If we knew where it grew in its native land we could make a more educated guess. When seed develop the the capsules dry and open at the top like little jars. You can carefully pick the whole stem and carry it to where you want more campion to grow. There you can sprinkle the seeds. Do this as soon as they are ripe.
Unwanted plants pull out without any effort.
Notice how the stems are arranged. Draw a simple diagram of one of the plants in the garden. Notice the leaves.