The most common reasons why gardenia leaves turn yellow include overwatering, poor soil drainage, and nutrient deficiencies such as iron or magnesium deficiency. Overwatering or soggy soil can cause root rot, impairing the plant's ability to take up nutrients, which leads to yellowing leaves. Iron deficiency disrupts chlorophyll synthesis and causes yellow leaves with green veins, particularly on new growth. Magnesium deficiency can also cause yellowing. Additionally, temperature stress, pH imbalance in the soil (gardenias prefer acidic soil), and pest or disease stress can contribute to yellow leaves. To fix yellow leaves on gardenias:

• Ensure the soil is well-draining and not waterlogged.
• Water only when the top inch of soil is dry, about 1 inch of water weekly.
• Apply chelated iron sprays if iron deficiency is suspected.
• Use fertilizers designed for acid-loving plants to correct nutrient imbalances.
• Check for pests like aphids or spider mites and treat as needed.
• Consider soil pH adjustments to maintain acidity between 5.0 and 6.5.

Yellowing can also be a natural aging process for older leaves but should not be widespread or accompanied by poor plant health signs.