From the Super Mario Wiki
The Greenhouse is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. The Greenhouse can be accessed by taking either of the north doors on the first floor of the Tower Lobby. Inside the Greenhouse are many wooden planters filled with different colored flowers. The room is covered in leaves and vines that have broken windows and crawl along the stone pathways. There are several dollies in this room and many vases with plants. Moss-like plants droop from the ceiling and there is a window to the far left that can peer into the West Garden. Both of the doors to the south of the Greenhouse lead back into the Tower Lobby.
During A Job for a Plumber, Luigi needs to go in this room to reach the other side of the Tower Lobby. Luigi is also ambushed by three Greenies with shovels when he reaches the other side. Pulling down the four moss-like plants that hang from the ceiling makes many bills appear. In The Pinwheel Gate and Graveyard Shift, Luigi can go back to the Greenhouse and pull off the moss from the ceiling again to make more bills appear. In Pool Party, the Greenhouse has three orange flowers that attack Luigi if he ventures too close. If Luigi defeats these orange flowers by ripping them from the ceiling when their attacks miss, many bills fall from the ceiling. In Hostile Intrusion, if Luigi enters the Greenhouse, many bills automatically fall from the ceiling.