Seeds from Hibiscus calyphyllus Lemon Yellow Rosemallow. his perennial shrub with its striking, large, lemon-yellow flowers and velvety leaves is perfect as an informal border or as a centerpiece in a small garden. A dense, perennial, rounded shrub; up to 9 feet high, the leaves are large,, light green, soft and velvety. The flowers are lemon-yellow, large, up to 4 inches in diameter, with a deep red to blackish center. Flowering time: all year round, but mainly from January to April. Hardy in zones 7a to 11.