From the Super Mario Wiki
The Boneyard (Japanese: 裏の空き地 Vacant Land of the Back) is one of the few locations in Luigi's Mansion located outside of the mansion. It is cleared in Area Two.
Spooky is found sleeping in the dog house. After Luigi captures both Spooky and a Skeleton Ghost, the Boneyard brightens. There are sparkles in Spooky's doghouse. If Luigi investigates with his Game Boy Horror, it transports him to the Graveyard.
If Luigi waters the bean, the bean sprouts into a small plant. After Luigi captures Bogmire, the bean grows and produces a bud. If Luigi waters it again, the bud grows and blooms into a gigantic flower. When he defeats Boolossus, the flower becomes a seed pod. Luigi then may water it a final time to make it open, revealing the rare Gold Diamond. With it are 2 Gold Bars, 20 Bills, and 30 Coins. If Luigi does not water it after defeating a boss, the plant whither and dies.
Ghosts in The Boneyard during the blackout