I am doing a cake for someone that wants smooth tree branches all around the cake and I am frosting the cake with buttercream. The branches need to be smooth and I wasn't sure if I need to add corn syrup or meringue powder to buttercream for the right look. I will be using a 4 or 5 tip to make them the right size! Any help would be appreciated!

Last year I made a cake with vines on it. The cake was iced with Cream Cheese frosting and I made the vines out of fondant. I don't know exactly the effect you are trying to get but maybe that would work. The flowers on the cake were of gumpaste. Here's a pic, so you can see if it might give you the effect you are looking for.