Growing Hydrangea From Seeds Step 1 Fill a planting flat for seeds two-thirds full with vermiculite or perlite potting soil. Step 2 Water the potting soil to increase the soil moisture. Step 3 Sprinkle four hydrangea seeds in each section of the flat and cover with plastic wrap to maintain soil moisture and humidity. Step 4 Place the planting flat with the hydrangea seeds in a bright location. Hydrangea seeds require ample sunlight to germinate, but you should avoid placing the flat in direct sunlight to maintain a stable temperature. Hydrangea seeds germinate in 10 to 14 days. Step 5 Fill 1-gallon potting containers with a mixture of peat soil and organic compost. Step 6 Transplant the hydrangea seedlings to 1-gallon potting containers once the plants develop three to four healthy leaves. Step 7 Place the individual potting containers in a sunny location and maintain soil moisture until you are ready to plant the hydrangeas in your landscape. Late spring -- after the risk of frost has passed -- is the best time to plant hydrangeas in your landscape.