Hello Nicole,
The use of jpgs for plants help me a lot to give a more realistic sense in my Sketch Up designs ! I recently found a very useful tutorial about it on Youtube this one :http://www.youtube.com/watch?v=rqWVKrPd_8E You will need images of the plants you want to add in your design proposal and Photoshop as well. In addition, a design in Sketch up looks more realistic when the edges in your design are not selected to be viewed. you can do this from View -> Edge Style -> Display Edges (not to be checked). I used these tips to create a roof garden; I upload here an image of this.