By about now most gardenia plants will have some yellowish leaves on them.
Not to worry though! This is just the cold winter temperatures and you don’t need to do anything.
… Yes, I know that the old books say that this is magnesium deficiency and you need to apply Epsom salts.
This is complete nonsense – just a silly Old Husband’s Tale.
When the warmer weather comes, give all gardenias some duck poo or chook poo or rose food.
In the meantime, just relax in a seat out in a sunny spot in the garden.