Most Gardenias perform best in a well-drained, humus-rich, acidic soil in a sunny or partly shaded position. Water and feed well to promote lush foliage and heavy flowering. In cool climates or during winter time in Australia, they should be moved to a heated greenhouse, as most gardenias are frost tender.
One problem that plagues gardeners is a gardenia bush with yellow leaves. Yellow leaves are a sign of chlorosis in plants. There are several causes and trying to determine the reason can involve a lot of trial and error - Chlorosis in plants simply means that the plant doesn’t have enough chlorophyll. This can be caused by poor drainage, root problems, pH too high, or not enough nutrients from the soil, or a combination of all of these.
The first thing people are advised to do is check the soil for too much water, if this isn't the case be lack of nitrogen, magnesium or iron.
According to the WA Department of Agriculture and Food, if nitrogen is deficient, the oldest leaves appear pale and lack the lustre of healthy ones. Yellowing appears at leaf tips and will affect all the leaves. Apply fertilisers high in nitrogen such as sulphate of ammonia or blood and bone. Organic matter and manures can also be added to the soil.
Magnesium is essential for photosynthesis and the formation of proteins and chlorophyll. It moves freely within the plant and is taken from the older leaves to supplement new growth. Lower leaves are first affected, yellowing from the tip downwards. Dead spots appear. Deficiencies occur on sandy acid soils and affect palms and citrus. To correct the deficiency apply magnesium sulphate (Epsom salts) or dolomite (magnesium-calcium-carbonate).
A condition often referred to as lime induced chlorosis affects acid loving plants that are unable to take up iron in alkaline soils. The newer leaves become pale green, yellow and, in severe cases, white. Veins remain green. This is a common occurrence on coastal alkaline soils. To correct the deficiency reduce the pH of soil with ammonium sulphate or agriculture sulphur, and apply iron sulphate. A foliar feed with a complete liquid fertiliser will help correct this problem.
It is recommended not to use any gardening tool to removed the yellow leaves, instead, use your hands.