Platycodon

The balloon flower, a campanula relative is a monotypic (one species in the genus) herbaceous perennial, with tidy foliage and large open bell-shaped flowers, from curious, angular, balloon-like buds.

Although similar to, and used by the same range of insects as, Campanula flowers, those of Platycodon are very much larger, and therefore a plant will not provide so much pollen and nectar resource to passing insects.