Pea gravel and pebbles can weigh down windowboxes. 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.
Window Box Ideas « Hyannis Country GardenThis box is a simple mix of whites and greens. White petunias, Sutera and Lobelia are almost covering the three dwarf boxwood plants that were in the box all winter.