Having waited for carrots seeds to germinate, however thinly you tried to sow them, they are bound to be too close together to give them room to develop into useable (or even prizewinning) roots. If you leave them most will remain very small and only a few will get to any size - and they will be twisted and forked as they fight for survival amongst the crowd.
Carrots have a strong smell when disturbed or crushed and it is said that the carrot fly can smell them from up to a mile away. When they lay their eggs next to the young carrot plant, the larva hatch out and feast on your carrots leaving tunnels just under the surface and making them uneatable.
Any thinning is therefore best left until the evening as the carrot fly tends to travel only in the heat of the day.
As the wind also disturbs the carrot leaves, preventative measures are recommended.
- Growing onions or any other highly scented crop either side of your carrots can disguise the small.
- Carrot flies tend to fly close to the ground so a barrier about 45cm. (18") high surrounding the carrot crop can stop them. Enviromesh or fleece is better than a solid barrier as the wind eddying round solid objects can blow the fly over into the crop.
- On the other hand you can grow carrots inside an Enviromesh or fleece enclosure or tunnel. That way the crop is completely protected. However, don't forget to weed them - and thin them in the evening!
Finally you could try growing a Carrot Fly resistant type - but you may find the taste is not as good as the traditional carrots.
- 'Grow carrots in containers or stacked tyres over 30" above the ground to prevent carrot fly.'
- 'When thinning carrots, water straight away to stop carrot fly.'