Also called shrub althea, Rose-of-Sharon (Hibiscus syriacus) is a deciduous shrub that was more popularly planted in American gardens in the early 1900s. Modern-day varieties display larger five-petaled blossoms that do not set seeds for a longer period across summer and fall. Plant Rose-of-Sharon in a sunny location in the garden in a fertile, moist and well-draining soil. It is regarded as a "cold hardy hibiscus" grown in U.S. Department of Agriculture hardiness zones 5 through 9.
An upright shrub that attains a rounded shape, Rose-of-Sharon potentially matures to 8 to 12 feet tall and 6 to 10 feet wide. Unusually cold winters may kill above-ground branches back, and new stems rejuvenate in spring. In fact, pruning back the branches annually in late winter encourages a heavier flowering display later that summer.
Rose-of-Sharon is native to temperate areas of northern India and western China.
The leaves emerge each mid-spring and mature into oval or diamond-shaped blades that are dark green. Each 2- to 4-inch-long leaf with coarse teeth may be shallowly lobed or, more obviously, three-lobed. The leaves are killed by fall or winter frosts; they do not turn colors before dropping off.
From midsummer to autumn, the branch tips produce wide-opening, trumpet-shaped flowers that range from 3 to 5 inches in diameter. In the wild, Rose-of-Sharon shrubs display pink flowers with a dark red center and white stamens topped in yellow anthers. All flowers have five petals that resemble crepe paper. Once flowers are pollinated by insects and seeds form, the shrub tends to cease blooming. Modern varieties of Rose-of-Sharon that develop more colorful and larger flowers tend to be sterile and therefore cannot make seeds. They will continually flower until a killing autumn frost, according to Dr. Michael Dirr of the University of Georgia.
Horticulturists have bred or selected differently colored flowers of Rose-of-Sharon shrubs and given them cultivar names. 'Blue Bird' bears lavender-blue flowers that close on rainy days. White-petaled flowers are produced on selections 'Helene,' 'Diana,' 'Lady Stanley,' 'Red Heart' and 'William R. Smith.' Rosy pink to lavender blooms occur on 'Minerva,' 'Aphrodite,' 'Meehanii' (also called 'Variegatus'), 'Pink Giant' and 'Blushing Bride.' Most have dark red centers in their flowers, but not always.