Professor David Mabberley, author of portable plant dictionary The Plant Book and member of the Royal Botanic Gardens, Kew Directorate, joins the team overseeing the Carmarthenshire attraction.
Other new trustees include taxonomic horticulturist Susyn Andrews and Professor Alistair Hetherington, the Melville Wills Professor of Botany at the University of Bristol.
Chairman of trustees Rob Jolliffe said: "We have looked at strengthening the board's representation from the fields of plant science, horticulture and education - the three fundamental pillars of what the garden aims to offer the public.
"The expertise these new trustees bring with them will complement the backgrounds and experience of the existing board members and strengthen the board's continuing role of providing support to the executive of the garden," he added.
Also joining the board are: National Museum of Wales head of learning Ceri Black; Ashdown Forest superintendent and clerk Hew Prendergast; ecological consultant Richard Pryce of Pryce Consultant Ecologists; and deputy leader and executive board member for resources Wyn Evans of Carmarthenshire County Council.