The courier will leave the box where they usually deliver to for that address.
If the recipient is not there when  the delivery is attempted, the courier will either: leave the arrangement in a covered, secure area of the address (porch, garage, or other entrance), or leave the bouquet with a neighbor and place a message on the recipient's door with this information.
Please note: Flowers cannot be left outside in extreme temperatures above 90 degrees F (32 C) or below 32 degrees F (0 C).