An attractive evergreen tropical that features large, glossy leaves and fragrant white blooms, the gardenia is a common sight in southern gardens or areas exposed to mild winters and warm summers. The shrubs thrive in U.S. Department of Agriculture hardiness zones 7 to 11 and are commonly grown as houseplants in colder areas of the U.S. As with other tropical plants, gardenias require regular care and attention to continue producing blooms that brighten the indoor or outdoor spot from May through Jun .
Plant gardenias in well-drained, mildly acidic soils with pH between 5.0 and 6.5 and full sun exposure. The plants thrive in areas with daytime temperatures between 18.3 and 23.9 degrees Celsius and nighttime temperatures consistent between 15.6 and 16.7 degrees Celsius. Improve the quality of heavy soils with shovelfuls of organic matter such as compost or manure prior to planting.
Irrigate gardenias one to two times a week so the soil remains evenly moist at all times. Provide a slow and steady stream of water to the plant, using a hose to prevent splashes. Allow the top 2 inches of soil to dry lightly before irrigating. Increase watering during dry periods to prevent water stress that causes buds to fall prematurely, reducing the blooms on the plant.
Spread a 2- to 3-inch-thick layer of organic mulch around the gardenia plant to help retain moisture and keep the roots cool. To prevent rot, space the layer of mulch 3 to 6 inches away from the plant.
Feed the gardenias 0.227kg. of 15-5-10 fertiliser per 100-foot area in spring. Spread the fertiliser evenly over the area. Reapply fertiliser when the gardenias begin to flower and once again when flowering ceases. Do not fertilise the shrubs in fall. Alternatively, feed the plants an acid-fertiliser (3:1:3) specifically formulated for azaleas at one-half the recommended strength every month from mid March to November.
Prune the shrub after blooming periods to increase vigour and maintain shape and aesthetic appeal. Clip off old, dried or diseased branches using sharp, sterilised pruning shears. Also remove wayward or long limbs to maintain shrub size and shape. Locate and cut old branches from the shrub's canopy to increase sunlight penetration.
Treat gardenia pests, such as aphids, mealy bugs or spider mites, with a homemade solution of 2 tbsp detergent mixed in 1 gallon of water. Spray the solution over the pests to deter them.
Inspect the foliage, stems and flowers of the shrub for symptoms of diseases such as sooty mould, leaf spot, canker and powdery mildew. Snip off infected parts with sharp, sterilised pruning equipment to prevent spread and treat with a registered fungicide at regular intervals.