🌱✗ Tested and false“Onions need light, sandy soil — you can't grow them on clay”
Onions grow across a wide range of soils, and sand is not mentioned as a requirement. The real need is for well-drained soil that has been manured the previous season rather than the current one. Heavy soil that drains poorly is a problem for many crops, not onions specifically.
What to do: Improve drainage with organic matter rather than trying to convert clay to sand. Manure the bed the season before planting, not the spring you put the sets in. Raised beds on heavy ground solve the drainage problem more cheaply than carting in sand.
In Scotland: Central Belt clay grows perfectly good onions if it is well drained. The key is winter preparation rather than soil type.
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- RHSFertile, well-drained soil; manure the previous autumn
“Onion sets always do better than growing from seed”
The word is always, and it is false. Seed-grown onions can produce larger bulbs with better storage life, and sets are more prone to bolting. Sets give a head start and are simpler to plant, but simplicity is not the same as growing better.
Our read Sets win on ease, not quality. The belief that they are categorically superior survives because most printed advice is aimed at beginners who want a result with the least effort, and sets reliably deliver that. The claim that they always do better is what turns serviceable advice into a myth.
What to do: Use sets for speed and simplicity, particularly on heavy soil where spring sowing is harder. Sow seed indoors in January or February for the biggest bulbs with the longest storage.
In Scotland: Autumn-planted sets overwintering in wet Scottish soil are a bolting risk. Spring-planted sets are the safer bet here.
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- RHSSets are more prone to bolting; seed-sown onions can produce larger bulbs
“Planting onions among your carrots keeps carrot fly away”
The theory is that onion scent masks the carrot smell guiding carrot fly to your row. Garden Organic tested it and got inconsistent results across trials. RHS repeats the claim in its companion planting feature, but its own carrot growing guide never mentions onions: there the advice is to cover the crop.
Our read When the body making a claim quietly leaves it out of its own growing guide for that crop, that tells you how much weight it really carries.
What to do: Cover the crop when you need protection. Sowing after mid-May dodges the first generation of carrot fly and lifting before late August dodges the second, and both have evidence behind them where the onion does not.
In Scotland: Carrot fly is patchy across Scotland, so in some gardens you will get away with no protection at all.
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- RHSRepeats the claim as traditional
- RHS carrot guideAdvises covering the crop; onions unmentioned
“Onions are ready when the tops fall over”
Lifting onions when the foliage topples over and turns yellow, typically in late summer, is the advice. The falling-over is a genuine indicator rather than folklore. After lifting, the onions need drying in the sun or under cover for storage.
What to do: Lift when the tops have yellowed and fallen, and lay them out to dry in the sun for a week or two, or under cover if the weather is wet. Store only undamaged, fully dried bulbs.
In Scotland: The drying part is harder here. A wet August means lifting the onions and bringing them indoors to dry, which takes longer but works. Do not leave lifted onions on wet soil.
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- RHSLift when foliage topples over and turns yellow; dry thoroughly for storage
“Onion sets are more likely to bolt than onions grown from seed”
This is a genuine trade-off, distinct from the general 'sets are easier' question. Sets do carry a higher bolting risk than seed-raised plants, though heat-treated sets close most of that gap.
What to do: Buy heat-treated sets to cut the bolting risk, or grow from seed if bolting has been a recurring problem on your plot. Snap off any flower stems as soon as they appear so the plant redirects energy to the bulb.
1 source
- RHS — How to grow onionsHowever, they are more prone to bolting (when a flower is produced, diverting energy away from the bulb). Using heat-treated sets greatly reduces the risk of bolting.
