From the Super Mario Wiki
When the player decides to stop here, they will meet Bob-omba, a female Bob-omb Buddy. Her quest, Flowers Are a Blast!, involves getting a pretty and healthy plant for her. She tells them that they can find one in the Desert Area. The player must go there and stop by the Piranha Plant icon and take a Piranha Plant. If the player returns to Bob-omba with the Piranha Plant, she will reward them with the Gaddget, Rochambeau. The ending states her piranha has grown a little too big.