Onions are one of the most reliable and useful crops you can grow on an allotment. They take up relatively little space, store well, and form the backbone of countless meals.
They’re also often misunderstood. Many problems with onions — bolting, small bulbs, rot in storage — come down to timing, spacing, or choosing the wrong growing method.
This guide covers how to grow onions successfully on an allotment, whether you’re starting from seed or using sets, and how to avoid the most common mistakes.
Choosing How to Grow Onions: Seed vs Sets
Before planting anything, it’s worth deciding how you want to grow your onions. Both methods work well, but they suit different situations.
Growing Onions from Seed
Growing onions from seed gives you the widest choice of varieties and is usually cheaper. Seed-grown onions are also less prone to bolting.
However, they take longer and need a bit more patience early on. They’re best suited to growers who are happy to start things under cover in winter or early spring.
Seeds are typically sown from January to March, depending on conditions and available light.
Growing Onions from Sets
Onion sets are small immature bulbs that are planted directly into the soil. They’re quicker, simpler, and ideal for beginners.
The trade-off is variety choice and a slightly higher risk of bolting, particularly if sets are planted too early or experience cold stress.
Sets are usually planted from March onwards, with overwintering sets planted in autumn.
When to Plant Onions on an Allotment
Timing is critical with onions.
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From seed: January to March under cover, or March to April outdoors if conditions are good
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From sets: March to April for spring planting, or September to October for overwintering varieties
Planting too early into cold, wet soil is one of the main causes of failure. Onions prefer steady conditions rather than rushing.
Where to Grow Onions
Onions need:
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full sun
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free-draining soil
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minimal competition
Heavy, waterlogged soil increases the risk of rot. If your plot is prone to sitting wet, raised beds or lighter areas of the allotment are a better choice.
Avoid growing onions where other alliums have recently been grown. Rotation helps reduce disease and improves overall crop health.
How to Plant Onions
Planting Onion Sets
Plant sets with the pointed tip just showing above the soil surface. They should be firm and upright, not buried deeply.
Space sets around 10–15cm apart, with rows roughly 25–30cm apart. Giving onions enough space is key to producing decent-sized bulbs.
Firm the soil gently around each set to stop birds pulling them out.
Planting Seed-Grown Onions
Seedlings should be planted out once they’re pencil-thick and conditions are settled.
Handle them gently, plant at the same depth they were growing before, and water in well. Avoid crowding — onions hate competition.
Looking After Onions as They Grow
Onions are low-maintenance once established, but they don’t like being ignored completely.
Keep beds weed-free, especially early on. Weeds compete directly with onions and quickly reduce bulb size.
Water during dry spells, particularly while bulbs are swelling. Inconsistent watering can lead to split bulbs or poor growth.
Feeding is rarely necessary if soil is reasonably fertile. Overfeeding often produces lush leaves at the expense of bulbs.
Common Onion Problems (and How to Avoid Them)
Bolting
Bolting happens when onions think they’ve experienced a winter. This is most common with sets planted too early or stressed by cold.
Plant at the right time and avoid very small or very large sets to reduce risk.
Small Bulbs
Usually caused by overcrowding, poor soil, or heavy weed competition. Space properly and keep beds clean.
Rot in Storage
This is often down to harvesting too early or storing onions before they’re fully dry. Good curing is essential.
Harvesting and Storing Onions
Onions are ready to harvest when the foliage starts to yellow and flop over naturally.
Lift them on a dry day and leave them to dry in the sun or a well-ventilated space for a couple of weeks. The skins should feel papery and dry.
Once cured, store onions somewhere cool, dry, and airy. Properly grown onions will keep for months.
How Onions Fit Into an Allotment Plan
Onions are ideal for early planting and early clearing. Beds can be reused later in the season for salads, spinach, or autumn crops.
They’re also a good crop for beginners because success is visible and problems are easy to spot early.
Grown well, onions reward patience far more than effort.
Final Thoughts
Onions aren’t difficult, but they are particular.
Get the timing right, give them space, and resist the urge to overdo it. Whether you grow from seed or sets, a small, well-managed patch of onions will outperform a crowded bed every time.