The two robbers approached the manager of the Garden Centre Group's centre in Great Gaddesden, near Hemel Hempstead, while he was locking up.
The pair emptied the safe, taking a laptop and mobile phone before getting away in what is thought to be a dark VW Golf, driven by a third man.
The robbery took place on 9 October, between 6.30pm and 6.45pm.
Detective constable Andrew Roseblade from the area crime group said: "I want to trace the occupants of the car or anyone who may have been driving along the Leighton Buzzard Road at the time and seen a vehicle matching the description.
"Also, I am appealing for anyone with any information on this crime to come forward."
Call 0845 33 00 222 with information.
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