From the Canterbury Scene progressive rock group "Caravan" on 1971 album "In the Land of Grey and Pink". Most likely a slang term used to describe a plant somewhat similar to tobacco. According to the song "In the Land of grey and Pink", the fantasy substance may be smoked in a similar fashion as tobacco or cannabis.
"...We'll pick our fill of punkweed, and smoke it 'till we bleed; that's all we'll need." --- "In the Land of Grey and Pink" (1971, Caravan)