Barretto's last book, published posthumously last month, is not only a comprehensive account of Hong Kong's wild orchids, but also a love story about the flowers and their guardian.
Smitten by the blooms, Barretto, a self-taught botanist, dedicated her life to documenting 125 new specimens growing on hillsides and shielding them from prying fingers.
A Hong Kong-born woman of Portuguese descent, she inherited her family's passion for all things botanical. She worked at the Tai Po District Office between 1950 and 1971, studying and identifying flowers brought in by surveyors.
Even without formal training, she became an expert in the diverse Orchidaceae family, which has more than 26,000 species. She read extensively and learned from horticultural specialists J.L. Young-saye and Hu Shiu-ying.
She also maintained a long-term correspondence with Phillip Cribb, from Britain's Royal Botanic Gardens. The pair exchanged ideas and asked each other many "very penetrating" questions, Cribb said.
"She always pointed out my mistakes in the letters," Cribb recalls. "She was often right because of her detailed knowledge of the morphology [structure] of the plants ... She had [the] instinct and persistence of a natural taxonomist."
In the 1960s, Barretto formed a group of wild-orchid enthusiasts, who spent their spare time on hillside expeditions, and naming specimens in the countryside. Their record of native orchids grew from about 70 at the start of their survey to around 120 in 1980.
Her list of writings includes educational articles for the South China Morning Post in the 1970s, a 34-page guide called Hong Kong Orchids in 1980 and a mammoth checklist of Kadoorie Farm's native orchids in 2003.
"She was conscious of the enormity of her task," says her son, Ruy. "She knew it would be days of extra work and correspondence."
At home in Tai Po Kau, Barretto grew her own orchid collection, which included a rare yellow-flowered Eulophia, and after retiring at the age of 55, she established an Orchid Haven at the Kadoorie Farm and Botanic Gardens. Today, the sanctuary features more than 65 local orchid species.
However, her passion was not always a walk in the park. On at least four occasions, Barretto had to step up and offer her expertise in government cases against illegal orchid traders.
The most difficult case was in 1989, when she helped the authorities figure out the origin of 7,000 stolen Lady's Slippers, a rare and unidentified species at the time.
"My mother had to analyse how and where they were stolen," Ruy says. "The orchids were collected in the wild from southwest China for private pleasure."
From the delicate Hong Kong Jewel Orchid to sweet vanillas, the aesthetic appeal of orchids is obvious.
They are often referred to as indicators of a healthy ecosystem since they are highly sensitive to disturbance, requiring very particular conditions to grow.
But their numbers in Hong Kong have fluctuated over the past 30 years, as their flowering cycles have fallen out of sync with the emergence of pollinators, or insects that spread pollen.
Still, thanks to Barretto's lifelong efforts, the flowers now enjoy more attention and government protection. "Modern-day terms like participation, networking, advocacy and awareness did not exist in my mother's time," Ruy says. "But she did it all perfectly and with great charm."