Mineral deficiency is one of the major causes of chlorosis in plants and in gardenias the most common deficiencies are magnesium, nitrogen, and iron, which result in the leaves turning from a glossy green shade to a bright yellow. While people commonly try to grow gardenias as houseplants, they can be finicky as they require high humidity. 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. The soil should not be wet. Water when the soil is slightly dry.
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.
Organic matter and manures can also be added to the soil to help balance. We do have an Aged Cow Manure product available here.
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 sulfate (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 the soil with ammonium sulfate or agriculture sulfur, and apply iron sulfate. A foliar feed with a complete liquid fertilizer will help correct this problem. We suggest our Founders Fish liquid fertilizer, available here.
It is recommended not to use any gardening tool to remove the yellow leaves, instead, use your hands.
The first thing I would try is reducing how much you are watering.