Police discovered the plants in flower boxes at a Green party district office in Gera, the Thüringische Landeszeitung newspaper reported Thursday.
Police are investigating possible drug law violations although preliminary reports were that the plants may only have been used for producing hemp. Prosecutors will decide whether charges should be laid, the newspaper reported.
But the discovery led to a firestorm of criticism from state politicians, including Wolfgang Fiedler, a state parliament member of the conservative Christian Democratic Union.
He told the newspaper that any Greens members who tolerated the cultivation of cannabis “must resign their high parliamentary office.”
One of the leading members of the Greens in Thuringia, Astrid Rothe-Beinlich, said she didn't know about the plants beforehand and had asked for an explanation from staff.
Asked how the plants had got there, a state party spokeswoman Daniela Hoffmann-Weber said: “That would interest us.”
“No one will seriously believe that we're making hemp here,” she said.
The discovery is the latest controversy to hit Greens this week in Germany. Earlier in the week, a party campaign manager in Berlin was arrested on drink driving charges.