That may not sound very threatening, but the report states that it was a large tree branch.
The man allegedly walked into the store around 11:30 on August 4 and demanded money. The clerk refused, probably having noticed that the robber was only armed with a tree branch. His moderately brave action infuriated the man, who then reportedly "struck the clerk several times with the tree branch," then fled. He tried to make a getaway on a motorcycle, but was blocked by an off-duty police officer who responded after seeing the man "being chased by the clerk and a small group of customers." (Again, this pursuit was probably due to the tree branch not being all that intimidating.) He was arrested and charged with robbery in the first degree.
This is certainly not the first time a robber who didn't have a gun available has turned to nature. In June 2008, a Florida man tried to rob a convenience store armed with a yucca plant. That again was slightly more threatening than it sounds, because he was wielding "Spanish bayonet," a member of the yucca family with sharp two-inch leaves that is apparently formidable enough to be used as a "security plant." Still, he too was unsuccessful, mainly because he was jumped by a customer armed with a bar stool.
The year before that, somebody robbed a bank in New Hampshire disguised as a tree. To the best of my knowledge, that guy is still at large.