The unnamed 34-year-old employee was opening up at 6.30am, when he was approached by a man dressed all in black with his face covered.
The man threatened the employee and forced him at gunpoint to open a room where the safe is kept. He then stole cash and left via a fire exit.
Det Sgt Luke Grove said: "This was a very frightening ordeal for the victim who thankfully was not harmed. His hands were bound during the incident but he managed to free himself to raise the alarm.
"I am appealing for anyone who saw a man walking or running across fields in the area of the garden centre, either before or following the incident, to come forward.
"We have limited information on his appearance as he was wearing black clothes and had his face covered."
Call 0800 555 111.