A metaphor is a comparison of two unlike things by saying one is the other. Plants are often used as metaphors of growth, death, and decay. In Macbeth, we see several examples of plants as a metaphor. One such metaphor is used by Ducan in the first act.
I have begun to plant thee, and will labor
To make thee full of growing. (Act 1, Scene 4, p. 17)
Here Duncan suggests that he is working for Macbeth’s well-being. He has begun to “plant” him for greater things. By this he means he has given him a better title, Thane of Cawdor. Macbeth interprets this as confirmation of the witches’ prophecies, however, and thinks Duncan is preparing him to be the next king. He misinterprets Duncan’s metaphor, and his generosity.
Shakespeare makes references to plants and growth, both of which symbolize the natural order. However, Shakespeare notes that weeds as well as useful plants can grow, thus indicating that ideas, good and bad, can also grow. The many references indicate the condition of the characters and settings in the play at any given time.