From the Super Mario Wiki
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The area has a doghouse in which Spooky sleeps inside of. When Luigi approaches Spooky, it starts barking and chases him around the Boneyard. A Mr. Bones gets irritated by the noise and rises out from the ground. He must suck it inside of his Poltergust 3000 and afterward do the same to Spooky. After Luigi captures both of them, the Boneyard lights up.
The Boneyard also has a patch of dirt with a large bean in it. If Luigi waters it, 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 may then 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 the flower after defeating either boss, the plant withers and dies.