The description as written here includes both Myriophyllum spicatum and Myriophyllum exalbescens as varieties. They can generally be distinguished by the number of leaflets with var. spicatum having more than twelve pair. It is important to distinguish between the two since var. spicatum is an extremely aggressive alien and var. exalbescens is native. See both pages 272 and 276 in Common Weeds of the United States.