Thin seedlings as soon as they are large enough to prick out with your fingers, tweezers, knife or a fork. This means for large seeds like peas and beans you would thin as soon as you see the green sprouts and perhaps the seedlings would be able to be transplanted. For tiny seedlings like carrots and beets you may need to wait a few weeks longer so you can grasp the stem and uproot it without disturbing the seedlings you want to keep. it is important to have room around each seedling for optimum growth. The seed pack will often tell you how many inches apart each seedling should be from it's neighbor. Don't forget you can add the seedlings of many crops to your salads. Keep planting those seeds, Marianne Binetti