Ask Mr. Smarty Plants

Determing the difference between Convallaria majalis (European Lily-of-the-Valley) and Convallaria majuscula (American lily of the valley) is not a simple matter.  In fact, there is an ungoing dispute as to whether they are truly distinct species or whether C. majuscula is a subspecies or a variety of  C. majalis as you will see from the following paper:

Gandhi, K.N., J.L. Reveal, and J.L. Zarucchi. 2012. Nomenclatural and taxonomic analysis of Convallaria majalis, C. majuscula, and C. montana (Ruscaceae/Liliaceae). Phytoneuron 2012-17: 1–4. Published 17 February 2012.

In this paper you will see that, in addition to its authors, there are others who believe that the American Lily of the Valley is actually a garden escapee  of the European Lily of the Valley that was able to establish itself in the wild—the two are the same species but differ at the the subspecies or variety level.

Although the USDA Plants Database still considers C. majalis and C. majuscula as distinct species, ITIS (Integrated Taxonomic Information System) lists C. majuscula as the unaccepted synonym for C. majalis var. montana, American lily of the valley and C. majalis var. majalis as the accepted name for European lily of the valley.  In our Native Plant Database you will still find American Lily of the Valley listed under C. majuscula because our database follows that of the USDA Plants Database.  You won't find C. majalis in our database at all because it is not native to North America according to the USDA Plant Database.

In the paper listed above (Ghandhi et al.), they propose the following names Convallaria majalis subsp. majalis (European Lily of the Valley) and Convallaria majalis subsp. majuscula (American Lily of the Valley).  And, although the paper notes that there is an overlap in features (length of bracts and pedicels, leaf blade size and venation, number of flowers per raceme, etc.) used to distinguish the American and European species, it offers these differences:

"1.  Plants forming dense colonies; leaves green until frost, veins faint; raceme (excluding scape) about half the length of the leaves; bracts 4-10 mm long, shorter than the pedicels; seeds almost globose................subsp. majalis

1. Plants scattered or forming small groups; leaves yellowing in late summer, veins strong; raceme (excluding scape) much shorter than the leaves; bracts 8-20 mm long, usually as long or longer than pedicels; seeds oblate or lenticular...............subsp. majuscula"

Now, I hope you aren't going to ask what the difference is between naming a plant a subspecies or calling it a variety because that is another whole can of complicated worms.  See: 

Clement W. Hamilton and Sarah H. Reichard.  Current Practice in the Use of Subspecies, Variety, and Forma in the Classification of Wild Plants.  Taxon. Vol. 41, No. 3 (Aug., 1992), pp. 485-498.