Yellow leaves is one of those catch all problems for gardenias.
* too much shade
* iron defficiency (aka, iron chlorosis) if the laveas are yellow and the leaf veins remain green. Apply iron chelated liquids. Ammend the soil at least once in
spring with either garden sulphur, green sand, iron sulphate, etc
* overwatering or soil is not draining well. Check the soil moisture content daily for a week or two and water when the soil feels almost dry or dry only.
* nematode damage
* some varieties loose some leaves naturally while others do that just as they start a
spring Flush
* check for pests unders the leaves
If the number of leaves yellowing exceeds 1/5 of the bush, something is afoot. If less then monitor soil moisture, mulch, TLC daily to see if it stops or improves.
