Carrots and other similar root crops are almost always seeded too closely together. It is important to thin carrots, they will not grow to maturity unless they have space to do so.
Thinning carrots is when you remove some of the baby plants in a row to free up space for the rest so that they can grow to full size. Ideally, thin carrots twice during the course of the season.
To thin, hold the carrot plant between your forefinger and thumb at the soil level. Pull firmly to dislodge the entire plant from the soil by the root. Mound more soil around the roots of the leftover plants. Carrots will discolor if the top of their root is exposed to sunlight.
Begin your first thinning when the plants are 4 inches tall. First look for the smallest and scrawniest plants to remove from the garden, or those that are growing right on top of one another. Thin the plants to about a thumb’s-width apart. Thin the carrots for a second time about a month later to about 1 1/2 to 2 inches apart.
During this second thinning, the carrots which are removed may be large enough to eat as a snack. Baby carrots are a delicious addition to salads and are also sweet and tender when roasted.
Carrots may be transplanted if there is an area of your garden where germination was spotty. Simply move the thinned carrot plants and plant them a thumb’s-width apart. Be sure to plant them very straight or you will end up with crazy crooked carrots! Mound soil around their roots, being careful to keep the tap root straight, and water in.
Source: CCF staff