Onions next to carrots: how much truth is in the myth?
Onions are said to keep carrot fly away — you will find it in many companion planting guides. Why that is only half the story, and what your garden journal has to do with it this October.
Carrots and onions are the classic dream team of companion planting. The scent of one is said to confuse the pests of the other. It is not quite that simple — but the pairing still makes sense when you use it the right way.
The myth of the onion scent
The tradition is old: carrot fly finds its host plant by smell. If you plant onions or leeks in the neighbouring row, their scent is said to mask the carrot smell. And the other way round, carrots are supposed to confuse the onion fly.
What is really behind it
The observation is not made up, but the effect is limited. A scent screen of onions does not protect reliably, especially once the onions are harvested or when the carrots are planted far away. So treat companion planting as one measure among several, not as a guarantee.
The pairing stays good for other reasons: the two crops root at different depths, hardly compete for nutrients and make good use of the space.
What stops carrot fly more reliably
A barrier works best: a fine-mesh crop net over the bed keeps the flies out mechanically. The fly stays close to the ground, so a net or plastic wall of about 80 centimetres in height also helps. Sowing late, after the main flight of the first generation in May, reduces infestation too.
By the way: parsley does not belong next to carrots. It is in the same family, is visited by the same pests and therefore makes a poor neighbour.
Right now in October: note down the damage
As you pull the last carrots, look closely: are the roots riddled with fine, rusty-brown tunnels? Those are the feeding tunnels of the larvae. Write down which bed it was, what grew next to it and whether you used a net.
Notes like these turn a myth into experience that fits your garden. After two or three years you will know which combination really works for you.
Planning for next year
Carrot fly overwinters in the soil. So move your carrots to a different spot next year; a break of at least three to four years in the same place makes sense. You can happily keep the onion rows next to them.
In Rootivo you record your observations in the garden journal, and the companion-planting lines layer in the bed planner shows you right away which neighbours fit when you plan.