We read "The Tiny Seed" by Eric Carle. Here is a video version if you do not have the book. It is a good book that helps go through the life cycle of a flower in a basic way.
2 Teaching Mommies has a great Flower Unit to check out.
Here is a Plant Life Cycle freebie.
Making Learning Fun has lots of flower themed activity ideas.
My son did a finger painting of flowers. He had a great time.
Here is what I did for a flower theme in 2011 with my older boys.