Barretto spent her childhood in Fanling where she played amidst wide open fields and rolling hills. She was home-schooled by her mother, who taught with the help of an American syllabus and an encyclopaedia set, while her father gave her a pony and a shotgun.
Barretto was a consummate archivist who typed out everything from scientific observations, accounts of her expeditions, correspondences to overseas scientists, to memos for her co-workers. With her typewriter, she put together many publications for KFBG, as well as a South China Morning Post column between 1974 and 1978.
Her earliest botanical expeditions began while working at the Tai Po District Office, where she often accompanied land surveyors as they drove their Land Rovers into the countryside for observations. She was also one of the earliest Hong Kong botanists to conduct fieldwork in Mainland China. Phillip Cribb from the Royal Botanic Gardens (Kew), remembered Barretto as being “very good at walking at right angles”, meaning that she dared to tread along untrodden paths, allowing her to chance upon species others did not find.