A Poppy is a resource used to restore Garrett's focus in Thief. Chewing the petals of the flower has the effect of sharpening Garrett's intuition. It costs 35 gold to purchase.
The white species appears to be a strange mutant variety empowered with Primal and has fungus like properties, capable of grow anywhere even underground, glowing with inner light. Where many poppies have sprouted one can almost find strange fungus-like vines or roots or other growths extending up the walls. Seed pods can be found in many locations where Primal is near, and often there is a haze of glowing pollen like dust in the air.