A survey of Britain In Bloom groups found 90% of them believed the main benefit of participating in the campaign was the 'creation and development' of communities.
Sue Biggs, director general of the RHS, said: "The report proves that gardening is not only a great social leveller but also the cohesive glue that binds neighbourhoods, reducing crime, transforming lives and enabling commerce to prosper.
"For the millions of volunteers through Britain in Bloom who annually green up and clean up where they live, this research highlights the incredible impact they have on their communities and how important the national campaign is to the neighbourhoods that take part."
Around half of the groups who responded to the survey said that the transformation of spaces that were eye-sores in the past had benefited the area.
Groups listed a reduction in crime, an increase in community spirit, feel for a place and sense of pride as benefits of participating in Britain in Bloom.
Nearly 70% of groups undertake litter picks, 33% care for street furniture and 25% remove graffiti with an estimated 115,000 trees, 352,000 shrubs and 21.6 million plants and bulbs planted by Bloom groups annually.