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2024 S.E Marshall & Co Limited
Growing your own onions, shallots and garlic is one of the most reliable and rewarding jobs in the veg garden.
They’re space-efficient, store beautifully and form the backbone of so many meals at home. On our allotment, they’re a non-negotiable crop every year and it’s something Oscar always gets involved with too.
Here’s exactly how I grow mine, step by step, from planting to harvest and storage.
Most people plant onion and shallot sets straight into the ground. And you absolutely can, but I like to give mine a head start in trays inside the greenhouse.
Why I Start Them in Trays
Warmer compost = quicker root development
Less risk of birds pulling sets out
Stronger root systems before planting out
Better spacing control
Fewer losses in cold, wet soil
It also makes the job easier with children. Oscar loves lining them up in trays and checking on their progress.
If you're looking for how to grow onions from sets successfully in the UK climate, this simple tweak makes a big difference.
Fill modular trays with good-quality multi-purpose compost.
Gently press each set into the compost so just the tip is showing.
Water lightly don’t drench them.
Keep them in a cool greenhouse or cold frame. Within a couple of weeks, you’ll see green shoots emerging.
I grow them on in trays until they reach around 15cm (6 inches) tall. By then, they have developed established roots and are much more resilient when planted out.
Once the soil is workable (not frozen or waterlogged), transplant them into well-prepared ground.
Soil Preparation for Onions and Shallots
Free-draining soil is essential.
Dig over the bed thoroughly
Remove weeds
Rake level and firm lightly
Add well-rotted compost
Avoid fresh manure, as it encourages soft growth and poor storage.
Onions benefit from higher phosphorus levels for root development (the “P” in N:P:K), but avoid overfeeding with nitrogen.
Spacing:
Onions: 10–15cm apart
Shallots: slightly wider spacing
Firm the soil gently around each plant and water in well.
One of the most common questions about growing onions from sets is how to stop them bolting.
Bolting (flowering too early) is usually triggered by stress especially cold snaps after growth has started or drought later in the season.
Here’s how I reduce the risk of onions bolting:
1. Start in Trays Stable early growth reduces transplant shock.
2. Avoid Early Planting in Freezing Soil
Cold soil followed by warmth is a common trigger.
3. Don’t Overfeed Nitrogen
Too much leafy growth early on can destabilise the plant.
4. Keep Growth Consistent
Water regularly during dry spells to prevent stress.
If an onion does bolt, remove the flower stem immediately but use that bulb first, as it won’t store well.
When to Harvest Onions and Shallots You’ll know they’re ready when:
The foliage turns yellow
The tops naturally flop over
Don’t bend them over artificially let them collapse on their own.
Lifting and Drying
Lift on a dry day
Shake off excess soil
Lay them out somewhere warm and airy
I dry mine in the greenhouse, on racks in the shed, or outside in sunshine (if the forecast is reliable).
Allow 2–3 weeks for proper drying, until:
Skins are papery
Necks are fully sealed
How to Store Homegrown Onions and Shallots
Once fully dry, you can store them in:
Mesh bags
Onion strings
Slatted crates
Keep them somewhere cool, dry and well ventilated.
Stored correctly, homegrown onions and shallots will last for months well into winter.
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