Hydrangea Flower Seeds Pack 100 SeedsHow to germinate? Fill a container with potting soil and add some peat moss to help retain moisture, then sprinkle the seeds on top of the soil and gently cover them with more potting soil. Keep the seeds moist for 10 14 days by keeping them under a clear plastic dome or plastic bag so they don't dry out, but you need to make sure that there is some room for air circulation so that mold doesn't grow either. Once they germinate, remove the dome or bag