In determining flower measurements as given in the key, mature (but not over-mature) flowers should be used; by the time the anthers are shed, the flowers have generally enlarged somewhat.
1. Perianth at anthesis 3–5 (–5.5) mm long, the deepest sinus usually less than half the length of the calyx.
1. Perianth at anthesis (5–) 6–9 mm long, the deepest sinus ca. half the length of the calyx.
All species found in Heuchera
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. July 23, 2019. https://www.michiganflora.net/genus.aspx?id=Heuchera.