"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.
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.
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.
This way is really tedious, but does work. The only advantage to doing it this way is that you can get 100-200 seedlings per flat instead of only 28 (or however many cells are in your flat). The disadvantage is the seedlings don’t transplant as successfully, and generally yields forked carrots.
Use the same setup as in the way shown above, but broadcast seeds so you have approximately 6-8 seeds per cell. Use potting mix that holds together well when wet, and tamp it down hard into the cells so that it will hold together when you push the soil block up out of the cell. Keep the seeds moist during the whole germination time.
When the carrot tops are about ¾” long, gently push the whole plug of starts out of the cell from underneath, and lay it on its side in a shallow container of water. Gently allow the potting mix to slump into the water, and separate out the carrots from the slurry of potting mix and water.
Lifting only by the leaves (NOT the stem), pick out the carrots seedlings and lay them on a plate with about ⅛” water on it, to keep the little seedlings wet.
In a moist garden bed, make holes spaced about 3” apart with your finger or a dibble. The holes need to be about an inch in diameter, because it is actually very difficulty to get a 2” long dangly, hairlike carrot root to go straight into the hole. It will want to cling to the side. If it clings, you’ll get a twisty weird carrot (but... that might be okay!).
When it’s in the hole to the right depth, lay it so it does stick to one side of the hole, and then backfill the hole with a little more potting mix, and water gently.