Up on the hill by the Old Mill the wind never quite quits, and in April it carries the smell of turned soil across West Chester Street. Nantucket Green Tree Inc. mows, trims and keeps lawns tended through the green months, and clears snow from drives and walks when the season turns. Town grass wakes early on the south-facing slopes and lags a week in the hollows near Lily Pond. Reading that difference — where to cut and where to wait — is most of the trade. In winter the same streets get the plow and the shovel, and a path down to the sidewalk by breakfast.