The term "Garden" in the shows title may actually mean the land of the dead. In episode nine, the name of the graveyard that Wirt and his friends enter is called "Eternal Garden". Near the end of the episode Wirt and Gregory are seen climbing over the "garden" wall, hence the title.
This episode finally reveals the reason Greg is wearing a teapot on his head: it's his elephant costume for Halloween (the spout is the trunk).
Wirt and Greg were from the modern time, and the reason for their "Puritan" outfits is because it was Halloween before they journeyed into the Unknown. This mini series premiered a week after Halloween in 2014.
The map shown on Wirt's bedroom wall at the beginning of the episode is the Mappa Mundi, the largest medieval (c.1300) map surviving, held at Hereford Cathedral.