Polygala verticillata var. ambigua (Nutt.) A.W.Wood
Family: Polygalaceae
whorled milkwort
Polygala verticillata var. ambigua image
from USDA Plants website  
Etymology: Polygala: from Greek polys, "many or much," and gala, "milk," since it was thought that the presence of some of the species in a pasture increased the yield of milk
Plants: annual forb
Conservation Status: Native