Closed Gentian (Gentiana andrewsii) Closed gentian earns its name because its petals never fully open. Its other common name is bottle gentian. The blooms "open" in late August and persist into September. Only large bumble bees are strong enough to force their way inside to get at the pollen and nectar, but find a rich reward. This ensures they will likely select another gentian and improve the plants' chances for pollination.