Location: Helvella dryophila and Helvella vespertina are western North American taxa, also reported from Michigan and Ontario. They both had been referred to as Helvella lacunosa, but the European Helvella lacunosa probably does not occur in western North America.
Nguyen et al. recommend distinguishing H. dryophila and H. vespertina with a combination of host and macro-morphological.
Helvella dryophila is primarily an oak associate. H. vespertina is a conifer associate.
“Helvella dryophila looks similar to H. vespertina, but the contrast in color between pileus and stipe is striking; the pileus is very dark and squat and rounded with distinct well defined grooves when young (FIG. 1A–C), and the ascomata are up to 85 mm high. We could not find any reliable microscopic differences.”
There is also some difference between the fruiting seasons. Helvella dryophila fruits in the spring (mid-December through April, June at higher elevations). Beug describes H. vespertina as “fall–fruiting”. But “fall” is imprecise. It is actually ""Oct–Mar (rarely in Apr)". Ngyuen, et al., at 1282.. Trudell says most common in fall, but can fruit “practically any time.” (H. lacunosa)