The retired marketing manager appeared on the popular BBC quiz show and correctly answered 10 questions on the Northumberland visitor attraction.
She then showed off her general knowledge to win the episode and move forward to the semi-finals.
Ian McAllister, head of marketing at The Alnwick Garden, said: “We were excited to tune into Mastermind this week to see Rosemary Duns answering questions on her specialist subject, The Alnwick Garden.
"It’s not often you get to see The Garden sandwiched between Marvel Films and John Cleese!
"We hope that Rosemary could hear the cheering from Alnwick when she won and we can’t wait to see what happens in the semi-finals!
"There’s always a job as a tour guide here for her and we wish her the very best of luck.”
The episode is available on BBC iPlayer.
How well do you know The Alnwick Garden? Here are the questions Ro was asked on Mastermind (answers at the end).
1 Which 18th century landscape designer was commissioned by the 1st Duke of Northumberland to redesign the grounds of Alnwick Castle including remodelling the River Aln to make it more like a lake?
2 After Jane Percy, Duchess of Northumberland, decided in 1996 to redevelop The Alnwick Garden, she immediately appointed the estate’s clerk of works to be the project director. What was his name?
3 What was the name of the Belgian landscape architect who, together with his son Peter, conceived the formal plans for Alnwick, the pair’s first garden design in Britain?
4 What name was given to the plot of land alongside Denwick Lane that Algernon Percy, the 4th Duke of Northumberland, bought in the mid-19th century to increase the size of the castle’s grounds?
5 The species of blossom tree commonly called the Great White Cherry, more than 320 of which form the Garden’s cherry orchard, has what Japanese name?
6 What’s the title of the television documentary first broadcast on BBC One in January 2003 which followed the development of phase one of the Garden?
7 The Alnwick Rose, which was unveiled at the Chelsea Flower Show in 2001, is the creation of which Shropshire rose breeder?
8 The sculptor, William Pie, designed eight water features in The Serpent Garden, including one named after which 17th century Italian mathematician?
9 Alnwick is renowned for its Poison Garden full of toxic plants. What plant genus represented there includes the deadly species known as Monkshood, Wolfsbane or Blue Rocket?
10 What was the name of the former SAS soldier and mountaineer who helped to design the elevated walkways around the Garden’s Treehouse to make them accessible to wheelchair users?
11 The Grand Cascade, the multi-tiered water feature that forms the backbone of the whole Garden scheme is bordered by a topiary hedge formed from which tree of the birch family?
12 What’s the name of the honey-coloured sandstone sourced from a nearby quarry that was used for the outer walls of The Grand Cascade and lower basin?
1 Capability Brown
2 Ian August
3 Jacques Wirtz
4 Goose Knows Field
6 Charlie and The Duchess
7 David Austin
10 Brummie Stokes