The yellow leaves on a gardenia are commonly caused by several factors including:

• Nutrient deficiencies, especially of iron and magnesium. Iron deficiency often results in yellow leaves with green veins (chlorosis), while magnesium deficiency tends to affect older leaves first. These can be treated by applying iron chelates or Epsom salts respectively.

• Overwatering or underwatering can lead to yellow leaves, with overwatering potentially causing root rot and disease.

• Incorrect soil pH is another factor; gardenias prefer acidic soil with pH between 5.0 and 6.5, and alkaline soils can reduce nutrient availability causing chlorosis.

• Sudden temperature changes or cold weather can stress the plant and lead to yellowing.

• Pests such as aphids, whiteflies, or spider mites may contribute to the problem but typically cause less severe damage.

To fix yellow leaves in gardenias, maintain consistent watering practices, ensure the soil is well-drained and acidic, and consider fertilizing with products formulated for acid-loving plants. Removing severely yellow or diseased leaves can also help the plant focus its energy on healthy growth.