Short-toothed Mountain Mint, Pycnanthemum muticum
If you want continuous buzzing of pollinators in your garden, this is a plant to grow. There are multiple species of mountain mint -- all native to North America. Two which grow well here are P. muticum and P. virginicum. The flowers are not showy by human standards but they apparently are for pollinators. They prefer sun but in my garden thrive in part shade. I've referenced a Penn State study below. Their Center for Pollinator Research has lots of great information and links of interest to us in the mid-Atlantic region. http://ento.psu.edu/pollinators
Penn State Extension Study, 2013
Evaluated 88 herbaceous pollinator plants
Mountain Mint Pycnanthemum muticum
#1 longevity - 10 weeks peak bloom
#1 pollinator diversity
#1 insect visitors – 78 in 2 min.
#1 bee and syrphid visits – 19 in 2 min.