From Bulbapedia, the community-driven Pokémon encyclopedia.
The Greenhouse (Japanese: 緑の館 Green House) is an anime-exclusive location in Hoenn. It is a Pokémon aromatherapy laboratory 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 Weepinbell, which they then mix with all kinds of herbs to create medicine. Besides a laboratory, The Greenhouse has a large garden designed to keep the resident Grass Pokémon happy. The leading therapist and director of The Greenhouse is Eliza.
When Ash and his friends found an ill Skitty, Brock came up with the idea to take it to The Greenhouse, which was nearby. Brock happened to know of the facility because he had seen an infomercial on it. When they arrived, Eliza accepted their request to heal Skitty and took them on a tour. After explaining her work, Eliza proceeded to examine Skitty and concluded that it was in fact fatigued. 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. However, the capture attempt was interrupted by Team Rocket, who stole many of the facility's fragrances with help of Seviper's Haze. Eventually, Team Rocket was found and defeated, and the stolen fragrances were safely returned to The Greenhouse.
Pokémon seen around The Greenhouse