Daniel,
IAD does have a flower shop in the arrivals hall, after you clear customs. Unfortunately, it closes at 9pm; in any case, given the usual arrival schedules of London and Buenos Aires (or Miami) flights to Washington, I’m guessing that she will arrive at the airport before you and be there to greet you as you leave customs.
Maybe you can arrange through Teleflora or the hotel concierge to have a bouquet of roses waiting in the hotel room? Other than presenting her with chocolate roses or jewelry at the airport, I can’t think of anything else that might be a rose substitute for the greeting – perhaps others reading can give you some creative ideas.
Cheers,
LR