Mr Melvin said working on the garden had been a ''privilege'' and he would keep a connection to it and the staff in one way or another.
The garden's site and volunteer supervisor, Kim Conroy, said Mr Melvin was the ''heart and soul'' of the garden and would be sorely missed.
But the man himself was a bit more humble when describing his impact on the garden.
''I'm just one cog in the wheel - just a very noisy cog.''
Raewyn Maskill will take over as head gardener.