|The Shakespeare Garden at the Huntington Library|
William Shakespeare's plays are filled with quotes about flowers, with the most famous probably being "That which we call a rose would smell as sweet" from Romeo and Juliet. Over time, gardeners developed a very special, formal version of an English garden termed a "Shakespeare garden." As the name implies, these gardens feature plants and flowers mentioned in the plays, or typical of the Elizabethan period but not mentioned.
Here in LA, there's a gorgeous "Shakespeare Garden" at the Huntington Library. (For information on the garden, go here.) There's a Shakespeare Garden in Central Park and also in San Francisco's Golden Gate Park. for a list of other, public gardens, see the Wikipedia article. Shakespeare's favorite flower (at least in terms of how often he used it) was probably the rose. the Garden Web has a section on Elizabethan roses, and there are lots of forums for gardeners who want to grow heirloom roses in their own gardens. "Elizabethan Roses" became a popular design motif for china.
Here's an excerpt from Folk-lore of Shakespeare that talks about his knowledge of plants.