Bottled gentian, Gentiana andrewsii
 
Only the strongest of bees can pry open the closed blue flowers and get to the nectar and pollen rewards. And these are the bumble bees. Others might take a shortcut and chew holes at the base of the flowers to "steal" some nectar without contributing to pollination. 
The bright blue flowers bloom late in the year, from August to October, but always stay closed.
 
Growing conditions:
In the wild, bottled gentian grows in moist meadows and damp prairies, along rivers, ditches, and forest edges. It prefers humus-rich, acidic soil but can tolerate more alkaline, limestone soil. It prefers part shade and moist, well-drained soil, but bottled gentian can be immersed in water for a short period of time. So, it is suitable for planting near downspouts, in rain gardens, along the pond, and at the edges of woodland gardens.