The 4,750 m2 building, designed by architect WilkinsonEyre, is being built at RHS Garden Wisley in Surrey. Once completed, it will provide research and educational facilities for horticultural science.
The project has secured £4m National Lottery funding and £1m from the local enterprise partnership.
The enabling works began in April and construction is due for completion in 2020.
The two-storey building will house plant specimens, laboratory space, library/archive, conference, learning and exhibition spaces, and a café.
Osborne has delivered similar projects in the past, including the library at the Royal Holloway University, where many of the design features were similar to the requirements of the RHS for this flagship project, the company says.
RHS programme director Andrew Jasper said: “This new centre will allow us to inspire the next generation of horticulturists and open up our previously unseen scientific work to our members, visitors, and the wider public.”
Osborne chief executive Andy Steele said: “Osborne has a track record and ethos of working collaboratively with customers to deeply understand their needs, challenges and timescales. We are excited to be part of this flagship project at Wisley, where we can make a difference to people’s lives and leave behind an important legacy.”