Q.how to grow rhododendron from seeds harvested from mature plants
when should you deadhead rhododendrons?
Certified GKH Gardening Expert
If you want seeds, then you should not deadhead them. This will kill off any forming seeds. When seeds pods are ripe and starting to drop seeds, then you can collect and deadhead.
The Rhododendron Society offers some insight into germinating seeds: https://www.rhododendron.org/v48n1p10.htm
This article will help you with the care of the shrub: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm