Carnations (Dianthus spp.) are perennial herbs and possibly the most popular cut flower in existence. Growing carnations in the home garden is easy. Give them at least 5 hours of sun a day and don't over-water them, and you will be rewarded with a garden full of sweet-smelling, colorful blooms. Most gardeners will deadhead carnations (pinch off dead and dying flowers) to encourage more blooms. Because you are planning on harvesting the seeds, resist the temptation to remove all fading flowers, leaving a few for their seeds.
Choose the healthiest and most attractive carnation plant in the garden from which to harvest seeds.
Place a small paper or cloth (not plastic) bag around the flower head and tie the bag with string or thread. This will keep pests from eating the seed and guard against the seeds falling to the soil or being blown by the wind before you have a chance to collect them.
Allow the carnation flowers to die back completely. They should be brown and dry.
Cut the flower head from the carnation plant.
Place newspapers on the work table and roll the flower heads between your hands to remove the tiny, black carnation seeds. Separate leaf and petal litter from the seeds.
Allow the seeds to sit out on the paper, in a single layer, for one to two days to be sure they are completely dry. Do not place them in the sun, but in a shady, dry area, out of the wind.
Store the seeds in a suitable container. These include a paper, foil or waxed envelope or bag or a glass bottle or jar. Do not use plastic containers or bags, as moisture can build up and the seeds may rot or mold.
Mark the storage container with the color and variety of carnation from which you collected the seed and the date of storage.