|I have a gardenia in a 24|
|Yellowing of the older leaves (innermost and lowest) is normal for your gardenia. Evergreens retain their leaves for a year or two but eventually these oldest leaves are shed in favor of new leaves so what you are seeing might be normal and the brown spots are incidental. On the other hand, the brown spots can indicate a problem.
Gardenias love a moist environment, but they don't like being drenched and having water on their leaves. If your gardenias have brown leaves or you notice brown spots on them, this is usually due to:
Poor drainage ? Gardenias love to be moist but not soaking wet. Be sure the plant is allowed to drain thoroughly between waterings.
Not enough humidity for indoor plants ? Try using a humidifier in the room where you keep your gardenias.
Not enough sunlight ? Gardenias need at least eight hours of full sunlight each day.
Pests ? Check your plants for pests underneath the leaves, in the soil (when you water your plants, do bugs rise out of the soil?), and on the stems. The type of pest will determine which pesticide you will use. Spider mites commonly cause this condition, and they can be treated with Neem.
Improper pH - Check your soil pH (it should be 5.0 to 5.5). Feed with Miracid if necessary.
At this point, since you've already treated with Ironite, I'd just wait and see what happens. You should see some improvement within a week or 10 days.
Good luck with your plant!