Following are screen shots from the Wisconsin PBS video about sack planters on the parking lot of a Salvation Army pantry. I have added further explanation and comments about this highly productive and inexpensive way to grow vegetables in the city.
A layer of gravel is poured into the bottom of the sack to create a base. I would add a layer of pourous nursery fabric or plastic screening to prevent soil from seeping into the gravel base. This retains air space between the pieces of gravel.
Position a can (with the top and bottom removed) in the middle of the sack on top of the gravel base. This will act as a mold or form for a gravel column that will go from the bottom to the top of the sack. Add soil until level with the top of the can mold.
An alternate method is to create a center core for the gravel column by using a piece of window screen material rolled into a round column shape to form the core. The screen retainer would rise from the gravel base to the top of the planter in one piece.
What is not quite clear in the video is that she lifts the can out of the soil at the bottom of the sack. The gravel stays on the bottom of the planter. Now reposition the can on top of the first soil/gravel layer, fill with soil around it, add gravel and repeat this procedure until reaching the top of the sack.
The planter is complete. Here you can see the gravel column that facilitates flow of water from top to bottom throughout the sack planter. Equally important are the pore spaces between the pieces of gravel that provide all important oxygen to the root system.
I have other ideas on using this basic system to make vertical planters like these even more productive and durable. Stay tuned.