Thorn Cross Young Offenders Institute in Warrington, Cheshire, won the Brockhouse Trophy for best show exhibit for its 1920s Film History garden, built to celebrate the 80th running of the show.
The young offenders' garden featured images of 1920s silent movie stars Charlie Chaplin, Laurel and Hardy and Buster Keaton.
Detainee Tom Collins said he and 20 others had grown 90 per cent of the plants used themselves. He added: "It opens your eyes to how creative you can be."
Thorn Cross sustainable development manager Keith Sowden said: "A lot of the lads have never done anything like this before. As we met the targets their confidence has grown with the chance of winning something. This gives them the opportunity to find work after Thorn Cross."
Detainees are working with Railtrack and United Utilities on placements in the final third of their sentences. They are also completing NPTC training for spray licences and turf maintenance.