For me, a garden confined to the windowsill alone feels inadequate—a real window garden must be a complete horticultural portrait. Consequently, vertical elements are necessary to generate eye movement. And here, gleaming glass shelves come to the rescue.
Three shelves are generally adequate for the average window.Align one shelf with the latch ledge above the windowsill, and another at the midway point between the two. Position the third shelf at an equal distance above the latch ledge.
I should probably mention that heavy, tempered glass is very expensive—often $100.00 per shelf. But you can ask your glass cutter to make shelves from half-inch-thick salvage glass. Such shelves can be had for about $10.00 each, and non-tempered glass (which I use) is fine.
Most of my shelves are 10-inches deep. They are held in place with 10-inch scrolled shelf supports.
You can find both new and salvage glass from your local glass-cutting shop. Shelf supports are available at any hardware store.