Tomatoes and other fast-growing plants may outgrow their pots before it’s time to move them into the garden. About a month after sowing or when seedlings are about 4’’ tall, gently remove one of the seedlings from its pot. If the roots are beginning to fill the space, then it’s time to transplant them into a larger pot. Avoid waiting too long, because root crowding can stunt plant growth and make it difficult for plants to recover after transplanting.