If somebody wants a plant that will bloom profusely and turn a balcony into a carpet of flowers, go for the Wave petunia.
Three years ago I had them planted in boxes sitting on the balcony and also boxes hanging from the railing and it was like a carpet of flowers cascading down. They hung about 3 feet past the balcony and the downstairs neigbors loved it. It was quite a site.
Also, in the northern Virginia area where I am at, they reseed themselves and live over the winter, even the very bad snowy winter we just went through.