The Great Sunflower Project

I am glad to be participating in the Great Sunflower Project. Farmers here in South Florida plant thier crops in September for a winter growing season. Not having a great deal of gardening knowledge, I would like to know if I can plant my sunflower seeds as soon as they are recieved in a few weeks or should I wait until fall to plant?

Another concern of mine is the hardness of my soil which is primarily made up a coral rock with a thin layer of dirt. Do I need to break up the coral rock when I plant or will the seedlings penetrate the pourus coral rock if planted in the thin soil layer above?

Sorry if these questions seem silly but my gardening expertese is nill. My only experience over the years has been the planting of trees on my property.