"Transplanting carrots doesn't work". Everybody says not to transplant carrots.
But what do you do when you work full-time, the days are in the high 90s, and the humidity is less than 10%? Transplanting may be the only way to get a crop, because otherwise you'd have to be home to mist your carrot seeds 3-4 times a day for 1-3 weeks in order to get decent germination.
The difficulty with transplanting carrots is that as soon as the seed germinates, it sends down an incredibly long - for its size - taproot. By the time the cotyledons (the long “seed leaves” that first emerge) are big enough to use as a handle for transplanting, the roots are already 2-3 inches long, and it's very difficult to handle these delicate roots.
So this year I have tried a couple of ways to start a midsummer carrot crop in my hot dry climate. Only one of them worked (the other yielded badly forked carrots, so I won't share.)
I have a basement seed starting operation that I use for growing my own starts in the spring. It consists of some sturdy wire shelving from Costco with cool-white fluorescent shop lights hanging from the shelves, illuminating the shelf below. The lights hang on little chains so I can adjust their height.
Until this year, I had never used them for carrots.
Start carrots in seed starting trays indoors, in good quality, finely textured and densely packed potting soil. Use seed starting trays that are at least 2” deep. Sow 2 seeds per cell. When the cotyledons emerge and get to be about ¾” long, carefully push the soil plug out of the tray and transplant the whole plug into the ground. If both carrots have germinated and survive transplanting, thin to one carrot. Or not.
It’s important that the soil in the cell stays packed together as you transplant, or the roots will be damaged. If this happens, your adult carrots will be forked, sometimes into 4 or 5 small stunted fingers resembling a hand.