The Duchess was welcomed by Lord-Lieutenant of Hampshire, Dame Mary Fagan DCVO JP, Chairman of Hampshire County Council, Councillor Ken Thornber CBE and Councillor Roy Perry a Trustee of Sir Harold Hillier Gardens and Leader of Hampshire County Council.
In 1964, Sir Harold Hillier celebrated the 100th anniversary of his grandfather having started the nursery business by planting the centenary border.
In the early part of the 21st century, the centenary border was becoming overgrown and the background shrubs and trees had become too large.
In a project that has been five years in planning and taken two years to complete with the help of more than 150 volunteers, the border has been enriched with even more plant diversity.
There are now 800 different types of plants in the Border, adding to the 42,000 plants in the gardens.
The Duchess planted a Sorbus Lemon Drop tree into the border and an identical tree will also be presented to Her Royal Highness as a gift. The tree was chosen to symbolise the 60th anniversary of the gardens this year as it was a cultivar of a tree selected at the Hillier Nursery around 60 years ago.
Councillor Roy Perry said: "It is a real honour. The Gardens have enjoyed Royal patronage since they were opened to the public in the late 1970s and long may that special relationship continue.
"The Gardens are so beautiful at this time of year and offer something for everyone, families and garden enthusiasts alike. The refurbishment and expansion of the Centenary Border is testament to the dedication and attention to detail of all those involved and will be a wonderful addition for all who come to enjoy these spectacular Gardens."
The Duchess later took part in a wine-tasting during a visit to a new vineyard in Hampshire. The Duchess, who is president of the United Kingdoms Vineyards Association, visited Hambledon Vineyard where she was met by owner and managing director Ian Kellett.
She toured the winery before attending a reception where she unveiled a plaque to celebrate the facility opening.
The duchess's father Major Bruce Shand was a vintner and her grandfather Philip Morton Shand wrote a book on wine-making.