Allow eight to 12 weeks after the bloom date for the fertile flower's seeds to mature. Watch for the hydrangea flower cluster to begin to fade and dry, and then remove it with a garden clipper. Use kitchen scissors to clip off the large, showy blossoms from the flower cluster. Store the small, remaining flowers in a paper bag for up to a week to dry. Shake the paper bag and pour the contents onto a plate. Use a sieve shaker if necessary to extract and clean the seeds.