Any of the genus Ageratum of tropical American herbs, annuals and perennials, from the sunflower family Asteraceae, tribe Eupatorieae; they are sometimes used in floral displays.
1888, George Washington Cable, Bonaventure, New York: Scribner, “Grande Pointe,” Chapter 2, pp. 77-78,
The way before him was little more than a bridle-path along the earth […] . The soft blue tufts of the ageratum were on each side continually.