Family: Malvaceae

There are around 200 species in this genus, varying from small, annual and perennial herbs through to woody shrubs and trees. Most come from the tropics, though a few originate from temperate regions of the world. The evergreen Asian hibiscus, H. rosa-sinensis, is one of the best known tropical flowers and, with good reason, the state flower of Hawaii, where many hybrids have been raised. H. syriacus is a much hardier, deciduous type and will tolerate more adverse conditions than the evergreen species.

Hibiscus flowers are mostly solitary and axil­lary, though in some species they are borne in sprays. Most blooms last for only one day, but modern hybrids of H. rosa-sinensis last much longer, even when cut. The flowers are usually large and showy, with a single stamen, and mostly marked with a deeper colour at the base. Many come naturally in double forms and these have several stamens. In fact, the earliest recorded varieties of H. rosa-sinensis were dou­bles, hence the common name, rose of China.

The fruit of the hibiscus is a dry capsule, often quite hairy and spiny. The leaves vary greatly according to the species, from the shiny, obtuse, entire leaves of H. rosa-sinensis to the deeply divided, hairy leaves of H. diversijolius.

Species

Some of the following species are unlikely to be available outside their native countries.

H. arnottianus, zone 10, from Hawaii, is an evergreen shrub or small tree, to 6 m (20 ft), with ovate, entire, mid-green leaves and delicate, white, fragrant flowers with a cen­tral, red staminal column.

H. diversifolius, zone 10, from Africa, Australia and the Pacific Islands, is an open, spreading shrub, to 3 m (10 ft), with deeply lobed, prickly leaves and very pale yellow flowers, to 18 cm (7 in) across, with purple centers.

H. heterophyllus, native rosella, zone 10, is a free-flowering Australian native. A tall, evergreen shrub, to 6 m (20 ft), it has simple or divided leaves and mostly white to pale pink flowers, to 12 cm (5 in) across, with maroon centers. Yellow variations, with red cen­ters, also occur.

H. insularis, zone 10, from Norfolk Island, is a tall, evergreen, spreading shrub, to 4 m (13 ft), forming a tangle of woody stems at the base. It has small, entire leaves and pale cream to soft lemon flowers, 10 cm (4 in) across, fading to a dull pink. The centre of the flowers is maroon.

H. moscheutos, swamp rose mallow, zone 5, from south-eastern and eastern United States, is an herbaceous perennial which grows to 2.5 m (8 ft). It has ovate leaves and pink or rose flowers. The flamboyant 'Southern Belle' hybrids produce huge flowers, to 35 cm (14 in) across, in many colours, including red, white, lavender and pink.

H. mutabilis, cotton rose or confederate rose mallow, zone 8, from China, may be a multi-branched shrub or small, deciduous tree, to 4 m (13 ft), with large, lobed leaves, hairy on the undersides, and double or single, white, autumn flowers, 18 cm (7 in) across, turning to deep pink.

H. rosa-sinensis, rose of China, Chinese hibiscus or Hawaiian hibiscus, zone 9, is an evergreen shrub or small tree, highly valued for its colourful flowers which are produced over a very long period. The many hybrids of this species have greatly increased the colour range as well as the size and quality of the blooms. The colours now range through reds, oranges, yellows, whites, pinks, browns and lavenders, as well as multi-colours, and are pro­duced in single, semi-double and double flowers up to 30 cm (12 in) across. In warm temperate zones they flower from late spring to late autumn, and in the tropics they produce blooms all year round. In cooler weather, the blooms will become smaller and deeper in colour and sometimes double blooms will revert to single. The red flowers of H. rosa-sinensis yield a dye. Some cultivars tend to be considerably hardier than others, but most are frost-tender. The taller growing cultivars are used for street plantings, hedges and windbreaks; the lower growing types are used as specimens, low hedges or as container plants. Some may be espaliered and others can be grown as standards. A very adaptable plant, H. rosa-sinensis can tolerate salt air but not front-line coastal exposure. Hundreds of cultivars are available, and more are being intro­duced all the time.

H. sabdariffa, Jamaican sorrel or rosella, zone 10, is a tropical annual or biennial, to 2 m (6 ft), with three-lobed leaves and small, yellow flowers. The calyx of the flower is used in jams and jellies.

H. schizopetalus, zone 10, from tropical eastern Africa, is an evergreen shrub, to 4 m (13 ft), with rounded, toothed, deep green leaves and deli­cate red or orange-red flowers, suspended on long stems. The petals are finely cut and recurved, and the staminal column hangs well below the flower.

H. syriacus, shrub althaea or rose of sharon, zone 5, is a beautiful, free-flow­ering, deciduous shrub or small tree, 3-5 m (10-16 ft) high, originally from China. A hardy species, it is useful for temperate-climate gar-dens as it flowers profusely from late summer into autumn. Its leaves take various forms, but are usually three-lobed or toothed. Its cultivars come in cooler colours like blue, pink, purple and white, and may be single or double and up to 16 cm (6 in) across. The petal base is generally blotched with red or maroon.

H. tiliaceus, cot­tonwood tree, zone 10, is a tropical, spreading, evergreen tree, to 12 m (40 ft), with large, heart-shaped, soft green leaves and yellow flowers, to 10 cm (4 in) across, with crimson centers, through the year.

Cultivation

Hibiscus prefer a rich, well-drained, sandy loam, with added animal manure. They like full sun and shelter from cold winds, par­ticularly in cooler districts. During the growing season, they need plenty of water and fertilizer to make sure the blooms are large and many. Regular pruning is necessary to encourage fresh, healthy growth which in turn produces the best blooms. H. rosa-sinensis should be reduced by one-third in early spring, in late spring for cooler districts; H. syriacus and H. mutabilis may be pruned back quite hard in winter; H. moscheutos needs the old canes removed; and other species should be trimmed after flowering.

Hibiscus should not be planted in damp, poorly drained soils as they are prone to attack by Phytophllrora root rot and collar rot. Insect pests such as aphids, whitefly, mealy bugs and scale insects may attack hibiscus and are particularly troublesome under glass. Before spraying, check that the chemical is suitable for hibiscus. Propagate from seed or from cuttings. Many annual and perennial species are grown from fresh, viable seed sown in spring. Some hibiscus have been found to germinate more readily if their seed is nicked with a razor blade. Sow seed in a commercial seed-raising mix, or sand and peat.

Increase H. mutabilis from hardwood cuttings taken in winter; H. syriacus from hardwood cuttings taken in winter or from firm tip cuttings in summer; and H. rosa-sinensis from hardwood cuttings, about 15 cm (6 in) long, taken in late winter or early spring, or from semi-hardwood in summer or autumn. Mild bottom heat can improve results. Some hybrids of H. rosa-sinensis are better if grafted by wedge or cleft graft onto rootstocks, such as H. arnottianus or H. rosa­-sinensis cultivar 'Ruth Wilcox', in spring. Strike cuttings in a mixture of coarse, washed river sand and coir peat, or perlite and coir peat. Hor­mone powder will help root formation.

Climate

There are species of hibiscus suited to various climatic zones, from cold and cool to subtropical and tropical climates. In much of the UK, H. syriacus is the usual garden plant and most others need to be grown under glass in an intermediate to warm greenhouse or con­servatory.