| | 1. Leaf blade sometimes doubly serrate, lower surface or veins short-hairy |
| U. minor |
| | 1' Leaf blade generally singly serrate, lower surface generally glabrous (or vein axils puberulent) |
| |
| | 2. Flowers appearing after leaves (in late summer to fall); calyx lobes deep; fruit wing opaque, brown |
| U. parvifolia |
| | 2' Flowers appearing before leaves (in spring); calyx lobes short, unequal; fruit wing ± translucent, whitish |
| U. pumila
|