The White Garden, at Kensington Palace, was planted to mark 20 years since Princess Diana died in a car crash.
The Duchess of Cambridge joined the princes on the garden tour.
A spokeswoman for Kensington Palace said: "The engagement will allow the princes to pay tribute to the life and work of their mother."
They met representatives from the causes and charities supported by Diana, including the Royal Marsden and Great Ormond Street hospitals, the National Aids Trust, Centrepoint youth homelessness charity and the Leprosy Mission.