Senior Vice President John Whitten said the bank's parent company, Herring Bank, does business in Oklahoma and Texas, both states where medical marijuana is not legal, The (Boulder) Daily Camera reported.
"There are unresolved issues with regulations, law enforcement and other agencies that need to get resolved before the industry can progress and become bankable," Whitten said.
A letter last week to operators of medical marijuana dispensaries asked them to shut down their accounts by the end of September, Whitten said.
Diane Czarkowski, one of the owners of Boulder Kind Care, said the dispensary, which now has an account at Colorado Springs State, has been through six other banks since it opened in October 2009. She said Kind Care is now talking to a local bank.
"I think a lot of people will be in a really bad position," she said. "They will be forced to do things under the radar, which is not helpful to the industry."