The short bit straight from Wikipedia is: Greenfingers is a 2000 British comedy film directed and written by Joel Hershman. It is loosely based on the true story about the award-winning prisoners of HMP Leyhill, a minimum-security prison in the Cotswolds, England,^[3] a story published in The New York Times in 1998.

While the prison was (and I think still is) minimum security, it was at the time of the real story an experiment in preparing murderers and the like for returning to outside life. So, you can already tell that the cast of prisoners is … quite interesting.

What with one thing and another, a small group of prisoners end up encouraged to enter a garden in the prestigious Hampton Court Show. While it doesn’t get the same screen time (so to speak) here in the US as the Chelsea Garden Show, it is a major Royal Horticultural Society Show. This link goes to their website and includes highlights from the 2022 show.

And the prison in the movie, Leyhill, did apparently continue to send entries to RHS shows.

Since then, I have come across an article from the Royal Horticultural Society with more information about prisoner gardeners. It’s apparently quite a Thing. I quote directly: “HM Prison Service Young Offenders Institution (HMPYOI) Thorn Cross, has been named the 2022 best-kept prison garden in England and Wales by the RHS and the prison service, receiving the Windlesham Trophy for its efforts. The Windlesham Trophy, devised by Lord Windlesham who was Chairman of the Parole Board at the time, was first awarded in 1983. It celebrates the horticultural success within prisons and promotes the benefits that green spaces have on wellbeing and rehabilitation. The competition is managed by the land-based sector (IRCPE – industries, retail, catering and physical education) of the prison service in partnership with the RHS.”

The RHS website has some additional information on projects related to prisons, including a WWI internment camp.