Takes a ground freeze to kill a carrot. Other wise they will keep growing until they start to send up a seed head.

Oh ... good news. Did some thinning today. Will give them more time.

Those lovely frosts actually help them to sweeten up a bit too :-)

I know. It is the very reason I plant carrots in July.

I also live in zone 5 WI. My carrots don't grow this late, but they are preserved in the soil until it freezes. I even had some make it thru winter and set seed the next year. It gets hard to harvest very soon unless you cover them with bales of hay to keep the ground from freezing.