Rose-of-Sharon (Hibiscus syriacus), also known as shrub althaea, is a hardy hibiscus. Unlike Hibiscus rosa-sinensis, which is cold hardy only to USDA Zone 8b, Rose-of-Sharon grows in Zones 5 through 9. Grow Rose-of-Sharon in full sun or very light shade in nutrient-rich, well-drained soil. Prune often to achieve fewer, larger blooms and prune little or not at all to produce many smaller blooms, according to University of Florida/U.S. Forest Service extension literature.
H. syriacus cv. Admiral Dewey flowers are single, white blooms. Admiral Dewey blooms from middle to late summer to early fall.
H. syriacus cv. Ardens is a semi-double flowering rose of Sharon with light purple blooms. It produces blossoms late in summer. Ardens reaches a height of eight to 12 feet with six- to 10-foot spread.
H. Syriacus cv. Blue Bird has large, single, pale-blue flowers. Blue Bird's flowers have a dark magenta eye (center). Blue Bird blooms from middle summer through early fall. Blue Bird grows six to eight feet tall with six-foot spread.
H. Syriacus cv. Diana is a rose of Sharon with large white flowers that remain open at night, unlike other cultivars. Diana is a heavy bloomer. Diana reaches a height of 10 feet with six-foot spread.
H. syriacus cv. Aphrodite has pink-mauve, single blooms with dark magenta eyes. Aphrodite blooms from summer through early fall. Aphrodite reaches a height of eight to 10 feet with a six- to eight-foot spread.
H. syriacus cv. Red Heart Rose-of-Sharon produces single, white flowers with scarlet eyes. Red Heart reaches a height of eight to 12 feet with six to 10-foot spread. Red Heart bears flowers from late summer through early fall.
H. syriacus cv. Minerva is a pink flowering Rose-of-Sharon that grows to 10 feet tall with six-foot spread. Minerva blooms from middle to late summer through early fall.