From Bulbapedia, the community-driven Pokémon encyclopedia.
The Greenhouse (Japanese: 緑の館 Green House) is a Pokémon aromatherapy laboratory which is located between Mauville City and Fallarbor Town. It appeared in I Feel Skitty!. At The Greenhouse, fragrances are used to heal Pokémon. They make the fragrances from Sweet Scents from Pokémon such as Oddish and Vileplume, which they then mix with all kinds of herbs and mints to create medicine. Besides a laboratory, The Greenhouse has a large garden. The leading therapist and director of The Greenhouse is Eliza.
When Ash and his friends found an ill Skitty, Brock came with the idea to take it to The Greenhouse which was nearby. Brock happened to know it because he had seen an infomercial on it. At the Greenhouse, they met Eliza. Eliza accepted their request to heal Skitty and took them for a tour through the building. After explaining her work, Eliza proceeded to examine Skitty. She concluded that it wasn't sick but just very tired. Inhaling one of Eliza's scents, Skitty quickly felt better.
May then challenged Skitty to a battle inside the laboratory as she wanted to catch it. She sent out Torchic to battle it, but right at that moment Team Rocket bursted in. With help of Seviper's Haze, Team Rocket managed to steal various fragrance bottles and got away in their balloon. Eventually, Team Rocket was found and defeated, and the stolen fragrances were safely returned to The Greenhouse.
Pokémon seen around The Greenhouse