Garden of the Dead (1972) 
Garden of the Dead is a silly, low budget Troma distributed zombie offering directed by John 'Grave of the Vampire' Hayes.
Inmates of a prison work-camp discover a barrel of experimental formaldehyde and proceed to get high off of the fumes. A group of them later attempt an escape and are chased down by the warden and his deputies, in which the prisoners' getaway truck crashes and they are all killed in the resulting firefight with the deputies. The warden then orders that the dead prisoners be buried in shallow graves in the woods.
The prisoners are not long buried when they start to reanimate and shamble back to the prison - at first you assume purely for vengeance - but later you discover that they're more interested in imbibing more of those refreshingly addictive formaldehyde fumes.
Dumb as it may be, Garden of the Dead is quite fun, and is a film in which they really went out of their way to concoct a bizarre mechanism for zombification. The resurrected dead also look pretty funky and even exhibit quasi-vampiristic qualities such as being afraid of light - allowing the effective use of the prison searchlights etc. to deter the marauding undead convict horde.
I was disappointed with the lack of a 'garden' though, at least in the conventional sense. I guess 'shallow graves near the prison grounds of the dead' didn't quite have the same ring to it.