Carrots and Onions: The Duo That Outsmarts the Carrot Fly
In late June the carrot fly's second generation takes flight — and right now it decides whether your carrots stay sound. One simple pairing helps more than most people think.
If your carrots end up with brown, corky tunnels inside by late summer, the carrot fly was almost always behind it. Late June kicks off its second — and often more damaging — flight, which makes this the perfect moment to talk about an old gardener’s trick: planting carrots and onions side by side. What sounds like kitchen romance has a very practical reason behind it.
How the carrot fly finds your carrots
The carrot fly is small and easy to miss, but it has a keen sense of smell. It tracks down your carrots by the scent of their foliage — especially when you bruise the leaves while thinning or weeding and the smell drifts into the air. It lays its eggs at the base of the plant, and the larvae then tunnel into the root, leaving behind those telltale brown, corky channels.
Late June brings the second flight, and it’s the sneaky one: its larvae hit exactly the carrots you’re growing for autumn and winter storage. Prevent it now and you save the main harvest.
Why the onion makes a good bodyguard
Onions — along with their relatives leek, garlic and chives — release sulphur-rich scent compounds. These settle like a veil over the carrot’s smell and throw the carrot fly off course: it simply finds fewer carrots to lay its eggs on.
The nice part is that it works both ways. The carrots’ scent, in turn, hides the onions from the onion fly. So the two of them guard each other — no magic, just two smells masking one another.
How to use the duo properly
The best approach is to plant carrots and onions in alternating rows right next to each other — carrots about five centimetres apart within the row, onions in the neighbouring one. That keeps the two scents close together at all times.
Timing matters: the protection only works while the onions are actively growing and giving off their smell. Once you harvest them, the effect fades. So plan a second row of onions or leeks to follow on, keeping the scented shield in place across the whole carrot season.
3 myths about carrot neighbours
Myth 1: ‘Companion planting replaces the netting.’ Not quite. The onion lowers the pressure, but the most reliable protection against the carrot fly is still a fine-mesh insect net (under 1.5 millimetres). Best of all, combine the two.
Myth 2: ‘Carrots get along with every herb.’ Quite the opposite — parsley, dill and celery are poor neighbours. They’re closely related to the carrot, attract the same pests and compete for the same space. Keep them well apart.
Myth 3: ‘One round of onions and you’re done.’ Sadly no: the masking only lasts while the onions are giving off scent. Without a follow-up crop in the bed, the carrots stand alone again in the second half of the season.
Planning the neighbourhood in your bed
You don’t have to memorise which plants get along and which don’t. In Rootivo, the companion-line layer in the bed planner shows you at a glance with green and red lines who pairs with whom: carrot–onion lights up green, carrot–dill red. That way you set up your rows correctly before the first seed even goes into the soil.
Im Lexikon: Karotte