Minecraft wiki entry for Quick_ways_to_get_food, associated text: A dandelion or blue orchid will give the "saturation" effect, promoting your stew to a superfood!
An oxeye daisy will let you regenerate a few points of health, while a lily-of-the-valley will poison you, doing several points of damage.
Other flowers offer various good or bad effects, but all lasting just a few seconds.