Gardenia

Identification and origin

Gardenia jasminoides belongs to the family Rubiaceae, the coffee family. It is native to a broad region stretching from the eastern Himalayas through central and southern China, Taiwan, and Indochina to southern Japan, where it often grows along stream banks.

It is a dense, rounded evergreen shrub. In cultivation it usually reaches 3–8 ft tall depending on the cultivar, with a similar spread; dwarf forms such as ‘Radicans’ stay at 6–18 in. The thick, glossy, dark green leaves, with deeply impressed veins, are about 2–4 in long.

The buds, twisted like a pinwheel, open into waxy flowers 2–4 in across, single or double, creamy white and aging to yellow. Their intense, jasmine-like fragrance is the plant’s main appeal. In temperate climates the flowers open from late spring to early summer, then more sporadically; in warm climates, bloom can be spread across the year.

The genus honors Alexander Garden, a Scottish physician and naturalist who settled in South Carolina in the 18th century; jasminoides means “like jasmine.”

Light

Indoors, give it a warm, very bright spot out of direct sun, which scorches the leaves or makes them wilt. A west-facing window works well during the growing season; in winter, move it to a south-facing window to make the most of the short days.

Outdoors it blooms best in full sun to part shade; in hot regions, morning sun and afternoon shade is ideal. Prolonged shade sharply reduces flowering.

Watering

In spring and summer, keep the potting mix constantly moist but never soggy. Check the surface often: as soon as it feels dry to the touch, water until the whole root ball is moist, then empty the saucer. In winter, water less often, but never let the root ball dry out completely. Too much water, too little, and above all irregular watering all cause bud drop.

Use rainwater whenever you can, especially if your tap water is hard: lime raises the pH and turns the foliage yellow. Let the water come to room temperature before using it.

The air needs to stay consistently humid: set the pot on a tray of damp gravel and, in hot weather, mist the leaves but never the flowers, since water stains the petals. In the garden, drip irrigation keeps water off the leaves and blooms, and a 2–3 in layer of organic mulch keeps the soil cool and moist.

Potting mix

In a container, use a peat-free potting mix for acid-loving plants that drains well and is rich in organic matter. The pH should stay between about 5 and 6.5; above that, the leaves turn yellow. In the ground, work in plenty of compost or ground bark and avoid planting next to concrete, which makes the soil alkaline.

Gardenias are heavy feeders. In spring and summer, apply a high-nitrogen liquid fertilizer every week, ideally one formulated for acid-loving plants; in winter, a balanced fertilizer with trace elements every five to six weeks is enough. In the garden, a slow-release azalea fertilizer in spring, repeated six weeks later, works well. Do not fertilize in fall: the tender new growth it triggers is easily damaged by cold. Too much fertilizer can also cause bud drop, and iron supplements correct yellowing of the young leaves.

Temperature

Indoors it likes 70–75 °F by day and slightly cooler nights, around 59–64 °F. In winter it does well at about 61 °F and tolerates 50–59 °F. Nights at 64 °F or above can prevent flowering, and sudden swings in temperature damage the buds, so keep it away from radiators, heating vents, and air currents.

It can spend the warm season outdoors in a bright, sheltered spot, but bring it back in before nights fall below 50 °F. In the ground it is grown in USDA zones 7b to 11b; cultivars such as ‘Kleim’s Hardy’, ‘Crown Jewel’, and ‘Frostproof’ handle cold better.

Repotting

Gardenias dislike crowded roots: repot every spring into a container only slightly larger, using fresh mix for acid-loving plants. The roots are shallow and resent disturbance, so handle the root ball gently. If repotting is not possible, replace the top 2 in of mix with fresh potting mix.

Pruning

Prune right after flowering, usually in midsummer, to remove straggly branches and keep a compact shape. Avoid hard pruning from fall onward, since it reduces the next season’s bloom. Indoors, a light trim in late winter is enough. Remove fading flowers regularly to encourage new ones.

Propagation

Take softwood cuttings in late winter or early spring, or semi-ripe cuttings in summer; heel cuttings are often especially successful. Dip the base in rooting hormone, insert the cuttings in a light mix based on sand or perlite, and keep it moist: rooting takes six to eight weeks.

Because flowering tends to decline after a few years, taking cuttings regularly keeps your plants young. Layering, seed, and grafting are also possible; nurseries sometimes graft gardenias onto Gardenia thunbergia, a nematode-resistant rootstock.

Common problems

• Bud drop: dry air, too much, too little, or irregular watering, too little light, nights that are too warm, temperature swings, excess fertilizer, poor drainage, thrips, or nematodes.
• Yellow young leaves: iron deficiency, often due to an overly alkaline mix or hard water, overwatering, or damaged roots. A few older leaves yellowing and dropping is normal.
• Leaves wilting while the mix is moist: the foliage is losing water faster than the roots can take it up, in intense heat or dry air.
• Whiteflies: the most common pest; pale, mottled leaves and sticky honeydew that becomes covered in sooty mold, a black fungus. Treat with insecticidal soap or horticultural oil.
• Aphids, scale insects, mealybugs, thrips, and spider mites: check the undersides of the leaves and treat at the first signs; getting rid of these sap-sucking insects also clears up sooty mold.
• Nematodes: these microscopic soil worms attack the roots, causing wilting and loss of vigor, especially in sandy soils.
• Root rot: poorly drained soil.
• Foliage diseases: powdery mildew, leaf spots, anthracnose.

The same symptom can have several causes: always weigh light, watering, temperature, humidity, and root health together.

Toxicity

Toxic to dogs, cats, and horses, and mildly toxic to people. The plant contains geniposide and gardenoside; eating it can cause mild vomiting, diarrhea, and hives. It does not cause contact dermatitis. Keep it out of reach of pets and small children, and if any part is swallowed, call your veterinarian or Poison Control.