Symphytum

The corolla changes from pink to blue as in Mertensia.

1. Upper leaves decurrent, with elongate narrow wings extending down the stem; calyx at anthesis ca. (5.5–) 6–9 mm long; pubescence spreading but not thorn-like; nutlets smooth and shiny between the ridges on the angles; connective between the anther sacs extending slightly beyond them.

S. officinale

1. Upper leaves not or scarcely (less than 1 cm) decurrent; calyx at anthesis ca. 3–4 mm long (soon elongating); pubescence of stem and inflorescence partly of strong recurved prickles or “thornlets” (± flattened and broad at base, like miniature rose prickles); nutlets papillose and coarsely reticulate between the angles; connective shorter than the anther sacs (the anthers appearing bifid at the apex).

S. asperum

All species found in Symphytum

Symphytum asperumPRICKLY COMFREY, ROUGH COMFREY 
Symphytum officinaleCOMMON COMFREY 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 11, 2017. http://www.michiganflora.net/genus.aspx?id=Symphytum.