It sure look like our (Brazoria County, Texas) native Penstemon tenuis, aka Brazos Penstemon or Gulf Coast Penstemon. It's the only Penstemon that grows here, requiring moist-wet soils. I did check Dave's Garden and supposely it's growing into New York State and Oak Park, Indiana although it's supposed to be hardy only to 7b. Guess no one told the plant.
Brazos Penstemon does have bright green, rather glossy leaves - the wider leaf base with the abrupt narrowing to the pointed tip looks the same. Flower color sounds right (when you say rosy-lavender). Height seems right for northern areas (here it's at least 36").
Was the plant from a nursery or wild? If wlld, it's not Brazos Penstemon.
