Thinning is the act of picking through a row of recently emerged seedlings and removing some of them to provide room for each plant in the row to mature properly. For instance, carrot seeds are planted in a straight row direct in the garden bed. Once the seedlings emerge, the row must be “thinned” so that each plant is 4-10cm away from its neighbour. For really big roots, you might thin to 10-14cm.