Acalypha gracilens
Family: Euphorbiaceae
short-stalk copper-leaf, slender three-seeded-mercury
Acalypha gracilens image
from USDA Plants website  
Etymology: Acalypha: from the Greek acklephes for "nettle," being an ancient name for a kind of nettle but applied by Linnaeus to this genus because of the nettle-like appearance of the leaves
Plants: erect, annual, 8" - 20" tall forb, usually either unbranched or with a few branches near the base; stems hairy
Leaves: stalk about 1/4 as long as the linear to oblong leaf blade
Flowers: 4 (male), 3(female)-parted, tiny, no petals, only sepals; inflorescence a cluster from the leaf axil; blooms July-Oct.
Fruits: capsule, 3 seeds
Habitat: full to partial sun; moist to dry; open woods, fields, meadows; in sandy soil
Conservation Status: Introduced - adventive