The last few weekends in Richmond have been REALLLLLLY hot. Not only has it been hot, it's also pretty humid. I delivered two cakes to outdoor receptions in the past few weeks, and both were covered in fondant. Both of the brides were reluctant to have fondant instead of buttercream, but my policy is that outdoors in the summer=fondant unless it's cool enough to use buttercream, and that's my call. It's a good thing, too, because sugar + heat + humidity= a big pile of mush.
If these cakes had been covered in buttercream, they wouldn't have survived the heat. The fondant was even looking very sticky because of the humidity, so I knew that the barrier that it provided between the weather and the cake was totally necessary. The one that I delivered last weekend was decorated with royal icing and fondant decorations, because royal icing doesn't melt. If I had decorated it with buttercream the borders would have melted and the flowers would have slid right off of the cake. It was so hot the bride decided to leave the cake inside until later in the reception (a smart choice), but even in the coolest room in the house it was pretty warm (Historic homes on the River are generally not air-conditioned.)
So ladies, if your baker tells you that the idea of an outdoor cake with buttercream in June or July is okay, question their judgment. In 95 degree heat with the kind of humidity that we have here, it's not a good idea unless you want to have the icing slide off of your cake.
I'm really looking forward to cooler weather!