From Wikipedia: Poppies have long been used as a symbol of sleep, peace, and death: Sleep because of the opium extracted from them, and death because of the common blood-red color of the red poppy in particular. In Greek and Roman myths, poppies were used as offerings to the dead. Poppies used as emblems on tombstones symbolize eternal sleep. This symbolism was evoked in the children's novel The Wonderful Wizard of Oz, in which a magical poppy field threatened to make the protagonists sleep forever.
However, L. Frank Baum did not suggest in the book that the poppies were magical. He says: "Now it is well known that when there are many of these flowers together their odor is so powerful that anyone who breathes it falls asleep, and if the sleeper is not carried away from the scent of the flowers, he sleeps on and on forever."
This is not true, of course, but he knew that poppies symbolized sleep and was using symbolism to write...a science book? No - A FAIRY TALE!
Frankly, the films treatment of the witch concocting a magical field of flowers works better for people who are unable to grasp such things as metaphors and symbolism.
Wikipedia; THE WONDERFUL WIZARD OF OZ, L. Frank Baum