White lilies are flowers that can be planted in a flower patch. They require a Farming level of 58, and grow from white lily seeds. When fully grown, white lilies protect all neighbouring allotments from disease. When a fully grown white lily is picked, it gives the player one white lily.
Unlike other flowers, the white lily cannot be used on anchovy oil to create imp repellent. It can, however, be used to make supercompost. Other than making supercompost, there is no known use for this item.
According to the book Gielinor's flora - flowers, white lilies are sweet-smelling, symbolising chastity and virtue. As such, it has become a symbol among followers of Saradomin, with some even believing that Saradomin himself created the lily to improve the crops of his most cherished followers.
For an exhaustive list of all known sources for this item, see here