Onions are a staple crop on UK allotments. They’re reliable, don’t take up much space, and store well. But once they’re harvested, many growers aren’t sure what should follow them — and it’s easy to make the soil work harder than it needs to.
What you plant after onions has a direct impact on soil health, pest pressure, and how well the bed performs the following season. Onions behave very differently in the soil to crops like potatoes or beans, so they need a different approach.
This guide explains what to plant after onions on a UK allotment, which crops work best, which to avoid, and how onions fit into a sensible crop rotation.
How onions affect the soil
Onions are relatively light feeders compared to many allotment crops, but they still influence the soil in a few important ways.
They:
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have shallow root systems
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don’t add organic matter back into the soil
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leave ground fairly firm and undisturbed
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are prone to disease build-up if repeated
By the time onions are lifted, the soil is usually tidy, workable, and not heavily depleted — but it hasn’t been improved either.
That makes onions a good preceding crop, as long as what follows makes sensible use of the conditions left behind.
The best crops to plant after onions
The most successful crops after onions are those that either:
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benefit from firm soil, or
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help rebuild structure and fertility
Brassicas are an excellent follow-on crop
Brassicas are one of the best choices to plant after onions.
Because onions don’t disturb the soil much, beds are often firm and well suited to brassicas, which prefer ground that hasn’t been freshly dug or overworked.
Good options include:
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cabbage
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kale
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broccoli
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cauliflower
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Brussels sprouts
Adding compost or well-rotted manure before planting brassicas helps compensate for the nutrients onions didn’t leave behind.
Potatoes also work well after onions
Potatoes are another strong option after onions, particularly in a rotation system.
Onions leave soil in good condition for potatoes to:
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break up structure
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suppress weeds
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reset the bed
This pairing works well on allotments where beds need occasional renovation without excessive digging.
Fast crops for the same season
If onions are harvested in summer, there’s often time to grow another crop before winter.
Quick-growing crops that suit onion-followed beds include:
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lettuce
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spinach
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radishes
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turnips
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salad leaves
These crops don’t place heavy demands on the soil and make good use of cleared space.
Crops to avoid planting straight after onions
While onions are fairly forgiving, repeating similar crops can cause long-term problems.
Avoid alliums as a follow-on
Onions belong to the allium family, along with leeks, garlic, and shallots.
Planting another allium straight after onions increases the risk of:
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soil-borne diseases
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pests such as onion fly
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declining yields over time
It’s best to leave at least two to three years before returning alliums to the same bed.
Be cautious with legumes
Legumes can grow after onions, but they’re often better placed after heavier feeders like potatoes.
If space is limited they can still work, but they’re not the most efficient pairing in a rotation system.
How onions fit into crop rotation
In most allotment rotation systems, onions are grouped with root crops.
A common and effective sequence is:
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Potatoes
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Brassicas
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Roots (including onions)
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Legumes
In this setup, onions benefit from following brassicas and are then followed by crops that either rebuild soil or reset structure.
If you’re running a simpler 3-year rotation, onions usually sit in the lighter-feeding group rather than alongside potatoes.
What if space is tight?
On smaller allotments, rotation isn’t always neat.
If space is limited:
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focus on avoiding repeating alliums in the same bed
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prioritise rotating crop families, not individual crops
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use fast crops to keep beds productive without stressing soil
Even imperfect rotation is better than none.
Common mistakes after harvesting onions
Most problems come from treating onions as neutral crops that don’t “count” in rotation.
Common mistakes include:
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planting garlic or leeks straight after onions
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assuming shallow roots mean no soil impact
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forgetting where onions were grown the previous year
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filling beds reactively without planning
These mistakes tend to show their effects over multiple seasons rather than immediately.
Planning what comes after onions
Because onions don’t dramatically change the soil, it’s easy to underestimate their role in rotation.
Keeping track of:
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where onions were grown
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which crops follow them
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how beds cycle year to year
makes long-term planning far easier and avoids accidental repetition.
Visual planning is especially useful once multiple beds and overlapping crops are involved.
Final thoughts
Onions may be modest crops above ground, but they still shape how an allotment performs over time.
Follow onions with brassicas or potatoes, avoid repeating alliums, and use fast crops to keep beds working between seasons. With a bit of planning, onion beds can stay productive without becoming problem areas.