Sweet Vernal Grass
Although summer is peak season for grass pollen, vernal is an early-blooming grass and is behind the sweet smell of freshly mown hay, according to The Encyclopedia of Herbs. The plant contains the natural compound coumarin, which is responsible for the grass's vanilla-like fragrance when cut and dried.
When does it peak? April to July
Where does it grow? In fields and on roadsides throughout the United States