A number of Eurasian elms are planted as shade and ornamental trees, and more may yet be found as escapes. Ulmus procera Salisb., English elm, has been reported from Ontario. It has corky winged twigs, like U. thomasii, but samaras glabrous except for the ciliate margin.
1. Leaf blades less than 5 (–7) cm long, the base only slightly if at all asymmetrical, the margin often simply serrate; samaras entirely glabrous; lateral buds not over 3 mm long; shrub or small tree, locally established as an escape from cultivation.
1. Leaf blades mostly longer than 7 cm, the base strongly asymmetrical and the margins doubly serrate; samaras pubescent, ciliate on margins (or both) or glabrous; lateral buds 4–10 mm or longer; native tree (except for the very uncommon U. glabra).
2. Samaras pubescent over the seed only, the wings glabrous and margins not ciliate; tips of buds, stipules, and perianth covered with dense red-brown hairs; leaves very harshly scabrous above; petioles densely pubescent; flowers nearly sessile.
2. Samaras glabrous on the sides, or if pubescent, then also densely ciliate; buds, stipules, and perianth glabrous to somewhat pubescent but with at most some red-brown hairs on bud-scales; leaves smooth to scabrous above; petioles glabrous to pubescent; flowers drooping on elongate pedicels or racemose (except in U. glabra).
3. Bark of older twigs coarsely winged with corky ridges; samaras pubescent over entire surface; mature leaves very smooth and glabrous above (± pilose when young).
3. Bark of older twigs not winged; samaras glabrous on the sides; mature leaves smooth to scabrous above.
4. Samara margins densely white-ciliate.
4. Samara margins lacking cilia.