| Ilex opaca var. opaca | American Holly |
| Ilex verticillata | Winterberry Holly |
| Impatiens capensis | Spotted Jewel-weed |
| Impatiens pallida | Pale Jewel-weed |
| Ionactis linariifolia | Flaxleaf Aster |
| Ipomoea hederacea | Ivy-leaf Morning-glory |
| Ipomoea lacunosa | Whitestar Morning-glory |
| Ipomoea pandurata | Big-root Morning-glory |
| Iris prismatica | Slender Blueflag Iris |
| Iris pseudacorus | Yellow Iris |
| Iris versicolor | Blueflag Iris |
| Isotria medeoloides | Small Whorled Pogonia |
| Isotria verticillata | Large Whorled Pogonia |
| Iva frutescens subsp. oraria | Northern Marsh Elder |