Gardenias -- also known as cape jasmine -- are grown primarily for the scent their flowers create. They can grow anywhere from 2 to 8 feet in circumference and have thick, glossy green leaves.
In semi-tropical areas -- USDA zones 7b and above -- a gardenia grows as an evergreen shrub that shouldn't drop its leaves in the fall as other deciduous plants do. If your gardenia is dropping its leaves in this climate, it may be due to a disease or insect infestation.
Gardenias are susceptible to some insect infestations. White fly and mealy bugs are two of the most common and can cause leaves to yellow and fall. Check the plant occasionally for evidence of bugs and treat with an insecticidal soap to control them.
Keeping gardenias healthy prevents loss of leaves. Plant it in a location that receives either full sun or light shade with moist, well-drained soil. Gardenias are acid-loving plants and the pH of the soil should be between 5 and 6. In the early spring, prune it to the desired shape and give your gardenia plant food once a month when it is growing actively.