Can be achieved by placing different Plants, it doesn't matter what type as long as it has the Plant indicator in the game.
More Plants will unlock while researching different things at the decoration shrine and you will also be able to unlock some art pieces via friendship levels next to being able to buying a plant litter box.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 Plants of the same Plant type: 2 stat, is not 4 stat, it's 2 stat.