The White Garden, at Kensington Palace in London - the former home of the Princess who died in a car crash in Paris-has been created to mark 20 years since she died on August 31, 1997 in a high-speed Paris car crash.
"The engagement will allow the princes to pay tribute to the life and work of their mother," Kensington Palace said in a statement.
"Together they will reflect on the significant achievement of the princess, and the legacy of her work which continues to resonate with so many today," it said.
Sheltering under large umbrellas against the pouring rain, the princes and the Duchess of Cambridge met representatives from the charities supported by Diana, including Great Ormond Street Hospital, the National Aids Trust and the Leprosy Mission.
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The White Garden has been inspired by memories of the Princess' life, style and image, such as her white "Elvis" Catherine Walker dress and features thousands of white flowers such as white roses, scented narcissi and forget-me-nots around the already existing Sunken Garden.
The royals were met by head gardener Sean Harkin and shown some of Diana's favourite flowers and were introduced to another gardener who knew their mother from her frequent visits to the garden of her former home.
The statue will also be erected somewhere in the grounds of Kensington Palace in the future.
Announcing the artwork in January this year, William and Harry had said, "It has been 20 years since our mother's death and the time is right to recognise her positive impact in the UK and around the world with a permanent statue".
"Our mother touched so many lives. We hope the statue will help all those who visit Kensington Palace to reflect on her life and her legacy," the Princes had said.
Diana had died in August 1997 in Paris when William was 15 and his brother was 12.
The brothers have used the anniversary of the tragedy to speak out about their memories of their mother and pay tribute to her.
"There's not a day that William and I don't wish that she was still around," Harry said during one of many television documentaries aired on British television in the last few weeks.