Natural

• Gardenia leaves naturally turn yellow during their dormant period, winter. However, if there are yellow leaves during the spring and summer it could simply be that the leaves are old. When Gardenias age, the older leaves begin to die off, so the plant reabsorbs the nutrients from the old leaves, which turns them yellow. The old leaves will then naturally fall off, or they can be pruned by hand.
  
  

Soil

• Gardenias that have yellowing leaves could be sitting in improper soil conditions. If the soil is too acidic or too alkaline, it can prevent Gardenias from absorbing all of the nutrients in the soil, even if they are plentiful in the soil around the roots. Alkaline soil can prevent the uptake of iron, which can cause yellowing in Gardenia leaves.
  
  

Water

• A simple cause for yellow leaves on Gardenias is that there is not enough or there is too much water in the ground near the roots. Too little water will disallow the plant roots from absorbing nutrients that are necessary to feed the plant and keep the leaves healthy; in response, the plant will absorb nutrients from within itself, usually from its leaves. If there is too much water present, it can drown the roots, killing off the Gardenia slowly.
  
  

Fertilization

• Inadequate fertilization can lead to a number of disorders in Gardenias, notably, yellowing of the leaves. If there is a deficiency of different micro-nutrients, especially iron, the leaves of Gardenias will turn yellow. To prevent or repair this damage, make sure that the fertilizer that you use has iron in it and that you fertilize three times per year.
  
  

Nematodes

• Nematodes are microscopic, parasitic roundworms that feed on the roots of Gardenias. Nematodes can cause wilting in leaves, yellowing in leaves, thin canopy, and a number of other problems with your Gardenias. To help with nematodes, apply organic matter, such as compost, liberally to the surrounding soil, to encourage the natural enemies of the nematodes to prosper.