If you have ordered seeds online, or would like the challenge of growing the hydrangea shrubs from seed, here are some basic steps to take. However, we do not recommend growing hydrangeas from seed. It is much easier to grow hydrangeas from cuttings.
Collecting Hydrangea Seeds
Hydrangeas produce seeds through their enormous blooms. However, the seeds themselves are very small. As you can see below, the seeds are the size of cracked pepper.
*Image Credit - Kevin Campbell - Finding Plant Treasures in the Foothills
Once the shrub blooms, allow 8 to 12 weeks for the flower to start to fade and dry. That is the ideal time to clip the flower heads and place them in a brown paper bag. After you have collected several blooms and placed the flowers in the bag, store the flowers in the bag for an additional 3-7 days to finish drying out.
Once the flowers are ready, shake the bag while holding it closed. That should cause the seeds to fall off of the florets. Again, these seeds will be very hard to find in the bag once you are done.
Germinating Hydrangea Seeds
You can sow hydrangea seeds immediately after collecting them in the fall. Or you can store the seeds in a plastic bag in a cool space until spring, and begin to germinate the seeds then.
Whichever time frame you follow, follow the same guidelines.
1. Surface sow the seeds in potting soil in a flat filled with potting soil. This means simply put the seeds on the top of the soil, and leave the seeds alone after that. Do not bury the seeds or mix it with the dirt.
2. The soil should be well drained but kept moist throughout.
3. Place the flat in a sunny spot but protected from the wind. The entire process should take around 14 days.
4. Once the seeds are small plants, you can follow the same steps when growing from hydrangea cuttings.