Hi Ali,
Much of what is said about these plant combinations is based on myth and legend with little or no scientific evidence to back it up.
Don’t get me wrong though, I do believe that growing certain flowers/veg close to specific crops does benefit those crops. Certain flowers attract specific predatory insects/pollinators which are of course of benefit to us.
On the specific question of the carrot fly you will often hear it suggested that growing Onions and Carrots together confuses the Carrot Fly. Unfortunately you need to grow a huge amount of Onions for the scent of the Onions to overwhelm the scent of the Carrots.
The surest way to deal with the Carrot Fly is to sow bearing in mind the Fly’s life cycle. Sowing in late May or early June avoids the first generation. The second generation hatches circa September and the surest way to deal with that is to cover the crop with fleece.
You will see it suggested that you should surround the crop with a 2′ high barrier of fleece or similar insect proof material. The theory behind that being that the Carrot fly cant fly that high. That may be the case but what happens if it gets blown higher on a windy day?
This year I’m turning over one of my raised beds in the middle of my plot to growing cut flowers, following Monty Don’s example, though I will be concentrating more on insect attracting varities more than purely decorative varieties. I think it will be interesting to see how many varieties of insect it attracts.
Don.