Hibiscus syriacus, (high-BISS-kuss seer-ee-AY-kuss) sometimes called rose of sharon or shrub althaea (also althea), is a member of the mallow (Malvaceae) family, and grows into a large shrub or small, multi-trunked tree that varies in size from 8 to 12 feet tall and 4 to 10 feet wide. Two- to four-inch flowers bloom in summer and may be white, magenta, violet, blue, pink, lavender, or a mixture of these colors. Flowers that are single have five petals, but others are double or semi-double and have many petals. Some selections sport a dark-colored splotch in the center.
Althaea is hardy in USDA Zones 5 to 9. Hailing from China and India, these deciduous plants are late to leaf out in the spring. The two- to four-inch, coarsely toothed leaves often have three lobes. Persistent brown seed capsules shaped like an oval that is pointed on one end develop following the flowers. Mature plants develop a deep tap root that increases wind tolerance, and moderate salt tolerance is a plus for gardens located near bodies of salt water. Althaea is # 8 on Operation Rubythroat's list of exotic plants that attract hummingbirds.
Growing Rose of Sharon
Althaea prefers acidic, well-drained soil, but is tolerant of many soils. Bloom is best in full sun, but some shade can be tolerated. Newly planted shrubs should be watered every day or so until they are established. After establishment, they will benefit from supplemental water during dry periods. Light pruning in spring will promote larger flowers but is not necessary for satisfactory performance. Once plants are well established, they are usually carefree.
Seeds sprout around established bushes, and plants can be found at almost any plant sale or from friends who have rose of sharon growing in their yards. Cuttings taken in summer root easily and are necessary if a plant is wanted that is exactly like the parent. Seeds do not come true to type.
Althaea Bud Drop and Other Possible Problems
Although Althaea is normally very easy to grow and little care is needed, some tendency for buds to drop is evident if soil moisture fluctuates greatly. Provide a good layer of mulch to help to keep the moisture level of the soil as constant as possible. Fertilize sparingly because too much fertilizer not only can cause buds to drop, but it may also increase aphid infestations on an abundance of tender, new growth. If aphids do appear, wash them off with a strong spray of water, or simply pinch off a heavily infested stems and discard them.
Infrequently, bacterial leaf spot may cause some leaves to fall. If reddish-orange fruiting bodies appear on the bark as a result of canker, prune out infected branches immediately. Not only will this malady kill the branches, but it may also kill the entire plant. Flowers have been known to be infected with blight caused by a fungal infection. Japanese beetles love the rose of sharon, and spider mites can attack, especially in hot, dry weather.
Kinds of Althaea
The Althaeas of my childhood were old-fashioned, pass-along plants that were traded among the country folk. These old-fashioned plants, unfortunately, have a serious drawback. Because seeds are set prolifically, invasiveness is a problem in many parts of the United States.
Sterile triploids have been introduced that have superior traits such as larger flowers and earlier bloom. The earliest of these selections, introduced by the National Arboretum during the '60s and '70s, set no seeds, so unwanted seedlings do not appear. Work continues on this species, and new cultivars are added to the list from time to time. Look for these (list below) and other sterile triploids.
With appropriate selection, gardeners can select cultivars of the old-fashioned Hibiscus syriacus, or rose of sharon as it is often called, that will add beauty to their landscapes for many years. By choosing sterile triploids, the plant can be included in gardens without the worry of contaminating the environment with yet another invasive species.•· 'Diana' - pure white
•· 'Helene' - white with maroon throat
•· 'Aphrodite' - dark pink with a dark red eyespot
•· 'Lucy' - reddish pink double flowers
•· 'Ardens' - blue-purple with a red throat
•· 'Morning Star' - pale gray-blue with pink throat
•· 'Jeanne d'Arc' - lush white, double blooms