Okay, so the title is misleading - this is a sedge, not a true grass, though it is grass-like. This beautiful plant is Carex pensylvanica, the Pennsylvania sedge.
The flowering tips are spectacular up close
It can host 36 species of caterpillars - quite a party!
Botanists teach amateur gardeners this useful mnemonic to help distinguish between grasses, sedges and rushes:
Sedges have edges,
Rushes are round,
Grasses have nodes from the top to the ground.
When ripe, the inflorescence looks more shaggy, like this