The amount of sun, shade and water you give a flower box will determine the success of the flowers but some good ideas for flowers that will bloom in hot afternoon sun are ivy geraniums, scavolea 'Blue Wonder', Sweet Potato vine and petunias.  If the window box is mostly shaded go for lobelia, inpatients, coleus and begonias especially the dragon wing begonias such as begonia boliviensis that can handle some sun and also flower in the shade. Window boxes cannot hold much soil so be sure to fertilize with a slow release plant food like Osmocote and check the soil for moisture by poking your finger about an inch down. During hot weather a window box might need watering twice a day. Keep growing, Marianne Binetti