Prairie Moon Nursery has a great description of this plant: “Bottle Gentian are slow-growing but long-lived and require little care once established. Bumblebees are the main pollinators because they are the only insects strong enough to pry open the closed flowers (see photo). Cream Gentian is the first Gentian to bloom in late summer. Bottle Gentian and others (see our website https://www.prairiemoon.com ) may wait until September or October to lend late-season color to mostly sunny sites in medium-wet to medium-dry soils. It is a great companion with other late bloomers such as: New England Aster (Aster novae-angliae), Obedient Plant (Physostegia virginiana), and Prairie Blazing Star (Liatris pycnostachya). Other common names include Closed Bottle Gentian, and Andrew’s Gentian.”
I have some bottle gentian in the back yard. It’s fun to watch bumblebees force their way in to get a juicy tidbit while unknowingly spreading pollen to other flowers.