An occasional specimen may appear ambiguous in leaf shape, but can be placed by checking characters of calyx, pubescence, and underground parts. The earliest (basal) leaves of H. appendiculatum and H. canadense tend to be pinnately divided although the later (cauline) leaves mostly have a basic palmate pattern; in H. virginianum all the leaves are pinnate. The early-formed pinnate leaves of all species in the spring are usually quite mottled with pale green or grayish, giving the leaf a water-splotched appearance.
1. Cauline leaves all deeply pinnate, with 4–6 lateral lobes; calyx without appendages or teeth in the sinuses.
1. Cauline leaves all or mostly palmately lobed (some at times broadly pinnately 4-lobed); calyx with a tiny appendage or tooth in each sinus (or this often lacking in H. canadense).
2. Sinus of calyx with a distinct reflexed appendage 0.5–1.5 mm long (enlarging in fruit); peduncles and pedicels densely pubescent with very short fine hairs plus stiff hairs 5–10 times as long; tap rooted biennial.
2. Sinus of calyx with no appendage or with an erect tooth not over 0.5 mm long; peduncles and pedicels glabrous or with sparse hairs not distinctly of two contrasting lengths; perennial from creeping rhizome.
All species found in Hydrophyllum
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 15, 2019. https://michiganflora.net/genus.aspx?id=Hydrophyllum.