The relevant paragraphs from The Wonderful Wizard of Oz –
They now came upon more and more of the big scarlet poppies, and fewer and fewer of the other flowers; and soon they found themselves in the midst of a great meadow of poppies. 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. But Dorothy did not know this, nor could she get away from the bright red flowers that were everywhere about; so presently her eyes grew heavy and she felt she must sit down to rest and to sleep.
… They carried the sleeping girl to a pretty spot beside the river, far enough from the poppy field to prevent her breathing any more of the poison of the flowers, and here they laid her gently on the soft grass and waited for the fresh breeze to waken her.
The opium poppy, Papaver somniferum, is the type of poppy from which opium and all refined opiates such as morphine and codeine are extracted. The name means, loosely, the "sleep-bringing poppy." But poppy seed is quite nice on a bagel, and poppy seed oil is quite healthy. In any event, these are clearly ornamental. The Tin Man need not worry about Dorothy.