Yellow Wood-Sorrel [Oxalis stricta] photos were taken at the North Carolina Arboretum, in Asheville, on 14 May 2017. It’s a member of the Wood-Sorrel (Oxalidaceae) family. Newcomb’s Wildflower Guide, pages 246-247, states that it blooms from Spring to Fall and is a “common weed of gardens and roadsides.”
USDA Plants Listing: Oxalis stricta