Mentha longifolia (L.) Huds.
Family: Lamiaceae
horse mint, wavy mint
Mentha longifolia image
from USDA Plants website  
Mentha longifolia image
Mentha longifolia image
University of Wisconsin - Madison (WIS-VP)  
Etymology: Mentha: Latin name for an unfortunate Greek nymph named Mentha who got herself turned into a mint plant, and a genus of culinary herbs named after her, this is one of the oldest plant names still in use
Plants: perennial forb, aromatic; stems square
Leaves: opposite
Flowers: flower drawing
Fruits: 1-seeded nutlet
Conservation Status: Introduced - adventive