| Common Name | Latin Name |
|---|---|
| Pale Indian Plantain | Arnoglossum atriplicifolium |
| Paleseed Plantain | see Hoary Plantain |
| Palmer Amaranth | Amaranthus palmeri |
| Palmer Pigweed | see Palmer Amaranth |
| Palmleaf Morningglory | Ipomoea wrightii |
| Parrotfeather | Myriophyllum aquaticum |
| Partridgepea | Cassia fasciculata |
| Pennsylvania Smartweed | Polygonum pensylvanicum |
| Penstemon digitalis | see Foxglove Penstemon |
| Perennial Lupine | Lupinus perennis |
| Perennial Pea | Lathyrus latifolius |
| Perennial Pepperweed | Lepidium latifolium |
| Perfoliate Pennycress | Thlaspi perfoliatum |
| Perilla Mint | Perilla frutescens |
| Persian Speedwell | Veronica persica |
| Pickerelweed | Pontederia cordata |
| Piedmont Bedstraw | Cruciata pedemontana |
| Pin Oak | Quercus palustris |
| Pineapple-weed | Matricaria matricarioides |
| Pinnate Tansymustard | Descurainia pinnata |
| Pitted Morningglory | Ipomoea lacunosa |
| Plumeless Thistle | Carduus acanthoides |
| Poison Hemlock | Conium maculatum |
| Poison Ivy | Toxicodendron radicans |
| Pokeberry | see Common Pokeweed |
| Poorjoe | Diodia teres |
| Potato Vine | see Chinese Yam |
| Prairie Mimosa | see Illinois Bundleflower |
| Prairie Rose | Rosa Setigera |
| Prairie Wild Rose | Rosa arkansana |
| Prairie beardtongue | see Trumpet Penstemon |
| Prickly Lettuce | Lactuca serriola |
| Prickly Pear | Opuntia humifusa |
| Prickly Sida | Sida spinosa |
| Prostrate Amaranth | see Prostrate Pigweed |
| Prostrate Knotweed | Polygonum aviculare |
| Prostrate Pigweed | Amaranthus blitoides |
| Puncturevine | Tribulus terrestris |
| Purple Amaranth | Amaranthus blitum |
| Purple Cockle | see Corn Cockle |
| Purple Cudweed | Gnaphalium purpureum |
| Purple Deadnettle | Lamium purpureum |
| Purple Feathergrass | see Chinese Fountain Grass |
| Purple Loosestrife | Lythrum salicaria |
| Purple Lovegrass | Eragrostis spectabilis |
| Purple Milkweed | Asclepias purpurascens |
| Purple Nutgrass | see Purple Nutsedge |
| Purple Nutsedge | Cyperus rotundus |
| Purple-Headed Sneezeweed | Helenium flexuosum |
| Purplehead Sneezeweed | see Purple-Headed Sneezeweed |
| Purpletop | Tridens flavus |
| Purslane Speedwell | Veronica peregrina |
