The Seedling Laboratory is one of the many rooms inside of the Haunted Towers, appearing in Luigi's Mansion: Dark Moon. It can be accessed by walking up the stalk inside the Botany Lab or by taking the door on the third floor of the Toolshed Stairs. The Seedling Laboratory is a rather large room, with:
Several drawers around the area, that give money or hearts.
A mechanism, in the center of the room, that allows Luigi to shoot seeds at the rows of potted plants to the north.
Several pipes and vines running through the area as well as shelves with potted plants.
In the mission B-2: The Pinwheel Gate, Luigi needs to go to the Seedling Laboratory in order to seek out one of Hiders that took a vane from the Pinwheel Gate. Before Luigi can suck up the Hider, he must fight two Greenies; after sucking up them, Luigi can finally found the Hider and get the first pinwheel vane after he defeats it. One of the posters here gives out a lot of money when pulled off and destroying all the potted plants with the seed-shooting machine causes 4 gold spiders to drop from the ceiling.
During B-3: Graveyard Shift, there are two potted plants in the large plots of dirt. The bottom left plot of dirt has two dandelions, a shovel, and a pitchfork while the top right plot has nothing. Luigi is attacked by three Slammers and a Greenie as well.
Starting on B-2: The Pinwheel Gate and all missions after, excluding B-4: Pool Party, Luigi can use the seed-shooting machine to kill a mouse scurrying along the rows of potted plants. Hitting the mouse causes it to drop the emerald it was dragging along with its tail.