There are two ways to add plants to your plans. The first (and easiest) way…
Vectorworks Tip #243 – Plants – Plant Shadows
Plant shadows can be individual to each plant, or can be connected to the document preferences. If you connect the plant shadows to the document preferences, it allows you to control all of the plant shadows on the entire document from one dialog box. If you create your plant shadows with a black color and a 50% opacity, the plant shadows appear to be gray and see-through.