Rorippa

1. Plant truly aquatic, the submersed leaves dissected in a bipinnate pattern into filiform segments (midvein present, the lateral segments again dissected), frequently detaching readily from the stem; petals white.

R. aquatica

1. Plant terrestrial or aquatic but even if in water the leaves with definite flat lobes (not bipinnately dissected) and not falling from the stem; petals yellow.

2. Uppermost leaves (including those at base of branches of the inflorescence) deeply pinnatifid (lobed nearly or quite to the midrib); fruit linear, the ovary soon becoming more than 5 times as long as wide, but not (in Michigan) forming ripe seed; petals 2.5–3.7 mm long, distinctly exceeding the sepals.

R. sylvestris

2. Uppermost leaves often merely toothed or shallowly lobed; fruit nearly spherical to short-oblong or cylindrical, no more than 5 times as long as wide, normally setting copious seed; petals 0.5–2.3 mm long, barely if at all longer than the sepals (often shorter).

3. Pedicels of mature fruit somewhat curved or arching downwards; petals ca. 1 mm long or shorter; leaves all glabrous beneath; mature fruit ca. 2–5.2 mm long, up to 1.7 mm thick, mostly broadest well below the middle, strongly tapering to ± acute apex.

R. curvipes

3. Pedicels of mature fruit usually ± straight, ascending or spreading; petals ca. (1–) 1.5–2.3 mm long; leaves glabrous or hispid beneath; mature fruit thicker, the largest on a plant (1.6–) 2–2.7 mm thick at or near the middle, not strongly tapering, the apex blunter.

R. palustris

All species found in Rorippa

Rorippa aquaticaLAKE CRESS 
Rorippa curvipesYELLOW CRESS 
Rorippa palustrisYELLOW CRESS 
Rorippa sylvestrisCREEPING YELLOW CRESS 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 27, 2017. http://michiganflora.net/genus.aspx?id=Rorippa.