The sweet scent of gardenias in flower heralds the beginning of summer. The genus Gardenia includes about 60 species from the tropics and subtropics of Africa, Asia, Australia and the Pacific. Europe fell under the gardenia’s spell when the first starry gardenia (Gardenia thunbergia) flowered at the Royal Botanic Gardens, Kew, in 1773. This small tree grows up to 5m high with large, short-lived, richly perfumed flowers. It has two mass flowerings, in spring and again in summer. But the number one gardenia is the common gardenia, Gardenia jasminoides (formerly G. augusta), a native of China and Japan. It flowers in summer, but also spot-flowers in spring and autumn. ‘Florida’ is the main variety grown, with the larger ‘Magnifica’ and the low-growing ‘Radicans’ coming second and third.
Gardenias are ideal for containers or garden borders. These shallow-rooted plants love frost-free, humid climates and grow best in well-dug, freely draining, compost-enriched, acidic soil. Coastal regions best suited to gardenias occur from Perth and Sydney northwards, but plants need to be protected from salt-laden sea breezes. For Victorian and South Australian gardeners, the cultivar ‘Grandiflora Star’ is the best choice. In the tropics, subtropics and inland regions, gardenias can have too much sunshine, and last longer in spots where they receive dappled afternoon shade.
A jacaranda or poinciana tree is good for protecting gardenias from intense sun. Gardenias need daytime temperatures above 10–15°C to start flowering, which is why many of the flower buds that form in warm autumn weather succumb to colder winter temperatures and drop off. They also need an acidic soil between pH 4.5 and 5.5. The Tahitian gardenia (G. taitensis) is the only species that copes with alkaline soils, but it can be hard to find and requires a warm climate. Neutral soils can be acidified by watering on iron chelates once or twice each year in late spring, summer or early autumn. Alternatively, you can rake in one handful of powdered sulfur per square metre in late spring or a month before planting, and mulch regularly with pine or she-oak needles to gradually acidify the soil.
Work in plenty of well-rotted manure, compost or a half-and-half mixture of both before planting. Fork this into the top 30–45cm of soil and then plant. In subsequent years, mulch with a 10cm layer of compost in spring to supplement the organic content of soil. Compost is great for encouraging earthworm activity. Alternatively, mulch with straw, lucerne, sugarcane or hay but keep it away from the base of the plants to avoid stem rot.
Planting in pots
The best way to enjoy gardenias in cool or inland regions is to grow them in pots. This allows you to move them to a sunny, sheltered position in winter, or a cooler spot with afternoon shade in summer.Plastic or ceramic pots are preferable to terracotta or concrete as they don’t allow the potting mix to dry out as rapidly. Use a potting mix for acid-loving plants. During the warmer months, gardenias grow steadily, and in southern Australia this may mean re-potting only every other year at the most. Congested roots stunt growth, and damaging their roots during re-potting can cause flower buds to drop. Planting, transplanting and repotting are best completed in spring.
Gardenias relish even moisture, but their roots rapidly rot in poorly drained soils, so if you use saucers under your pots always tip any water out of them. Feed gardenias monthly from late spring to the end of summer, when they actively take up nutrients. Use poultry manure sparingly or feed with a flower and fruit fertiliser or seaweed tonic.
The tree gardenia (Rothmannia globosa), from South Africa, can often be found in heritage gardens. Tough and slow-growing, this plant makes an attractively shaped small tree, growing to about 6m high. Its fragrant, cream to white red-speckled flowers bloom in one large show in spring, and are followed by masses of macadamia-like seed pods. A very old and lovely specimen can be seen at Camden Park House in Sydney’s south-west, home of wool pioneers John and Elizabeth Macarthur.
What’s the problem?
Gardenia problems are common during winter and early spring. Flower bud drop and yellow foliage usually occurs because of the cold. Adding nutrients at this stage could be a waste. Don’t panic. Just wait until growth resumes during warmer weather and see if the leaves return to a deep, glossy green.
Why have the flower buds dropped off?
Overwatering, drought, inadequate sun, or high or low night temperatures can be the problem. Choose an ideal location and provide consistent care. It may also be sap-sucking pests such as thrips or aphids. Spray buds and flowers with horticultural spray oil three times in spring or summer, three weeks apart.
Why are leaves dropping off?
Older, lower leaves turn yellow and fall as new leaves and shoots are produced in warm weather. Expect some winter leaf drop in cooler climates.
Why are the leaves going yellow?
If leaves yellow into the warmer months, it’s likely to be a lack of iron and/or magnesium. Magnesium deficiency first affects old leaves. Sprinkle a teaspoon of Epsom salts around the plant and water in. Iron deficiency first affects new leaves. Apply iron chelates. Treat both in late spring and again in summer.
Why have the leaves turned black?
Sooty mould is a harmless fungus that coats leaves. It grows on honeydew, a sticky, sweet substance excreted by scale and mealybug. Spray entire plants with horticultural spray oil. The mould will crumble away or use a soft cloth and soapy water to gently remove it.
Why didn’t my gardenia flower?
Gardenias flower on new growth. Prune or shape your plants, especially hedges, in late August at the latest. Why do the flowers turn brown? Flowers naturally darken from white to yellow as they mature. Drought and intense sun cause flowers to burn, brown and crisp. Protect gardenias from hot afternoon sun, especially in inland, tropical or subtropical climates.
The common gardenia grows about 1m tall and wide, with double white flowers produced primarily in the warmer months. It’s heavily perfumed.
Sometimes sold as ‘Golden Magic’, this is a vigorous hybrid 1–2m tall and wide with double pure white flowers that turn lemon, then finally gold, before falling.
This compact plant is ideal for pots, growing 60–90cm tall and 1m wide, with double, white flowers.
A spectacular plant growing 1–2m tall and wide, with large, double, white flowers and robust growth, this gardenia is great in gardens.
Lead Image from