | Simple rows in beds or allotments |
| Row spacing | Around 15–20cm apart | Allows easier weeding and airflow |
| Module clumps | 4–6 plants per clump | Reliable planting from trays |
| Clump spacing | Around 10–12cm apart | Small bunches for easy harvesting |
| Square foot growing | Around 16 single plants or 4 small clumps | Raised beds and small spaces |
Spring onions are also useful as gap fillers between slower-growing crops, provided they still get enough light and are not crowded out.
Growing Spring Onions in Pots
Spring onions are well suited to containers because they do not need deep soil or much root space. A trough, window box, large pot or shallow grow bag will all work.
A container around 15–20cm deep is usually enough. Long troughs are often more useful than deep pots because they give you more growing area without wasting compost.
Use a peat-free multi-purpose compost and make sure the container has good drainage. Spring onions like steady moisture but do not want to sit in cold, waterlogged compost.
You can either:
- Sow directly into shallow rows, covering the seed lightly and keeping the compost moist while it germinates.
- Plant module-grown clumps around 10–12cm apart for a more reliable start.
Containers are particularly useful near the kitchen door, where you can harvest spring onions little and often as you need them.
You can also use spare space in grow bags, but avoid planting spring onions where larger crops will quickly shade them out. Tomatoes, courgettes and other vigorous plants can easily overwhelm such a small crop once their foliage fills out.
The main thing to watch is watering. Containers dry out faster than open ground, and consistently dry compost can leave spring onions thin and slow-growing. Keep the compost evenly moist, harvest them young and sow another small batch if you want a steady supply through the season.
Watering, Feeding and Care
Spring onions need very little attention once established. The two jobs that matter most are steady watering and keeping weeds down.
Watering
Keep the soil or compost evenly moist, especially while seeds are germinating and young plants are establishing.
In open ground, water during dry spells rather than little and often. Containers need checking more regularly because compost dries out much faster.
Aim for moist soil, not soggy conditions. Spring onions dislike sitting in cold, waterlogged ground.
Feeding
Spring onions are not hungry plants. If the soil has been improved with compost, they usually need little or no extra feeding.
Container-grown plants in tired compost may benefit from a light feed, but good moisture, enough light and low competition matter far more than regular fertiliser.
Weeding
Keep the area around spring onions clear, particularly while the plants are young. Their narrow leaves and fine roots are easily outcompeted by weeds for light, moisture and space.
A light mulch can help retain moisture and suppress weeds once the plants are established, but avoid covering small seedlings.
Common Spring Onion Problems
Spring onions are usually straightforward once established. Most problems happen while they are young and come down to cold soil, old seed, overcrowding, inconsistent moisture, weeds or early pest damage.
| Problem | Most likely cause | Quick fix |
|---|---|---|
| Poor germination | Cold soil, old seed or sowing too deep | Use fresh seed and sow about 1cm deep |
| Leggy seedlings | Too little light indoors | Move to brighter, cooler conditions |
| Thin growth | Overcrowding, weeds, shade or dry spells | Thin, weed and water consistently |
| Slug damage | Damp weather and tender seedlings | Protect young plants early |
| Bolting | Stress, drought, cold or plants left too long | Keep growth steady and harvest young |
| Rust or mildew | Damp, crowded growth | Improve spacing and airflow |
Poor Germination
Patchy germination is probably the most common spring onion problem. Typical causes include cold or waterlogged soil, ageing seed, sowing too deeply and allowing the seedbed to dry out.
For more reliable germination:
- Use fresh seed.
- Sow around 1cm deep.
- Keep the soil or compost evenly moist.
- Avoid cold, waterlogged ground.
- Start a batch in modules if direct sowing repeatedly fails.
Seedlings started indoors can also become pale and floppy if they do not get enough light. Move them into brighter, cooler conditions once they germinate and harden them off before planting outside.
Slugs and Snails
Slugs and snails are most troublesome while spring onions are small. If seedlings disappear overnight, check for slug trails, chewed plants and hiding places around pots, trays and weedy bed edges.
Keep the area around young plants reasonably clear, raise trays off the ground where possible and protect early sowings if slugs are active. Once spring onions are established, they are usually less vulnerable.
Bolting
Bolting is when a spring onion starts producing a flower rather than staying tender and leafy. It is usually associated with stress such as cold, drought or being left to grow for too long.
