Dianthus plants are easy to propagate from stem cuttings, and this is an inexpensive way to replace older plants that are coming to the end of their lifespan. Propagation works best in spring or summer when plant cells are dividing quickly, so rooting happens more easily. Take several cuttings from healthy fresh stems that have not yet flowered.
You will need a length of stem with around five sets of leaves on it, cutting it from the main plant just below a node where a set of leaves join the stem. Remove all of the lower sets of leaves from your stems, and prepare a container of moist potting soil to root the stems in. Ensure the potting soil is moist, and make several holes in the soil with a pencil. Dip the raw end of the stems in rooting hormone, and then insert them into the premade holes.
They should be spaced apart enough so that the leaves are not touching, and inserted deep enough so that only half of the stem is left poking out from the soil. Cover the stems and container with a plastic bag to create a humid environment and set it in a bright spot away from direct sunlight, such as a windowsill. Maintain moist soil and wait for the stems to root. Once new growth occurs on the visible parts of the stem, this is an indication that roots present and getting stronger, and they can be transplanted to a more permanent home in a larger container or directly into the ground