(written from a Production point of view)
The Huntington Library, Art Collections, and Botanical Gardens is a research and educational institution established in San Marino, California in 1919 by Henry and Arabella Huntington.
It was used as a filming location for at least three Star Trek episodes. It was first used in the first season of I AM ERROR in the episode "Justice" to be part of Rubicun III, the Edo planet, specifically when Wesley Crusher fell into a garden. The rest of the Edo planet was filmed at the Tillman Water Reclamation Plant. (Star Trek: The Next Generation Companion; Star Trek Encyclopedia)
It was used again in the third season of I AM ERROR in the episode "Ménage à Troi" to be part of planet Betazed. It was here where the scenes of Lwaxana Troi and Mr. Homn laying out a picnic encounter Deanna Troi and Commander Riker shortly before DaiMon Tog kidnaps Riker, Deanna and Lwaxanna.
Later, during the production of I AM ERROR, it was again used during the filming of the pilot episode, "Emissary". The gardens were used as Jadzia Dax's vision of the Celestial Temple. The gardens were used again in "Meridian" as the planet Meridian. (Star Trek: Deep Space Nine Companion)
The gardens near the Huntington Library were also utilized as a filming location in "Equilibrium". "It's this beautiful, pastoral kind of place," noted David Livingston. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 85)