It seems that heather has particular psychotropic properties when fermented.
"There is considerable evidence that fermented heather was one of the sacred beverages of the Picts and Celts and an integral part of Druidic sacred life and ceremonies. Its use is ancient in fermentation...Considered to be the first fermented beverage in the British isles, it has enjoyed wide historical use throughout ancient Druid lands."
"Heather mead, for millennia, was considered to be a uniques fermentation that that produced remarkable effects in those that drank it"
Sacred and Herbal Healing Beers
Stephen Harrod Buhner