To give your seedlings the best chance to grow you need to thin or remove some of the seedlings. This decreases the crowding of roots and allows the nutrients to reach only one plant.
You want to thin out the seedlings after the first true leaves appear. True leaves are not the first set of leaves that grow from the seedling but the second set. The second set of leaves are the true leaves, these leaves are miniature versions of the mature plant's leaves.
Once the first set of true leaves appear you can remove all other seedlings in the pot that surround this plant. To remove the seedlings you want to clip them off at the soil line with a scissors. Do not pull, tug or rip out the unwanted plant. If you pull out the unwanted seedlings you can pull out the plant you wanted to save because the roots are intertwined.
You might be upset to just cut off your precious seedlings but this is the easiest way to assure your seedling growing stronger.
An alternate way to separate seedlings is to remove the bunch of seedlings from the container and carefully pull the seedlings apart. Then you can plant the individual plants in their own containers. This method can result in damaged plants that do not thrive.
If you are new to growing plants from seed you will want to take the easy way by clipping the seedlings off.
Grow little seedlings grow!
