Plants are often not a specific item of dream interpretation because most of the time they function simply as background scenery. The exception to this rule is when a particular type of plant is identified in the dream.
Plants that are significant are those that have historic importance in literature or your personal experience. For example, you may dream of visiting a friend who is sitting in a thicket of hemlock. Obviously, this plant is significant because of the implications of hemlock and suicide in ancient lore.
Other plants that may be significant are those that remind you of a childhood memory, a particular place, or a particular person. In those cases, identifying the relationship of your current circumstances with your memories is important.