Plastic Bottles instead of Pea Gravel Pea gravel and pebbles can weigh down window boxes. Empty water bottles are a lightweight option. Place rinsed bottles inside the planter and add soil on top of them. The space between the bottles will allow proper drainage without the extra weight.
Gorgeous window boxes full of hydrangeas, and good looking shutters in Charleston, South Carolina. How to grow hydrangeas in pots/containers: http://www.hgtvgardens.com/hydrangeas/growing-hydrangeas-in-pots