Rose of Sharon

Identification and origin

Hibiscus syriacus belongs to the mallow family, Malvaceae, which also includes hollyhocks, cotton, and okra. Despite its name, it does not come from Syria: it is native to southern China and Taiwan. Grown for a very long time, it has naturalized in many parts of Europe, Asia, and North America.

It is a fast-growing deciduous shrub with an upright habit, often multi-stemmed, which can also be trained as a small tree. It usually reaches 8–12 ft tall and 6–10 ft wide. The alternate, simple leaves are 2–4 in long, three-lobed, coarsely toothed, smooth, and sometimes glossy, turning a modest yellow in fall. It is one of the last shrubs in the garden to leaf out in spring, so don’t write it off too soon.

The flowers, 2–4 in across, have five crepe-textured petals and a prominent central column of stamens, much like a hollyhock. They form on the current season’s growth, from early summer into fall. Each flower lasts only one day, but new ones keep opening for weeks. Single or double, they come in white, pink, red, lavender, violet-blue, or two-tone, often with a dark red eye. They attract butterflies, bees, and hummingbirds, and they are edible. The fruit is a brown, five-valved capsule about 1 in long that stays on the branches all winter.

There are many cultivars: ‘Diana’, a triploid with large pure white flowers that stay open at night; ‘Blue Bird’, sky blue with a small red center; ‘Red Heart’, white with a red center; ‘Aphrodite’, dark pink with a deep red center; ‘Lavender Chiffon’, lavender with a lacy center; ‘Jeanne d’Arc’, double white; ‘Lucy’, double red; and ‘Purpureus Variegatus’, with yellow-variegated foliage.

Light

Plant it in full sun, with at least six hours of direct sun a day, or in partial shade. It loves heat: a south- or west-facing spot sheltered from wind suits it well. In shade, flowering drops off noticeably and the branches stretch and weaken. It tolerates urban air pollution and moderate salt, which makes it a good shrub for city and coastal gardens.

Watering

Once established, rose of Sharon handles dry spells, but it blooms more freely when the soil stays moist through summer. New plantings need more attention: during the first growing season, water whenever the soil feels dry below the surface, soaking the whole root zone, until the shrub is well rooted.

After that, pay most attention during long hot, dry stretches in summer. Feel the soil at the base of the shrub, and if it is dry deep down, water thoroughly to support flowering. On the other hand, avoid soil that stays soggy: the shrub does not tolerate extremely wet ground, and excess water can lead to edema and rot.

Potting mix

This shrub adapts to almost any soil: clay, loam, sand, or chalk, from acidic to alkaline. It does best, though, in moist, well-drained ground rich in organic matter, so work well-rotted compost into the planting hole. Only extremely dry or constantly waterlogged soils cause trouble. It also tolerates road salt and growing near black walnut trees.

Temperature

This is a hardy shrub, grown in USDA zones 5a to 9b, and it withstands winters down to about 5 °F. It enjoys hot summers, when it blooms best. Winter cold is not a problem, since the shrub is leafless then; what usually catches gardeners off guard is how late it wakes up in spring. Wait until new shoots appear before judging any damage.

Repotting

Rose of Sharon is grown mostly in the ground. Choose the site with its mature size in mind, because it fills out quickly. For a hedge, space plants 6–12 ft apart. Set it in soil enriched with organic matter, and water regularly through the first growing season.

Young plants raised from cuttings can be moved into a larger pot as soon as their roots have formed, then planted in their permanent spot once they are well developed.

Pruning

Although it can grow without pruning, rose of Sharon blooms far better when pruned every year in early spring. Because it flowers on the current season’s growth, a fairly hard pruning at that time encourages vigorous new stems and more, larger flowers.

If older wood is not removed regularly, the flowers get smaller over the years. Each year, cut a portion of the oldest stems back to the base to rejuvenate the shrub. In summer, remove spent flowers and seed capsules before they ripen to prevent self-sown seedlings, which can be numerous. Pruned well, the shrub stays bushy enough to serve as a hedge or screen.

Propagation

Cuttings are the most reliable way to reproduce a cultivar. Take semi-hardwood cuttings, pencil-thick and 5–6 in long, from firm current-season growth. Strip the lower leaves and insert them in a mix of three parts sand to one part peat or a substitute, kept lightly moist. Roots usually form in four to five weeks; then pot up the young plants.

Seed is easy too: collect it when the capsules turn brown and start to split, soak it overnight, and sow fresh seed in fall or sow in spring after the last frost. Some cultivars readily self-seed.

Common problems

• Flowers getting smaller every year: old wood never renewed, or the spring pruning skipped.
• Sparse flowering: too little sun, long summer drought, or pruning done too late.
• Foliage suffering in summer: prolonged drought or, on the contrary, soggy soil causing edema and wood rot.
• Invasive self-sown seedlings: capsules left on the shrub; the species has become invasive in some regions.
• Leaf spots, cankers, rust, powdery mildew, gray mold: diseases favored by humidity; remove affected parts and improve air flow.
• Pests: aphids, whiteflies, and Japanese beetles in summer; under glass, mealybugs and scale insects.
• Bare shrub in spring: often just late leaf-out, which is normal for this species.

The same symptom can have several causes, so always look at exposure, watering, pruning, and the condition of the branches together before drawing conclusions.

Toxicity

No toxicity has been reported for dogs, cats, or horses, or for people. The flowers are even edible. As with any plant, an animal that chews a large amount can still have a mild, short-lived upset stomach.