When the leaves on your gardenia plants begin to turn yellow it's usually caused by a mineral deficiency that causes chlorosis in gardenia plants and other plants as well.
The PH level of the plants soil is effected by the mineral deficiency which prevents the gardenia plants from absorbing iron, nitrogen, mannesium, and zinc which the gardenia plants need to thrive.
If your gardenia pants are turning yellow you can fix the problem by using some fertilizer high in magnesium or even adding some Epsom salt to the gardenia plants soil and this will correct the PH level of the soil.