The easiest way to propagate a dianthus plant is through stem cuttings in the spring or summer. Cut a length of stem with approximately five sets of leaves just below a node. Dip the stems in rooting hormone, and place them in small holes in moist potting soil. Cover the stems with a plastic bag, creating a humid environment, and set the pot where it will receive bright, indirect sunlight. Once new growth occurs, you can transfer your plants to a more permanent home.