Perhaps the most important step taken by the last family to own privately what is today known as Waveny Park was in hiring the renowned Olmsted Brothers landscape architecture firm to design its grounds and gardens, a local expert said Wednesday. Led by the Brooklyn-born founder of Texaco Oil Company, the Lapham family not only built Waveny House but oversaw creation of the carefully cultivated area immediately around it, including the walled garden, according to Keith Simpson, a New Canaan-based landscape architect. Yet since New Canaan acquired the property in 1967, its main house, outbuildings and grounds all have needed regular repair and upkeep, such as when Simpson and the Garden Club restored the walled garden east of the prominent brick structure in 1982. “But it’s only a small area,” Simpson told more than 100 listeners gathered in the Visitors Center at the New Canaan Nature Center for an hour-long talk on the cherished public park. “More places need attention.