Cold stratification is a process used to break the dormancy of dogwood tree seeds and promote germination. It involves subjecting the seeds to a period of cold, moist conditions that simulate winter. This process is necessary because dogwood seeds have a natural dormancy mechanism that prevents them from germinating immediately after being shed from the tree. Cold stratification can be achieved by soaking the seeds in water for 24 hours, mixing them with moist sand, and storing them in a plastic bag in the refrigerator for about 90 days. This mimics the natural winter conditions that the seeds would experience in the ground, allowing them to break dormancy and germinate when planted.