Tropical hibiscus

Identification and origin

Hibiscus rosa-sinensis, whose epithet means “rose of China,” belongs to the mallow family, Malvaceae. It is often said to come from tropical Asia or southern China, but it is not known in the wild: botanists treat it as a cultigen, a plant that arose in cultivation from hybrids made in the South Pacific, hence the spelling Hibiscus × rosa-sinensis. It is recorded only as introduced, from Florida to India and the Pacific islands, and even on Madeira and Sardinia.

It is an upright evergreen shrub with alternate, oval, toothed, glossy leaves 2–5 in long. Planted in the ground in a warm climate, it reaches 4–10 ft tall; in a pot, up to 6.5 ft, and it can live for about twenty years.

The solitary flowers are 4–8 in across. Their five petals surround a long column of fused stamens, tipped by five velvety stigmas. Each flower lasts only a day or two, then closes and drops, but others take over. There are some 5,000 varieties, single or double, solid or with a contrasting eye, ranging from red to yellow by way of pink, orange, and white. ‘Cooperi’ stands out for its foliage marbled olive green and white, sometimes tinged pink, and its red flowers.

Light

Light drives flowering: the more the plant gets, the more it blooms. Indoors, give it at least four to five hours of direct sun at a south- or west-facing window. In winter, poor light makes leaves and buds drop, and only long days keep it flowering through the cold season.

It benefits greatly from a summer outdoors, in full sun or light afternoon shade. Acclimate it gradually: first on a porch, then in the filtered light under a tree, and finally in full sun. Reverse the process in fall.

Watering

Keep the potting mix moist but never soggy, and never let the plant dry out to the point of wilting. Feel the surface: when it starts to dry, water until it runs out of the drainage holes, which flushes out built-up salts, then empty the saucer.

Outdoors in summer, a pot can need water almost every day. In winter, when growth slows, let the surface dry a little more between waterings. Hibiscus likes high humidity: during the heating season, set the pot on a tray of damp gravel or clay pebbles.

Potting mix

It needs a rich mix that holds moisture yet drains well: a peat-free houseplant mix or a loam-based mix, in a pot with drainage holes.

It is a heavy feeder. During growth, use a balanced fertilizer: outdoors in summer, at half strength every two to three weeks; indoors, at half strength about once a month in spring and summer. Cut back in winter and stop altogether if the plant is resting. Yellow leaves can signal that it needs fertilizer.

Temperature

Hibiscus blooms best at 64–75 °F during the day; below 55 °F in the daytime, buds drop or fail to form. In winter, nights should not fall below 45–50 °F. A winter rest of two to three months in a bright room at about 55 °F, with little water, suits it well. It handles sudden swings poorly: keep it away from radiators, cold air currents, and windowpanes on frosty nights.

Outdoors, shelter it from wind and bring it in before temperatures drop below 54 °F. In the ground it grows only where there is no frost, in USDA zones 9a to 11b; at the cold edge of that range it can freeze to the ground and regrow from the roots. Some varieties thrive in frost-free Mediterranean climates if watered in the dry season; in France, only the most sheltered spots on the Riviera suit it year-round.

Repotting

Repot in late winter or early spring into a slightly larger pot. Repot young plants every year, as well as newly bought ones, which often come in pots that are too small.

Pruning

Flowers form on the current year’s growth, so pruning keeps the plant compact and free-flowering. In late winter, shorten the previous season’s shoots to leave a framework: by a third to a half of the plant’s height, and cut vigorous stems back to 2–3 in from their base. Remove weak shoots and pinch young plants to make them branch. It tolerates hard pruning well, especially after a difficult winter, but late pruning removes buds and delays flowering.

Propagation

Cuttings are the easiest method. In late spring or early summer, take tip or semi-ripe cuttings 3–5 in long from the strongest shoots. Dip the base in rooting hormone, tap off the excess, and insert them in a small pot filled with a light, sandy mix.

Keep them barely moist in a well-lit spot, inside a plastic bag or a small propagator that keeps the air humid. Bottom heat of 75–81 °F speeds rooting, which takes two to five weeks. Then move each rooted cutting to a 4 in pot, pinch its tip, and leave it undisturbed until spring.

Common problems

• Bud drop: too cold or too hot, cold air currents, too little or too much water, low light, or a recent move. In warm regions, insect larvae can also bore into the buds.
• Yellow leaves: abrupt changes in soil moisture or temperature, overwatering, need for fertilizer; some yellowing is normal in spring and fall.
• Leaves and buds dropping in winter: not enough light; reduce watering so the plant settles into semi-dormancy.
• Small, puckered, brittle new leaves in rosettes: boron deficiency, favored by leached soil or high pH.
• Pests: aphids, spider mites, whiteflies, mealybugs, and scale, often clustered under leaves and on buds; black sooty mold reveals their honeydew. Wash the foliage, treat with insecticidal soap, and inspect the plant before bringing it indoors in fall.

A single symptom can have several causes: check light, temperature, watering, and roots together before acting.

Toxicity

No toxicity has been reported for this species, either in pets or in people: veterinary references do not list it among harmful plants, and its flowers are even edible. Rose of Sharon (Hibiscus syriacus), a close relative that is also called rose of China, is listed as non-toxic to dogs, cats, and horses. If a large amount is eaten, or if in doubt, call a veterinarian.