| Nettle family | Urticaceae | Annual or perennial herbs; leaves various, simple, stipulate in most; flowers small and greenish, mostly unisexual and plants monoecious or dioecious, regular, in axillary or terminal clusters; fruit an achene. |
| Nightshade family | Solanaceae | Annual or perennial herbs, or woody vines; leaves alternate, simple to compound; flowers bisexual, regular, solitary or in cymes; fruit a capsule or berry. |
| Oleaster family | Elaeagnaceae | Shrubs with silvery-scaly pubescence, sometimes thorny; leaves alternate, simple, usually silvery below, entire or serrate; flowers mostly bisexual, in racemes; fruit drupelike but an achene surrounded by flesh. |
| Olive family | Oleaceae | Deciduous or semi-evergreen shrubs or trees; leaves opposite, simple or compound; flowers bisexual or unisexual, regular, in various inflorescences; fruit a drupe, capsule, or samara. |
| Onion family | Alliaceae | The Alliaceae was once included in the monocot family Liliaceae, but is now recognized by many as a separate plant family. The family includes herbaceous (nonwoody) monocot plants that are generally perennial but not evergreen. |
| Orchid family | Orchidaceae | Perennial herbs; leaves various, simple and parallel-veined, sometimes absent; flowers bisexual, irregular, solitary or in spikes or racemes; fruit a capsule; seeds lack endosperm, requiring contact with a symbiotic fungus for successful germination; largest family of flowering plants (rivaled only by the Aster family), with possibly over 30,000 species. |
| Passion-Flower family | Passifloraceae | Perennial herbs climbing by tendrils; leaves alternate, palmately lobed, stipulate; flowers bisexual, regular, solitary in leaf axils with conspicuous fringed corona; fruit a berry. |
| Pawpaw family | Annonaceae | Small deciduous trees; leaves alternate, distichous, simple and entire; flowers bisexual, regular, solitary; fruit a bananalike berry. |
| Peony family | Paeoniaceae | Perennial herbs or sometimes shrubby plants; leaves alternate; flowers bisexual, regular; large seeds first red in color, later turning shiny black and bearing a fleshy appendage called an aril. |
| Phlox family | Polemoniaceae | Biennial or perennial herbs; leaves opposite or alternate, simple or compound; flowers bisexual, regular, in panicles or cymes; fruit a capsule. |
| Pickerel-Weed family | Pontederiaceae | Perennial herbs, rooted or floating in water; leaves basal or alternate, simple; flowers bisexual, irregular or nearly regular, solitary or in spikes; fruit a capsule. |
| Pine family | Pinaceae | Trees, all monoecious; leaves needlelike or linear, evergreen in our native species; cone-bearing. |
| Pink family | Caryophyllaceae | Annual or perennial herbs, often with swollen nodes; leaves opposite, simple, entire, stipules scarious or absent; flowers mostly bisexual, regular, solitary or in various clusters; fruit a capsule or 1-seeded utricle. |
| Plantain family | Plantaginaceae | Annual, biennial, or perennial herbs; leaves basal, simple, parallel-veined; flowers bisexual, regular, in spikes or heads; fruit a circumscissile capsule. |
| Pokeweed family | Phytolaccaceae | Perennial herbs to 9 feet; leaves alternate, simple, entire; flowers bisexual, regular, in racemes; fruit a juicy, purplish berry. |
| Pondweed family | Potamogetonaceae | Perennial herbs, rooted in water, totally submersed or with floating leaves; leaves alternate, simple, filiform to elliptic and parallel-veined, stipulate; flowers bisexual, regular, in axillary spikes; fruit an aggregate of achenes. |
| Poppy family | Papaveraceae | Annual, biennial, or perennial herbs, typically with colored or milky sap containing toxic alkaloids; leaves alternate or basal, often lobed or dissected; flowers bisexual, regular, solitary or in cymes; fruits a capsule dehiscent by valves or pores. |
| Primrose family | Primulaceae | Annual or perennial herbs; leaves various, simple and entire in most; flowers bisexual, regular, in various clusters; fruit a capsule with free central placentation. |
| Purslane family | Portulacaceae | Annual or perennial herbs, often succulent; leaves various, simple and entire, stipulate in some; flowers bisexual, regular, in racemes, cymes, or sessile glomerules; fruit a capsule. |
| Quassia family | Simaroubaceae | Trees with bitter oder; leaves alternate, pinnately compound; flowers unisexual (plants dioecious), regular, in large panicles; fruit a samara. |
| Rose family | Rosaceae | Annual or perennial herbs, woody vines, shrubs, and trees; leaves mostly alternate, simple to compound, usually conspicuously stipulate; flowers mostly bisexual, regular; fruit types vary. |
| Royal Fern family | Osmundaceae | Rhizomes stout, covered with persistent leaf bases but lacking scales; leaves variously compound, scaleless; sporangia in dense clusters on reduced, stalklike pinnae, these on separate leaves from those bearing sterile pinnae, or in different portions of the leaf; sporangia with annulus incomplete. |
| Rush family | Juncaceae | Annual or perennial herbs; stems usually round, pithy or hollow; leaves basal or alternate, the blade flat or terete, the sheath open or closed; flowers bisexual, regular, in terminal or lateral inflorescences; fruit a capsule. |