Gardenias are an indoor plant, grown for their attractive foliage and highly scented showy flowers. Although often considered to be difficult, the advice given here will help guide you to growing success.
Grow gardenias in a compost for acid loving plants. Gardenias can be grown in beds, but growing them in containers allows the plants to be moved to more suitable seasonal sites and makes it easier to control pests.
Over summer, place in a light situation but shade from direct sun, as the plants may be scorched or the leaves may wilt, even if the compost is moist. A west-facing window sill is ideal. Night temperatures should remain as far as possible between 15–18°C (59–65°F) with a day temperature of 21-24°C (70-75°F) as fluctuations can damage flower buds. Ventilate to reduce the temperature and provide a free airflow but without draughts. Bright light is needed but hot direct mid-day sun should be avoided by careful positioning or by some shading of glasshouses and conservatories.
In winter, place the plant in as much light as possible such as in a south facing window of a heated conservatory or glasshouse. Provide a minimum temperature of 16°C (61°F) if possible, although 10–15 °C (50–59 °F) is acceptable.
Watering and feeding
Gardenias grow best in conditions of high and uniform humidity. Stand the container on tray of gravel, expanded clay granules (Hydroleca) or recycled lightweight aggregate (Hortag) and keep moist, with the water level slightly below the surface of the gravel. Mist the leaves frequently in hot weather, but not when in flower, as water on the flowers may discolour them.
Over spring and summer keep the compost moist, but not saturated. Over winter, when the plants are not growing, reduce the watering, without allowing the compost to dry out between watering. Use rainwater where possible, and always at room temperature.
Gardenias require high nutrient levels from six weeks after potting; use a high nitrogen liquid feed every week in spring and summer. Over winter, feed with a balanced fertiliser with trace elements at five or six-week intervals.
Pruning and training
Keep the plants in shape by light pruning, in February or March as soon as flowering is ended.
Gardenias are easy to propagate, and indeed it is a good idea to take cuttings, as you may find that after two or three years, plants may have fewer and smaller flowers. So it is always good to have replacement stock in hand.
Take softwood cuttings in late winter or early spring. Slightly riper, harder shoots (semi-ripe) can be used as cuttings in summer; heel cuttings are often especially successful.
Once rooted, pot on into single pots, and repot annually. Avoid allowing the plants to become pot bound.
Gardenia jasminoides 'Kleim's Hardy': Said to be hardy enough to grow outdoors, although this is not universally accepted.
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Gardenia problems include:
Yellowing of leaves: This can particularly affect the new growth. This is most likely to be iron deficiency but could be root damage caused by over watering. If overwatering is not to blame, apply chelated (sequestered) iron. You may need to treat the plants at at fortnightly intervals if you are using hard water.
Too much, or too little water: Watering is key; too much water leads to yellowing of leaves and leaves fall prematurely. Too little water causes leaf browning at the tips and edges.
Yellow growing points (chlorotic): This and slow growth are caused by lack of light and low temperatures.
No formation of flowers: Can be due to hot nights of 18°C (65°F) or more, and a dry atmosphere. Provide humidity by placing plants on a gravel tray and provide ventilation to reduce temperatures.
Bud drop: Is usually due to extreme or fluctuating temperatures, or over- or under-watering. Choose a location kept at a constant temperature. Buds are also prone to drop when watering is irregular; avoid both dry conditions and waterlogging.
Wilting of leaves, even though compost is moist: Indicates that moisture is being lost through the leaves more rapidly than it can be taken up by the roots. Move plant to a shadier position and mist the leaves three times a day, with rainwater, until the plant has recovered. Stand plant on a gravel tray over spring and summer.
Pests: There are also a number of pests that will cause problems, including red spider mite, mealybug, scale insects and aphids.