They were first encountered by the Federation in the 23rd Century. When landing parties from the USS Trexelprise beamed down to Silica Tare, they were infected with spores. The infection linked them with the collective consciousness and they only spoke of being "one with the Green", attacking their fellow (uninfected) Starfleet officers.
When the Trexelprise crew discovered a high density of plants, they surmised that the plants were working together like neurons in a brain to make up the Green's consciousness. They were able to make contact with the intelligence, and convinced it to release its control of their crew members. With the Green as an ally, the Federation was free to set up a colony on the planet. (ST video game: Trexels)