Rosarius Poppus is one of the less-utilized flowers in the Herblore skill, it is a required ingredient in 5 different potions. It is used when crafting the Defence, Stamina, Super defence, Super stamina and Extreme defence potions, and gives 5 XP when harvested.
You can find Rosarius Poppus in Draynor Village, Varrock and Lumbridge as well as many other spots.
The best spot to gather large amounts of Rosarius poppus is outside of the Draynor Village bank.