Hi Matt
The Place Plant tool creates a hybrid object. Hybrid objects, whether they are plants, hardscapes or tables and chairs, can only be placed on the layer plane, or parallel to the layer plane. The cannot be placed on angled planes such as the vertical face of a wall or extrude.
However, you may be able to place this plant purely as a 3D view.
You could create a 3D plane, which you extrude from a working plane set against the wall face. You can then apply the image prop texture onto this plane, but its effectiveness will be dependent upon your view. If you are looking head on at the wall, you will just see the top edge of the image prop, which will look like a line or two crossed lines.