I need to do this cake for a wedding.
How do I go about making the rose garlands? I know how to make roses but making them stay on the cake is another story. Should I use buttercream or something else?
If making the roses from BC, use a good crusting BC recipe. Let them air dry at least overnight. Then use BC as the glue to hold them in place.
The air drying allows the moisture to escape from the BC rose, making it very lightweight. A freshly made rose still has all of that moisture in it and has quite a bit of weight. (compare the two in your hands .... you'll be amazed.)
It's the only method I ever knew ever since I started making cakes and it holds up fine.
here's an example: http://cakecentral.com/gallery/1496730