The mansion's greenhouse is home to a multitude of plants that most would deem too dangerous to keep. Of course for Ellen, there's no other plants worth keeping!
Inside it's glass walls, Ellen experiments with mad botany, and rears a myriad of nasty plants, flowers, bushes, and trees into a lovely, diabolical garden. Her favorite plants being her own Nepenthes sinestros.
The first mention of the greenhouse is in "Pet's Revenge", where Morella had since been moved after she sprouted from a Balm encrusted beaker. It is also where Edgar accidentally let Morella bite Pet.
It's a more common location in the TV series, and Ellen has a wider variety of carnivorous plants to attend to. It is assumed that they are of the Nepenthes family, but it is never officially stated for all of them.