Longwood Gardens does rent out its facilities for meetings and social functions. It does not rent out the building or the grounds for wedding ceremonies, but it does allow wedding photos to be taken on the property.
Longwood is an incredible backdrop for wedding photographs as you can see by looking at the picture above. The grounds make for incomparable settings for romantic once in a lifetime photos. The ground are filled with incredible specimen trees, fountains, topiaries, and vast lawns.
The gardens do have guidelines that govern wedding photography. You can see these here.