The Plant Room is located inside the Green Button Door ( located in the Inside ) if you need help to get the button visit the following section: Button Locations.
When you enter you will see at your left that there is a room where you can find Mandrake (NPC)and in the other room you can see a door, when you enter in it you will find a place with 8 pots and a big door at the end.
To unlock the big door, you need to plant all the 8 types of seed in plots (The ones in Topple Hill, The Town of Right and Wrong, Abandoned Orchard and Farm Fortress. When all of the 8 plants are harvested, they will be added in the Plant Room plots and will let you in the big door.