Heaven's Garden is the center of Heaven.
This location does not have an official name, nor a fixed appearance. For example, Zachariah refers to it as "Mission Control", and the angels presumably see it as God's Throne Room. Others are known to see it as the Garden of Eden.
This place is tended to by Joshua, who refers to himself as a gardener, whose job is to "trim the hedges".
When Sam and Dean Winchester are sent to Heaven after being killed, Castiel urges them to find Joshua, who is said to speak with God, and communication with God would undoubtedly be heplful. Sam and Dean traveled along the Axis Mundi towards the center of Heaven. During a confrontation with Zachariah, Joshua arrived to pick up them.
Upon arrival, Sam and Dean perceived the garden as the Cleveland Botanical Garden, which they went on a school trip.
In 2017, Joshua abdicates his role as the garden's caretaker to take charge of Heaven. His attempts to have Lucifer's son killed lead to his death at the hands of Dagon. Following his demise, it is unknown who takes care of The Garden.