Anthony Tavernor has been restoring Plas Cadnant Hidden Gardens, near Menai Bridge, on Anglesey for 20 years, describing it as his life's project.
But after days of heavy rain, a "tidal wave" of flood water swept through the garden in the early hours of Saturday.
It came as floods caused "chaos" across north Wales.
Former farmer Mr Tavernor has been restoring the 10-acres of garden and buildings at Plas Cadnant since buying the then overgrown 200-acre estate in 1996.
It now attracts visitors - both local and from abroad - and its fans include the Prince of Wales, who Mr Tavernor said had hoped to visit the estate.
But the force of the water from the flooded River Cadnant has now washed away many precious plants, including some rare botanical species.
It also demolished a wall dating back some 200 years, as well stone obelisks, platforms and seats.
"The garden sits down in the valley so a huge amount of water came down from the fields like a tidal wave, devastating everything in its path," said Mr Tavernor.
"I'm just devastated. This has been my life's project - my purpose in life - for the last 20 years and I live and dream it,
"It's everything. I'm so emotionally involved with the garden. We've had so much support from people since we put the pictures on our Facebook page. It's almost like a bereavement."
But he insisted he would carry on his work, despite the setback, and aimed to open as normal for visitors in February.
"I was hoping to have a legacy for the future generations," he added.
"I'm sure a lot of people would abandon it but I'm going to persevere."
Days of rain caused floods which led to roads being closed, trains disrupted and homes evacuated on Boxing Day.
Four flood warnings remain in place across north and west Wales, along with dozens of flood alerts.

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In an error-strewn display, the 2012 bronze medallist needed 102 points from his closing dive to reach the final.
However, he managed only 50.40 to finish in last place with 403.25 - well short of the personal best of 571.85 he set in Friday's preliminary round.
"It's really hard to accept. I've worked so hard," said Daley, 22.
"Yesterday I scored the highest score ever in an Olympic Games. Today it wasn't meant to be. That's what diving does sometimes.
"I truly am heartbroken because I feel like I am in the peak physical condition and I could have won."
British Diving performance director Alexei Evangulov had said Daley was "in the best shape and best form of his career" before the Olympics.
Daley, who won a bronze medal in the 10m synchronised platform with Dan Goodfellow, suggested he would compete again in Tokyo in 2020.
"I'm so happy with how GB have done, so proud to be part of it and I wanted to be able to stand on top of that podium, so it will be another four years' hard work," he said.
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China's Chen Aisen, 20, claimed his second gold of the Games by winning the final, which took place later on Saturday.
Chen, who also won the 10m synchro alongside Lin Yue, scored 585.30 points to finish ahead of Mexico's German Sanchez (532.70) and defending champion David Boudia of the United States (525.25).
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Polish national Daria Pionko, 21, was found in Springwell Road, Holbeck, at about 01:30 GMT on Wednesday.
Police said Miss Pionko, who had been living in Leeds, was a sex worker. She was taken to hospital for treatment, but died a short time later.
A 38-year-old man was arrested in connection with the incident.
Miss Pionko had been found unconscious with injuries consistent with an assault, West Yorkshire Police said.
Officers found her in a defined managed area where sex workers operate during set hours, the force said.
The force appealed for witnesses to come forward and in particular anyone who was in the Springwell Road area between 20:00 Tuesday and about 01:30 Wednesday.
A historical walled garden has been "devastated" by floods which have washed away a 200-year-old wall and rare plants, its owner has said.
[NEXT_CONCEPT]
Britain's Tom Daley said he was "heartbroken" after a shock semi-final elimination in the Olympic 10m platform diving competition.
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A woman who died after being found seriously injured in Leeds, sparking a murder inquiry, has been named by police.