I have several varieties of butterfly bush. I am trying to get a more "treelike" base that will stand upright. I pruned them, leaving about 3 feet of base branches. One of the butterfly bushes has new growth growing on old branches, and most have new growth growing up from it's base. These butterfly bushes will serve as a background for the rest of my garden. Have I pruned them correctly to achieve the treelike goal? Most articles I have found mention whacking them down to the ground. All advice appreciated. Thanks.