Phlox divaricata

Phlox divaricata, the wild blue phlox, woodland phlox, or wild sweet william, is a species of flowering plant in the family Polemoniaceae, native to forests and fields in eastern North America.
Etymology.
The species name "divaricata" means "with a spreading and straggling habit".
Description.
Wild blue phlox is a semi-evergreen perennial growing tall with opposite, unstalked, hairy leaves in length and ovate-lanceolate in shape. Flowers appear in late spring and early summer. They are pleasantly fragrant and in diameter, with five petals fused at the base into a thin tube. The petals are a variety of pastel colors: blue-lavender, light purple, pink, or white.
There are two subspecies: ssp. "divaricata", with petals notched at the tip, and ssp. "laphamii", without a notch.
Ecology.
The flowers produce nectar at the base of the long, narrow corolla tube, and pollen near the end of the corolla tube. Only butterflies, moths, skippers, and long-tongued bees have long enough tongues to drink the nectar. Short-tongued bees and flower flies are unable to reach the nectar, but may gather or feed on pollen.
Phlox is self-incompatible, so it requires cross-pollination to produce seed. Butterflies are the most effective pollinators. As they insert their proboscis into the flower to drink nectar, it contacts the anthers and picks up pollen. As they coil the proboscis before moving to the next flower, most of the pollen falls off, but some remains to be transferred to the stigma of the next flower that they drink nectar from.
Cultivation.
Cultivated varieties have various colours, including blue ('Blue Moon'), lavender ('Clouds of Perfume') and white ('Fuller's White', 'White Perfume'). 
"Phlox divaricata" and the lavender-flowered cultivar 'Chattahoochee' have both won the Royal Horticultural Society's Award of Garden Merit.