The grotto is a unique part of the Temple of Melitele in Ellander. It is hewn from the rock face behind the temple and has a roof made of translucent crystal. In it grow all sorts of plants and creatures useful in medicines and magic which are found nowhere else. Nenneke and the other priestesses tend the greenhouse.
| Plants were everywhere. They grew out of beds hewn into the bedrock and filled with peat, in enormous chests, troughs and flowerpots. They climbed up rocks, up wooden trellises adn stakes.
— page 207, The Last Wish (UK edition)
In the short story "The Voice of Reason", Nenneke explains to Geralt that climate changes have changed the nature of sunlight in the world and that is why only the unique climate conditions in the grotto allow some of the plants to grow at all any more.