What's Rose of Sharon? How to Bloom Without Seedling Mess

Rose of Sharon is a deciduous, multi-stemmed shrub in the mallow family that NC State Extension lists at 8 to 12 feet tall and 6 to 10 feet wide, hardy in Zones 5 through 9. Clemson Extension's Bob Polomski places its bloom in July, August, and early September, when few other shrubs are flowering. Virginia Tech Extension lists white, red, purple, violet, and blue flowers, single or double.

It is a different plant from tropical hibiscus and from the herbaceous hardy hibiscus that dies back to the ground. Rose of Sharon keeps a woody frame through winter and grows new flowering shoots from it each year.

Those new shoots are why the timing works the way it does. Clemson notes that the flowers form on the current season's branches, so a plant that looks bare in late spring is not behind schedule. It has not grown the wood that will carry the flowers yet.

Seedlings are the complaint that makes rose of Sharon feel like work. Virginia Tech Extension notes that it self-sows and produces populations of seedlings near the parent, and NC State reports that it has been called invasive in some states, including Virginia and Kentucky. The seed capsules hang on through winter and drop their seed over months.

The usual answer is a low-seed cultivar, and the sources disagree about how well that works.

The Oregon State study explains part of the gap. The four National Arboretum cultivars were introduced as sterile triploids, but flow cytometry found 'Aphrodite', 'Diana', and 'Minerva' to be ordinary tetraploids, and 'Aphrodite' produced about 10 seedlings per pollinated flower, the highest of the nine cultivars tested. Only 'Pink Giant', a hexaploid, came close to sterile, with 0.08 seedlings per pollinated flower as a female parent. The researchers noted the cultivars may have looked nearly sterile when first released and somehow regained fertility, and that plants from different sources may differ.

Two practical conclusions follow. A low-seed cultivar is still worth choosing, since a plant that sets less seed in the garden saves weeding time even if it is not sterile. And no cultivar name removes the need to pull seedlings, especially near a woodland edge or in a state that has reported the plant spreading.

Seedlings are easy to pull when they are small and become a digging job once their stems turn woody. Two checks a year catch most of them, one in late summer or fall when the young plants are easy to see, and one in spring before the new crop gets established.

On a large shrub, removing every capsule is unrealistic, which is why cultivar choice and regular pulling do more than deadheading.

Clemson notes that full sun gives the best flower display, although the shrub will grow well in part shade. In a shaded spot, expect fewer flowers rather than a failing plant.

The shrub is not fussy about soil. NC State recommends moist, well-drained soil and says to avoid extremes of wet or dry, and Virginia Tech lists sandy, loam, or clay soils, a pH range of 3.7 to 7.3, and salt tolerance, adding that it transplants readily. Steady moisture matters most in the first couple of summers, while roots spread beyond the planting hole.

NC State notes that the flowers attract butterflies, bees, and hummingbirds, which is a reason to avoid spraying open flowers. When the choice is between summer shrubs for a warm garden, crepe myrtle fills the same July-to-September slot in Zones 6 to 9, while rose of Sharon is hardy a zone colder.

Prune in late winter or early spring, before new growth begins. Clemson's pruning guide groups rose of Sharon with the summer-flowering shrubs that flower on the current season's growth, so cuts made then do not remove any flower buds.

The amount of pruning changes the flowers. NC State notes that spring pruning encourages flowers on new growth and increases flower size, and Virginia Tech goes further, recommending that about one-third of the wood, including the oldest branches and any weak growth, come out at ground level each year. Virginia Tech also notes that flowers get smaller over time unless older wood is pruned out regularly.

The timing is the opposite of forsythia and other spring bloomers, which flower on last year's wood and are pruned right after flowering. Mixing up those two rules costs flowers on one shrub or the other.

Virginia Tech's list is short and specific. Aphids gather on new growth, spider mites show up in hot, dry locations, and leaf spots, blights, Japanese beetles, and whiteflies are the other problems it names.

Japanese beetles arrive during the bloom season, so any control has to account for the pollinators visiting the same flowers. Handpicking suits a single shrub, and the plants Japanese beetles avoid guide covers the larger yard plan.

Virginia Tech suggests rose of Sharon for borders and mass plantings, and its upright 8- to 12-foot frame makes it a summer screen behind perennials or along a fence. A narrow strip by a front walk is a poor fit for the full-size plant, since it needs its 6- to 10-foot width. Its late-summer flowers also pair well with tall perennials in a seasonal privacy screen.

The ASPCA lists rose of Sharon as non-toxic to dogs, cats, and horses, and NC State's toolbox says the same. The listing covers Hibiscus syriacus. It does not extend to other hibiscus or to a shrub recently sprayed for aphids or beetles.