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Mr Stuart (84), originally from the Catlins, was working at the Dunedin Botanic Garden with his twin brother when their boss was promoted to the gardens in Christchurch, taking his two young apprentices with him.
There, Christchurch-born Mrs Stuart (85, nee Barton) was a florist and head of the propagating department.
"She was my boss ... the only girl amongst 18 boys," Mr Stuart recalled.
Romance quickly bloomed between the two, helped by Mrs Stuart's daily delivery of flowers to Mr Stuart while he was in hospital with rheumatic fever.
They married at St Peter's Anglican Church in Christchurch in 1952, and lived in Clinton for a while where Mr Stuart had set up a nursery. They then moved to manage the family farm at Waipahi, where Mrs Stuart wrote her thesis and became the second woman in New Zealand to gain her national diploma in horticulture.
They had three children, son Lynn, and daughters Jane and Kate.
In 1984 they bought half a hectare of bog land in Wanaka, where Mr Stuart had spent many fishing holidays.
"It was just a wilderness," he said of the property.
"All the locals thought we were mad," Mrs Stuart added.
With the help of some friends they transformed the property, and after deciding they were "too young to retire", they opened a floral studio and a garden centre there.
Business boomed, but after about 10 years, they sold the lease. A beauty therapist business now operates from the studio on their property.
Still not ready to retire, the couple established a wholesale nursery business on the outskirts of Wanaka which supplies plants throughout New Zealand.
"Keith was bored and started growing plants as a hobby, which turned into a business," Mrs Stuart said.
Despite recently going through cancer treatment, Mr Stuart is back working at the nursery, while Mrs Stuart - a keen waterskier - takes care of the book work and plant orders.
The active pair, who have nine grandchildren, have been busy this week sprucing up their garden for today's celebrations with nearly 80 guests.