Urticaceae, the Nettle Family
Herbs or trees, with small inconspicuous apetalous flowers.
| 1a. Trees or tall shrubs — 2. | |
| 1b. Herbs — 7. | |
| 2a. Leaves oblong-ovate to lanceolate, serrate — 3. | |
| 2b. Leaves broadly ovate to rotund, some of them lobed (Mulberry) — 6. | |
| 3a. Leaves thick, coarsely and doubly serrate, broadest near the middle (Elm) — 4. | |
| 3b. Leaves thin, simply serrate, broadest distinctly below the middle | Hackberry, Celtis occidentalis. |
| 4a. Some of the branches with flat corky wings; leaves smooth above | Cork Elm, Ulmus racemosa. |
| 4b. Branches without corky wings; leaves more or less rough above — 5. | |
| 5a. Petioles and axillary buds glabrous | White Elm, Ulmus americana. |
| 5b. Petioles and axillary buds pubescent with rusty hairs | Slippery Elm, Ulmus fulva. |
| 6a. Leaves rough above | Red Mulberry, Morus rubra. |
| 6b. Leaves smooth above | White Mulberry, Morus alba. |
| 7a. Leaves alternate — 8. | |
| 7b. Leaves opposite — 9. | |
| 8a. Leaves 2-5 cm. long, stems pubescent | Pellitory, Parietaria pennsylvanica. |
| 8b. Leaves 8-20 cm. long; stem armed with stinging hairs | Wood Nettle, Laportea canadensis. |
| 9a. Twining plant; leaves serrate or cleft | Hop, Humulus lupulus. |
| 9b. Erect plant; leaves palmately compound | Hemp, Cannabis sativa. |
| 9c. Erect plants; leaves not lobed or compound — 10. | |
| 10a. Stems armed with stinging hairs — 11. | |
| 10b. Stems glabrous or rough, but not with stinging hairs — 12. | |
| 11a. Leaves ovate, with a heart-shape base | Stinging Nettle, Urtica dioica. |
| 11b. Leaves lanceolate or ovate-lanceolate, not heart-shape at base | Slender Nettle, Urtica gracilis. |
| 12a. Stems glabrous, pellucid | Clearweed, Pilea pumila. |
| 12b. Stems rough, opaque | False Nettle, Boehmeria cylindrica. |