How about some scopolomine!

Scopolamine or “The Devils Breath” as it has become known, is a part hallucinogenic part delirium inducing drug native to the northern area of South America including Ecuador, Venezuela and of course Colombia. Its effects vary from being mildly euphoric (When mixed with opiates) to causing a zombie-like state that renders your power of free will useless....

It is known to give euphoria and with enough in the blood the inmate would have flash backs as it is a truth serum. But, it would kill him while hallucinating so that should be okay.

I would not expect to find too many of these plants in the neighborhood. They are dangerous plants.

Deadly drug: (click here) Scopolamine is made from the Borrachero tree, which blooms with deceptively beautiful white and yellow flowers

They are mostly tropical. If your neighborhood has one it could be removed for the danger it possesses. 

Or maybe some strychnine.

The inmate can care for the tree before he dies and bond with a kinship. The bonding process would provide a warm fuzzy feeling inside. It could be grown in the yard and provide shade on hot days. Then it can be harvested to provide enough poison to kill the inmates on death row.