He will join fellow ambassadors Alan Titchmarsh, Mary Berry, Adam Frost, James Wong and Nick Knowles.
Beardshaw will join Adam Frost on Friday 10 April to launch RHS Britain in Bloom 2015 with a community transformation project, and he has other projects already planned over the year, according to the RHS.
Sue Biggs, RHS Director General, said Beardshaw is "one of Britain’s best-known and best-loved gardeners" and has "long been involved with, and championed, community gardening projects".
She added: "RHS Britain in Bloom volunteers invest upwards of 10 million hours a year into greening up and cleaning up Britain’s villages, towns and cities. These volunteers create safer, happier environments, where crime is reduced, and transform disused spaces into beautiful places where communities come together and engage with each other."
Beardshaw said: "I’ve been involved in many community gardening projects over the years and have seen first-hand the incredibly positive impact they can have on people living in the area and involved in gardening. They create friendships in streets where people didn’t even say hello, and make brighter, more beautiful places to live. It’ll be a privilege to celebrate the work of the Bloom volunteers and encourage more people to get involved."