Carrots (Daucus carota) grow readily from seed directly sown in a sunny garden bed. It's difficult to properly space the small seeds, which aren't much bigger than a grain of sand, when planting. Thinning the seedlings after they emerge gives the remaining plants room to form their large, flavorful roots. Carrots seedlings need thinning after they emerge and produce their second set of leaves, which is usually between two and four weeks after you sow them in spring.
Water the carrot bed before thinning until the top 6 inches of soil is evenly moist. The moisture makes it easier to remove the unwanted seedlings without disturbing the remaining carrot sprouts.
Measure the distance between the seedlings. Carrots require about 3 inches between plants in the row, so denser rows will require thinning to this required distance.
Grasp the unwanted seedling near its base and pull it straight up and out of the soil. If the carrots are spaced extremely close together, cut through the stem of the unwanted seedlings with a small pair of shears at soil level. Cutting off the top kills the root but doesn't disturb any seedlings growing near the thinned plant.
Apply one-fourth of a cup of 21-0-0 fertilizer to every 10-foot row about six weeks after the seeds germinate, or about four weeks after thinning. Sprinkle the fertilizer on top of the soil 6 inches away from the seedlings and water immediately after application so the nutrients soak into the soil.