Director Sue Biggs puts the increase down to "being much more open in telling people what we're doing".
Many of the membership gains came from the new RHS Experience stands at the Chelsea, Hampton Court and Tatton Park shows. The stands showed off RHS work in art, library, science, Britain in Bloom and community.
Biggs added: "We've been so quiet until now." She said anecdotal evidence showed the RHS was attracting younger members who wanted to know "how to do grow your own properly".
RHS Wisley's September show attracted 31,332 people, similar to 2010 (as were other RHS shows), but over four days rather than three. The actress Jane Asher opened the show, which had 30 floral exhibitors.