'Astra Semi-Double White' Platycodon (Platycodon grandiflorum 'Astra Semi-Double White') is an unusually beautiful "Balloon Flower" that bears large (to 3 1/4 inches wide), fully double, starlike white flowers. The plant is compact, grows to 6 inches tall, and blooms continuously. It's also perennial in zone 5, which means it has a chance to carry over year-to-year with milder Ozarks winters.