While there are 200 different species of gardenias in the world, most people encounter the cape jasmine or common gardenia (Gardenia jasminoides or Gardenia augusta) as a potted houseplant, or growing in a garden setting. The plant is known for its glossy green leaves and fragrant white flowers. Many varieties of the common gardenia are grown, mainly types with double-form flowers, which have extra rows of petals to make their blooms resemble those of roses or camellias. Grow common gardenia outdoors where winters are mild, as in U.S. Department of Agriculture hardiness zones 8 and warmer, or as a houseplant elsewhere.
Provide a moist, well-draining soil that is not alkaline in pH and is rich in organic matter for the common gardenia to grow. Whether grown indoors or out, the soil must be acidic (pH less than 7.0), otherwise the gardenia will have trouble growing and maintaining healthy green leaves. Add organic matter to sand and clay soils to improve fertility, texture and drainage qualities.
Plant your shrub or place the containerized gardenia plant where it receives six hours of sunlight from dawn to midday, then protective shade from trees or a building. While gardenias can handle a lot of direct sunlight, it is important that the foliage is shielded from intense, hot sun rays in the heat of the summer afternoon. Too little sunlight or stagnant air can lead to mildew on leaves.
Maintain a high ambient humidity around the gardenia shrub, especially in the growing season from spring to fall. Place the plants in a wind-sheltered part of the garden or near a water feature, and out of wind-swept areas, to elevate humidity and help retain foliage and develop summertime flower buds. Don't spray the foliage with water as that can increase mildew fungal problems. In regions with chilly winters, keep the plants out of areas where cold, drying winds can reach the plants. Cold winds (which naturally have less moisture in them) can also lead to premature abortion of young flower buds.
Water and fertilize your common garden shrub freely when there are new leaves and flowers present during the growing season to keep the soil evenly moist, but never soggy. Use a well-balanced fertilizer formula (such as 10-10-10 with micro nutrients) in granular or water-soluble form according to product directions in spring and summer. Do not fertilize plants in the winter. Reduce watering in winter to keep the soil slightly drier when the plants are not actively growing.
Prune back errant branches or tips on the gardenia in spring using hand-held pruners. Do this just before or as the shrub begins to display its newest leaf and stem growth. Make pruning cuts 1/4 inch above a lower branch junction, lower healthy leaf or dormant bud. Prune away any dead or diseased branches anytime you encounter them, regardless of season.