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.
Making the most of small space; this is a great design to bring nature into every city corner. Designed and built by Belderbos Landscapes I like the planters with dwarf boxwood. COuld use against the front of house or on either patio.