If the delivery is not left at the delivery address, FedEx will either try to deliver the package later in the day or the next morning. Typically, there is a door tag left at the door or delivery location that specifies the next steps of the delivery.
If they continue to have problems with the delivery, they will hold the flowers at the nearest FedEx shipping facility.
It is up to the individual FedEx driver if they leave the package at the door. If they feel it is unsafe to leave it, they will reattempt delivery at a later time.