President Thomas Jefferson’s passion for botany shaped his instructions to Meriwether Lewis to observe the soil, face of the country, and its vegetable productions, especially those not in the U.S., including when particular plants flowered or lost their leaves. During the expedition, Meriwether Lewis and William Clark collected 178 plants new to science, and most of these plants are now at the Academy of Natural Sciences in Philadelphia, Pennsylvania. William Clark wrote on July 16, 1806, to observe “the Silkgrass, Sunflower & Wild indigo all in blume.” Indians taught the explorers to forage for plant berries and roots, and several new plants were named for Meriwether Lewis and William Clark. The plants identified here include serviceberry, silver buffaloberry, ponderosa pine, cottonwood, blue flax, Lewis’ monkeyflower, bitterroot, golden currant, chokecherry, camas lily, mockorange, and elkhorns.