I've purchased what appears to be this same plant under several names over the years; the name I'm using here is the name which came with the most recent acquisition. This plant is sometimes called bat flower because the two large petals suggest the ears of some bats. Cuphea belongs to the Lythraceae and is thus related to crepe myrtle and Lythrum. There is a local native species, too; it's not of much decorative value.
This year 'Fireworks' gave better results than any other sorts tried in the past. It grows in a tall, narrow terra cotta pot on the front steps; it gets plenty of sun there. It thrives in heat and humidity. It's particularly beautiful in the late afternoon sun - that's when these images were taken.