Minecraft wiki entry for Flower, associated text: Any mob killed by the wither drops one wither rose upon death. If possible, the wither rose is placed on the block where the mob died. If the flower cannot be placed there, or if /gamerule mobGriefing is set to false, the flower is dropped as an item instead. Wither roses are dropped even when /gamerule doMobLoot is set to false.[1][2]