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Gardenias, I love Gardenias! Have you tried a fertilizer for acid loving bushes? You can go to a nursery or even The Home Depot or Lowe's and explain what's going on, and then they can recommend what you should do for them.
sounds like too much water. Pine straw has acidity in it and helps transfer the nutrients properly to acid loving plants. Fertilizer could also be needed. Plant nutrient deficiencies vary and sometimes the necrosis in the leaves can identify the issue or which nutrient is lacking...