From the Super Mario Wiki
The Plant Nursery is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. The Plant Nursery can be accessed by taking either of the north doors on the second floor of the Tower Lobby. Inside the Plant Nursery, there are four sets of wooden planters: one by each of the four walls of the room. The contents of the planters differs between missions. There are pipes with running water, a sink, and several shelves with pots and sacks. Both doors to the south lead back to the Tower Lobby. Later in the game, a pathway to the Rumpus Room becomes available.
During B-1: A Job for a Plumber, Luigi needs to go in this room to get to the other side of the second floor. Inside this room, Luigi needs to kill the Flytraps guarding a treasure chest with a key. In B-2: The Pinwheel Gate, the Plant Nursery has two dried plants that can be watered. Flashing the Strobulb on these plants after they have been watered will cause nine bills to pop out from each plant. In B-4: Pool Party, the Plant Nursery will be overrun with foliage. A treasure chest filled with many coins, bills, and Gold Bars is made invisible on the middle planter by Spirit Balls. Luigi also needs to water the sprout in the middle of the room to cause a large stalk to grow. This stalk breaks the ceiling and creates a way to Rumpus Room. In B-Bonus: Hostile Intrusion, the Plant Nursery may be one of the rooms that has ghosts.