Water gardenia with this once a month for vigorous blooming and no yellow leaves

What causes yellowing and leaf drop in gardenias? You said you mixed it with river sand, and the river sand made up a large portion. Such soil is very good for planting gardenias, because it has good water permeability. Even if you water too much, root rot is not likely to occur, so the yellowing that appeared is clearly not caused by root rot.

Please enter an image description

Its leaves clearly start to yellow from the veins and gradually spread to the whole leaf, but the amount is not large, only a few leaves occasionally, which fully proves that it is caused by a lack of fertilizer, because your river sand makes up a large portion of the potting soil. River sand has no nutrients; it increases the water permeability of the soil, so root rot is less likely to occur, but fertilizer will be lacking. I suggest you supplement fertilizer regularly, two to three times a month. Supplementing fertilizer can keep its leaves from turning yellow. If only one or two leaves occasionally drop in a month, that is normal aging of old leaves. If there are rather many, I suggest you quickly give it some fertilizer, choosing a fast-acting fertilizer such as Flower Duoduo water-soluble fertilizer or other nutrient solution types, all of which are fine.

Please enter an image description

One more point here: the soil you mixed with river sand has no acidic substances in it, so you need to water it with ferrous sulfate once a month. Of course, if you truly love growing gardenias, I suggest you buy an acidic nutrient solution and give it to her a few times a month. That way you can supplement nutrients and also avoid a lack of acidic substances. It is much better than using ferrous sulfate.

Please enter an image description