Good drainage is critical so do whatever you can to insure the soil doesn't stay soggy for very long. Raised planting beds usually works well.
Feed the gardenias with a plant food for acid loving plants such as Miracid or an Azalea/Camellia food. The Mexican heather will do fine with just a standard turf type fertilizer and that is pretty economical too. Use a tablespoon spread evenly throughout a 2 foot circular area around each plant.
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