Hardy Hibiscus Thrive Where Other Shrubs Fail
by Fil Jessee
For a spectacular floral show in summer’s hottest weather, few shrubs can outshine hardy hibiscus. The tall and woody “Rose of Sharon” or althea (Hibiscus syriacus) thrives in poor soils which dry out for extended periods of time, and several species of mallows flourish in soggy places unfit for most other shrubs. But all require full sun and heat to flower well.
The upright growth of althea is especially stunning when planted in odd-numbered groupings of three or more. And these deciduous shrubs are equally well suited for use as fast-growing low-maintenance screens. Their only nemesis is the annual assault of Japanese beetles which quickly spoil their attractive foliage and devastate open blooms as well as flower buds. Fortunately, however, these pests can be controlled by spraying the shrubs with liquid Sevin or applying a good systemic insecticide to the roots from late May to mid-July. Thereafter, grasshoppers, caterpillars, and aphids can sometimes be troublesome but usually to a lesser degree.
Like crape myrtles, altheas greatly benefit from an annual late winter pruning to thin out weaker branches and reduce the length of long branches by at least a third. This will encourage the shrubs to produce more vigorous new growth and, consequently, larger flowers.
Mallows, on the other hand, are considered die-back woody perennials and require cutting back to the ground before new growth begins to emerge from their bases. And for these plants, the size and abundance of flowers can be greatly enhanced by frequent applications of liquid fertilizer and water during periods of drought. If their thirsty root systems are allowed to dry out for more than a day, mallows will show their displeasure by shedding leaves and dropping buds.
In selecting the best varieties of althea, many gardeners are inclined to make choices based on color rather than growth habit. Consequently, they later regret the fact that their selections produce a copious number of volunteers which can turn well-groomed beds and hedge rows into weedy looking thickets. This problem can be eliminated, however, by choosing triploid hybrids or double-flowering cultivars which produce little if any fertile seed. Among the best of these are “Diana” known for large pure white flowers which remain open at night, Aphrodite” with dark pink blooms accented by a prominent deep red eye, “Helene” with white flowers punctuated by a reddish purple blush at the base of the petals, and “Minerva” which sports lavender flowers with a pinkish blush and dark red throat. And the old-fashioned “Blushing Bride” with puffy white flowers and pink accents remains the most popular double.
In the die-back category, the tallest are the Confederate Rose (H. mutabilis) and the Swamp Mallow (H. coccineus). The former can stretch to a height of seven feet or more in a single season, and is easily identified by its large maple-like foliage and huge double pink blooms in fall. This species puts on quite a show just prior to the earliest frost when all other hibiscus begin to slow down or cease blooming altogether. The latter has an open and airy appearance with unmistakable marijuana-like foliage and deep rose or white flowers which closely resemble the blooms of the tropical hibiscus.
The largest flowers by far are produced by the common garden mallows. These can be as big as dinner plates and, thanks to extensive hybridization in recent years, now come in broad array of colors including some beautiful two-tones. The plants seldom reach heights of morethan five feet and, therefore, mix well with many other perennials.
Among the finest of the older named varieties, Southern Belle and Disco Belle are commonly available at most local garden centers. But some pretty exciting new hybrids can be obtained by mail order. Of these, “Kopper King” has stunning dark purple leaves which form a beautiful backdrop for its foot-wide light pink flowers. Another beauty is appropriately named “Fireball” and sports abundant brilliant red flowers on compact four-foot plants. And one of my personal favorites is “Raspberry Rose” which produces numerous raspberry-red flowers on large plants which reach seven feet tall and ten feet across.
When in active growth, all hardy hibiscus species are thirsty plants and should be mulched and well-watered throughout the summer. Most mallows will even thrive in boggy conditions, but altheas prefer dryer locations and well-drained soil. Both will greatly benefit from regular feeding with a balanced liquid fertilizer while forming new growth and, then, a switch to a flower producing formula when buds begin to form.
When the first flush of flowers begins to fade, cutting the plants back by a foot or two will often encourage the growth of secondary branching and an encore of blooms. This is especially beneficial for old-fashioned altheas which, otherwise, will spend most of their late summer energy on developing seed pods rather than flowers.
Propagation of both mallows and altheas is easily accomplished by placing semi-woody cuttings in sand under glass or plastic in shade. New plants can also be started from seed, but offspring obtained in this manner may not produce the same characteristics of parent plants.