From the Super Mario Wiki
The Gardener's Lab is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. The Gardener's Lab can be accessed by taking the elevator down from the Gardener's Dwelling. The Gardener's Lab is a fairly large room shaped like an upside-down T. To the far left, there are many plant specimens in containment, specifically a Flytrap and an Orange Flower. There is a prototype Pixelator inside by these plants as well as a table with small stacks of paper and various test tubes. On the right side, there are Bulbs blooming. Using the Bulbs, Luigi can go back to the Gardener's Dwelling. The door on the north end of the hall leads to the Sewer.
During The Pinwheel Gate, Luigi needs to go to the Gardener's Lab to chase after a Hider that had stolen a pinwheel vane. However, Luigi will need to defeat two Greenies before he can fight the Hider. During Graveyard Shift, Luigi needs to escort a Toad assistant to the Pixelator in the Gardener's Lab. Both the Flytrap and the Orange Flower are missing from the containment tubes. In Pool Party, the containment tube with the Flytrap is now broken, but nothing much has changed. In Hostile Intrusion, the Gardener's Lab may be one of the rooms that is infested with ghosts.