Gardens of Equestria is a megaspell designed by Twilight Sparkle, actually one of her two biggest projects while in charge of the Ministry of Arcane Sciences. It takes the form of a tall chamber, located deep inside Spike's Cave. In the middle of the chamber rests one of the three Crusader maneframes.
It is described as such: "The chamber was a chimney. Staring upwards, I could see far above us a rough circle of night’s sky, twinkling with stars. The super-maneframe was pointed towards that hole like a colossal magical wand. [...] A platform radiated out from the base of the Crusader like a six-pointed star, each point ending in a dais. Upon each dais rested a fine pillow upon which sat a single piece of jewellery, [the Elements of Harmony]."
It requires the six Elements of Harmony, and six respective bearers, to function, and is said to have the power to purge the entire land of Equestria from all sorts of magical contamination, including radiation and Taint.
During the War
The Elements were placed in Spike's Cave where a Crusader Mainframe was housed, and all the Elements were put in their respective places.
Two hundred years later, the six elemental bearers (the Elements of Magic and Generosity are left unknown) have come together to wield their Elements (which then transform into Golden Pipbucks with the bearers' respective Cutie Marks) and activate the Gardens of Equestria, which purge the Equestrian Wasteland of its many environmental hazards.
Despite finally solving many of the problems of the Equestrian Wasteland, its activation had created several of its own problems. Due to the fact that it purged all radiation from the lands, it meant that Ghouls could not heal themselves, and thus had to make expeditions to the outskirts of Equestria in order to get irradiated water, dubbed Aqua Cura. Another issue stemming from the elimination of all radiation in the Wasteland was that it made Red Eye's radiation engines absolutely useless, as they no longer had any fuel sources, causing the rebuilt Equestrian civiliation to depend solely on a limited supply of energy cells.
Notes and Trivia
The Gardens Of Equestria were based off of "The Garden of Eden Creation Kit", GECK for short, that could purge the entire Capital Wasteland of the Great Wars effects. It was very unstable and would kill the player if it was activated in Vault 87, if the player can survive the radiation, or just pick it up for use on Project Purity.