The events, which began at RHS halls in London in 2014 will branch out to Blackheath on 3 May and Marylebone on 6 September.
Secret Garden Sundays feature food producer stalls and growers, focussing on attracting a younger audience and grow your own.
The RHS events gave free rent to exhibitors after low early visitor numbers.
The first RHS Secret Sunday at RHS halls is on 1 March.
RHS show season begins with London plant and potato fair at RHS halls on 20-21 February.
Meanwhile, the RHS has dropped its 'floral continents' theme for Chelsea Flower Show floral marquee. The idea, initiated by the BBC, was deemed unworkable as nurseries placed in the overseas half of the marquee could equally be in the UK half, and vice versa.
M&S and Interflora are running UK-themed exhibits. See more in Horticulture Week 20 February.