As early as you can see which ones are doing well, or you are inclined to do it. Thinning carrots can be very frustrating, but giving them some space usually is rewarding in the end. I'm never ruthless enough at thinning them out. Try using scissors to cut off the tops so you don't disturb the roots of the ones you save. You can try to plan it so you save enough carrots to pull small carrots inbetween the ones you leave to get big. You can pull them out as wanted, steadily thinning them out all summer, but you want to thin enough so the roots don't touch.

Even then, I've seen people ignore thinning them and pull out perfectly edible Red Cored Chanteney in clumps, if the soil is decent.

You could also take a rake and run it cross ways through the carrots to thin.

A carrot is still a carrot and needs to be a few inches away from its buddies next to it. Those round Parisan type carrots can grow 1.5 to 2 inches in diameter. Thin them when they are still young.

And while I think those round carrots are cute, if I am going to use up valuable garden space for 60+ days then I would rather grow a longer carrot. Older carrot varieties like Danver's Half Long and Red Cored Chantenay do just fine in amended clay soil.

Rodney

I'll second that carrots will still produce somewhat even if you don't thin them, though not ideal. I usually thin them as soon as I see true leaves.

I've tried the pelleted seed to avoid thinning, but the germination rates were terrible, so it defeated the purpose.

Thanks, everybody. I'll let you know what happens.

There is a u tube video about a guy growing carrots and he didn't thin them at all! The carrots were so packed that I was surprised they grew at all; and the biggest surprise was the carrots were a decent size & length, but he had to pull with all his might to get them out.