Early Nantes, Nantes or Nantes 2 all have a beautiful flavour and I usually plant them under mesh to prevent the carrot fly leaving its eggs for its young to have a feast. Iím all for companion planting but I think trying to put it off by smells is a bit hit and miss compared to covering.
However, covering inhibits other friendly insects too, a bit like antibiotics killing all bacteria, whether good or bad. So one year, on the recommendation of a normally reliable source I erected a barrier about 2 foot six inches tall, as shown in this picture. The otherwise stupendous crop was decimated by carrot fly. It was heartbreaking.
Complete coverage works but the foxes do like trampling the netting I think they flit across it like Crouching Tiger Hidden Dragon.Because of the foxes I don't eat the carrots raw and always scrape them before cooking. I wonder if a substantial wooden structure with meshed sides and top would deter them. Once they get interested you have generally had it. One chewed through a wire fence which was only about eight feet long: it could easily have gone round it. However I gained some satisfaction one time when an enormous bulge in a similar structure indicated that a fox had probably hit it midair at full tilt.
If you are interested in finding out more about the carrot fly follow this interesting link:
Carrot Fly Conference
Carrots: Planting Guide
Late Feb / Early March
when ready, the tops are usually feelable, so I gauge when they are big enough.