The RHS will launch a programme of regional growth in 2012 financed by the sale of Lawrence Hall in London to Westminster School for £18m.
RHS director Sue Biggs said the sale would help generate the cash to develop the RHS's 10 new objectives, including "transforming communities" and being a world-leading horticultural society. She described the 999-year leasehold sale as "a wonderful happenstance moment" that would benefit the society and the school.
The hall's team will continue to organise events for groups such as gardening societies at RHS Lindley Hall, which the society will keep. The four RHS London shows will continue at both halls during school holidays.
The RHS will also start a fundraising campaign to raise a further £9m.
Rather than take over a garden to add to RHS Wisley, Hyde Hall, Harlow Carr and Garden Rosemoor, Biggs said the society wanted to offer more to its existing 380,000 membership nationwide.
This includes a first RHS Urban Garden "for local residents, offering educational initiatives including school visits, workshops and inspirational gardening courses". The location could be a brownfield site in a UK city "outside our heartland" and not in London.
The RHS will also build regional centres across the UK, based at RHS gardens or partner colleges and gardens. The first two will open this year and are designed to work with community gardening supporting Britain in Bloom, It's Your Neighbourhood, local gardening clubs and schools involved in the society's Campaign for School Gardening.
Plans also include developing the Lindley Library and Hall to create a new space for a library and gallery, as well as the creation of a "world-class research centre for the benefit of all gardeners and the environment".
At RHS Wisley, the society plans to create two more teaching rooms and an exhibition area and event facility. Additional spaces will enable nurseries to promote British-grown, rare and unusual plants, with a dedicated room to enjoy the most extensive range of horticultural books on sale for gardeners in the UK. A new restaurant will be added and the entrance will be revamped.
At Hyde Hall, Essex, plans include the UK's largest cultivated perennial meadow, a Mediterranean edible garden and a new learning centre.
In my view
Peter Seabrook, gardening writer and broadcaster
"The £18m sale looks like a good price but I'm worried that it won't be invested to the same return that our benefactors left us. Could we not build a walled garden and glasshouses for fruit and veg at Hyde Hall that will last 100 years?"