Keep plants growing steadily and harvest them while young. Successional sowing also helps because you always have another younger batch coming through.
For overwintering, use a hardy variety rather than expecting a summer variety to cope with unsuitable conditions.
Onion Pests and Diseases
Onion fly and allium leaf miner can both affect spring onions and other alliums. Where these pests are already a problem, fine insect mesh is one of the most practical ways to protect young crops. Remove badly affected plants and avoid leaving infested allium material around the bed.
Rust and mildew are more likely in damp, crowded growth. Avoid overcrowding, keep weeds down and give plants enough space and airflow.
White rot is more serious. It can affect onions, garlic, leeks, shallots and spring onions, and can persist in contaminated soil for a long time. Affected plants may yellow, wilt and rot around the roots or base.
If you suspect white rot:
- Avoid growing alliums in the affected patch.
- Take care not to move contaminated soil elsewhere.
- Clean tools and boots after working in the area.
- Do not compost badly affected plants.
For most spring onion growers, the main priorities are much simpler: use fresh seed, sow shallowly, avoid overcrowding, keep seedlings evenly moist and protect young plants from slugs.
When and How to Harvest Spring Onions
Spring onions are usually ready to harvest around 8–12 weeks after sowing, although cold weather and slower early growth can push this back.
Rather than relying on an exact date, harvest them when they are a useful size in the kitchen. Look for:
- Stems around pencil thickness
- A firm white base
- Healthy green tops
- A size you are happy to use
Homegrown spring onions do not need to reach supermarket size. They are often at their best when harvested a little younger, while the stems are still tender and the flavour is mild.
How to Harvest Spring Onions
Grip the plant low down near the base and pull gently from the soil. If the ground is dry or firm, water first or loosen it with a hand fork to avoid snapping the stems.
With module-grown clumps, you can either pull the whole clump or take the largest stems first and leave the smaller ones to grow on.
Spring onions are best harvested young rather than left in the ground to bulk up like storage onions. If they are left too long, the stems can become tougher and plants may start to flower.
For a steady supply, sow small batches every 2–3 weeks and harvest the biggest spring onions first.
Use them fresh where possible. They will keep in the fridge for a few days, but spring onions are much better treated as a quick, regularly sown crop than something to store long term.
Companion Planting for Spring Onions
Spring onions are useful companion plants mainly because they are small, upright and quick to harvest. They fit easily into gaps between slower-growing crops without taking up much room.
They work particularly well along bed edges, in raised beds and between young crops before larger plants fill the space.
The main thing is to avoid crowding them out. Spring onions still need enough light, moisture and airflow to grow well, so treat companion planting as a way to use spare space rather than relying on particular crop combinations.
Spring Onion FAQs
Yes, although they need plenty of light once they germinate. Spring onions can be started on a bright windowsill or under cover, but seedlings grown somewhere too warm and dark can quickly become long and floppy. Move them to bright, cooler conditions as soon as possible.
Yes. If the white base still has roots attached, place it in water or compost and it will usually produce another flush of green growth. It is useful for a quick regrow, although sowing fresh seed is more reliable if you want a proper crop.
Yes. Stand the rooted base in a small jar with enough water to cover the roots and keep it somewhere bright. Change the water regularly. You should get some fresh green growth, but for stronger long-term growth, move the rooted base into compost.
Yes. Spring onions can still be sown in late summer, and hardy varieties can be sown into late summer or early autumn for overwintering. Standard summer varieties are less reliable as temperatures and light levels fall.
Yes, but choose a hardy variety intended for autumn sowing. Sow in late summer or early autumn so the plants have time to establish before winter. In colder gardens, a cloche, fleece or sheltered position can give them extra protection.
Most spring onions take around 8–12 weeks from sowing to harvest, although this varies with the weather and time of year. Warm summer sowings can develop faster, while early crops may take longer in cold soil and lower light.
In the UK, spring onion and salad onion are usually used interchangeably. Some varieties form a slightly more swollen base, while bunching onions tend to remain straighter and form long white stems, but they are generally grown and used in much the same way.
Because spring onions are harvested relatively quickly, you can often follow them with another short-season crop such as lettuce, radish, rocket, spinach, beetroot or carrots, depending on the time of year.
If you have had problems with allium pests or diseases, avoid immediately following them with onions, garlic, leeks or shallots in the same patch.
